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Vol 30: The Classics
SCIENTIFIC PAPERS
PHYSICS • CHEMISTRY
ASTRONOMY . GEOLOGY
WITH INTRODUCTIONS AND NOTES
VOLUME 30
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124392
P F COLLIER & SON
NEW YORK
Copyright 1910
By P. F. Collier & Son
Copyright 1906
By Harper & Brothers
, . . ' '
Designed, Printed, and Bound at
Qfyt Cottier Pre**, «$eto gork
CONTENTS
THE FORCES OF MATTER
PAGl
Lecture I. — The Force of Gravitation 5
Lecture II. — Gravitation — Cohesion 21
Lecture III. — Cohesion — Chemical Affinity 36
Lecture IV. — Chemical Affinity 50
Lecture V. — Electricity 62
Lecture VI. — The Correlation of the Physical Forces . 75
THE CHEMICAL HISTORY OF A CANDLE
Lecture I. — A Candle : The Flame — Its Sources — Struc-
ture — Mobility — Brightness 89
Lecture II. — Brightness of the Flame — Air Necessary
for Combustion — Production of Water 104
Lecture III. — Products : Water from the Combustion-
Nature of Water — A Compound — Hydrogen . . .117
Lecture IV. — Hydrogen in the Candle — Burns into
Water — The Other Part of Water — Oxygen . . .133
Lecture V. — Oxygen Present in the Air — Nature of the
Atmosphere — Its Properties — Other Products from
the Candle — Carbonic Acid — Its Properties .... 147
Lecture VI. — Carbon or Charcoal — Coal-Gas — Respira-
tion and its Analogy to the Burning of a Candle —
Conclusion 163
ON THE CONSERVATION OF FORCE
Introduction to a Series of Lectures Delivered at
Carlsruhe in the Winter of 1862-1863 181
H c 1 vol. xxx — a
2 CONTENTS
ICE AND GLACIERS
PAGE
A Lecture Delivered at Frankfort-on-the-Main, and
at Heidelberg, in February, 1865 221
THE WAVE THEORY OF LIGHT
[A Lecture Delivered at the Academy of Music, Phila-
delphia, Under the Auspices of the Franklin
Institute, September 29TH, 1884] 263
THE TIDES
[Evening Lecture to the British Association at the
Southampton Meeting, Friday, August 25TH, 1882] . 287
THE EXTENT OF THE UNIVERSE 325
GEOGRAPHICAL EVOLUTION 339
INTRODUCTORY NOTE
Michael Faraday was the son of a blacksmith, and was
born at Newington Butts, near London, September 22, 1791.
He began life as an errand-boy to a bookbinder and stationer,
to whom he was later bound apprentice. After eight years in
this business, he zvas engaged by Sir Humphry Davy as his
laboratory assistant at the Royal Institution, and in 1813-15
he traveled extensively on the Continent with his master, and
saw some of the most famous scientists of Europe. Shortly
after his return to the Royal Institution, he began to make
contributions of his own to science, his first paper appearing in
1816. He became director of the laboratory in 1823, and pro-
fessor of chemistry in 1833; rising rapidly, through the number
and importance of his discoveries, to a most distinguished
position. But he was working at too great pressure, and in 1841
his health gave way, so that for some three years he could not
work at all. He recovered, however, and made some of his
most important discoveries after this interruption ; and was
offered, but declined, the presidency of both the Royal Society
and the Royal Institution. He died August 25, 1867.
It was characteristic of Faraday's devotion to the enlargement
of the bounds of human knowledge that on his discovery of
magneto-electricity he abandoned the commercial work by
which he had added to his small salary, in order to reserve all his
energies for research. This financial loss was in part made up
later by a pension of £300 a year from the British Government.
Faraday's parents were members of the obscure religious de-
nomination of the Sandemanians, and Faraday himself, shortly
after his marriage, at the age of thirty, joined the same sect,
to which he adhered till his death. Religion and science he kept
strictly apart, believing that the data of science were of an
entirely different nature from the direct communications between
God and the soul on which his religious faith was based.
The discoveries made by Faraday were so numerous, and often
demand so detailed a knowledge of chemistry and physics before
they can be understood, that it is impossible to attempt to de-
scribe or even enumerate them here. Among the most im-
portant are the discovery of magneto-electric induction, of the
3
INTRODUCTION
law of electro-chemical decomposition, of the magnetisation of
light, and of diamagnetism. Round each of these are grouped
numbers of derivative but still highly important additions to
scientific knowledge, and together they form so vast an achieve-
ment as to lead his successor, Tyndall, to say, "Taking him for
all and all, I think it will be conceded that Michael Faraday
was the greatest experimental philosopher the world has ever
seen; and I will add the opinion, that the progress of future
research will tend, not to dim or to diminish, but to enhance
and glorify the labours of this mighty investigator."
In spite of the highly technical nature of his work in research,
Faraday was remarkably gifted as an expounder of science to
popular audiences; and his lectures at the Royal Institution,
especially those to younger audiences, were justly famous. The
following examples are classics in the department of clear and
fascinating scientific exposition.
THE FORCES OF MATTER
By Michael Faraday
LECTURE I
THE FORCE OF GRAVITATION
IT grieves me much to think that I may have been a cause
of disturbance to your Christmas arrangements, for
nothing is more satisfactory to my mind than to perform
what I undertake; but such things are not always left to
our own power, and we must submit to circumstances as they
are appointed. I will to-day do my best, and will ask you
to bear with me if I am unable to give more than a few
words; and, as a substitute, I will endeavor to make the
illustrations of the sense I try to express as full as possible;
and if we find by the end of this lecture that we may be
justified in continuing them, thinking that next week our
power shall be greater, why, then, with submission to you,
we will take such course as you may think fit, either to go
on or discontinue them; and although I now feel much
weakened by the pressure of the illness (a mere cold) upon
me, both in facility of expression and clearness of thought, I
shall here claim, as I always have done on these occasions,
the right of addressing myself to the younger members of the
audience; and for this purpose, therefore, unfitted as it may
seem for an elderly, infirm man to do so, I will return to
second childhood, and become, as it were, young again among
the young.
Let us now consider, for a little while, how wonderfully
we stand upon this world. Here it is we are born, bred, and
live, and yet we view these things with an almost entire
absence of wonder to ourselves respecting the way in which
all this happens. So small, indeed, is our wonder, that we are
never taken by surprise; and I do think that, to a young
5
6 FARADAY
person of ten, fifteen, or twenty years of age, perhaps the
first sight of a cataract or a mountain would occasion him
more surprise than he had ever felt concerning the means
of his own existence; how he came here; how he lives;
by what means he stands upright; and through what means
he moves about from place to place. Hence, we come into
this world, we live, and depart from it, without our thoughts
being called specifically to consider how all this takes place;
and were it not for the exertions of some few inquiring
minds, who have looked into these things, and ascertained
the very beautiful laws and conditions by which we do live
and stand upon the earth, we should hardly be aware that
there was any thing wonderful in it. These inquiries, which
have occupied philosophers from the earliest days, when
they first began to find out the laws by which we grow, and
exist, and enjoy ourselves, up to the present time, have
shown us that all this was effected in consequence of the
existence of certain forces, or abilities to do things, or
powers, that are so common that nothing can be more so;
for nothing is commoner than the wonderful powers by
which we are enabled to stand upright: they are essential
to our existence every moment.
It is my purpose to-day to make you acquainted with some
of these powers; not the vital ones, but some of the more
elementary, and what we call physical powers; and, in the
outset, what can I do to bring to your minds a notion of
neither more nor less than that which I mean by the word
power or force? Suppose I take this sheet of paper, and
place it upright on one edge, resting against a support
before me (as the roughest possible illustration of something
to be disturbed), and suppose I then pull this piece of string
which is attached to it. I pull the paper over. I have
therefore brought into use a power of doing so — the power
of my hand carried on through this string in a way which
is very remarkable when we come to analyze it; and it is
by means of these powers conjointly (for there are
several powers here employed) that I pull the paper
over. Again, if I give it a push upon the other side,
I bring into play a power, but a very different exertion of
power from the former; or, if I take now this bit of shell-
THE FORCE OF GRAVITATION 7
lac [a stick of shell-lac about 12 inches long and 1 1-2
in diameter], and rub it with flannel, and hold it an inch
or so in front of the upper part of this upright sheet, the
paper is immediately moved towards the shell-lac, and by
now drawing the latter away, the paper falls over without
having been touched by any thing. You see, in the first
illustration I produced an effect than which nothing could
be commoner; I pull it over now, not by means of that
string or the pull of my hand, but by some action in this
shell-lac. The shell-lac, therefore, has a power wherewith it
acts upon the sheet of paper; and, as an illustration of the
exercise of another kind of power, I might use gunpowder
with which to throw it over.
Now I want you to endeavor to comprehend that when
I am speaking of a power or force, I am speaking of that
which I used just now to pull over this piece of paper.
I will not embarrass you at present with the name of that
power, but it is clear there was a something in the shell-lac
which acted by attraction, and pulled the paper over; this,
then, is one of those things which we call power, or force;
and you will now be able to recognize it as such in whatever
form I show it to you. We are not to suppose that there are
so very many different powers ; on the contrary, it is wonder-
ful to think how few are the powers by which all the phe-
nomena of nature are governed. There is an illustration of
another kind of power in that lamp; there is a power of
heat — a power of doing something, but not the same power
as that which pulled the paper over ; and so, by degrees, we
find that there are certain other powers (not many) in the
various bodies around us; and thus, beginning with the
simplest experiments of pushing and pulling, I shall gradually
proceed to distinguish these powers one from the other,
and compare the way in which they combine together.
This world upon which we stand (and we have not much
need to travel out of the world for illustrations of our
subject; but the mind of man is not confined like the matter
of his body, and thus he may and does travel outward, for
wherever his sight can pierce, there his observations can
penetrate) is pretty nearly a round globe, having its sur-
face disposed in a manner of which this terrestrial globe
8 FARADAY
by my side is a rough model; so much is land and so much
is water, and by looking at it here we see in a sort of map
or picture how the world is formed upon its surface. Then,
when we come to examine farther, I refer you to this
sectional diagram of the geological strata of the earth, in
which there is a more elaborate view of what is beneath
the surface of our globe. And, when we come to dig into
or examine it (as man does for his own instruction and
advantage, in a variety of ways), we see that it is made up
of different kinds of matter, subject to a very few powers;
and all disposed in this strange and wonderful way, which
gives to man a history — and such a history — as to what there
is in those veins, in those rocks, the ores, the water-
springs, the atmosphere around, and all varieties of ma-
terial substances, held together by means of forces in
one great mass, 8,000 miles in diameter, that the mind is
overwhelmed in contemplation of the wonderful history re-
lated by these strata (some of which are fine and thin like
sheets of paper), all formed in succession by the forces
of which I have spoken.
I now shall try to help your attention to what I may
say by directing to-day our thoughts to one kind of power.
You see what I mean by the term matter — any of these
things that I can lay hold of with the hand, or in a bag
(for I may take hold of the air by inclosing it in a bag) —
they are all portions of matter with which we have to deal
at present, generally or particularly, as I may require to
illustrate my subject. Here is the sort of matter which we
call water — it is there ice [pointing to a block of ice
upon the table], there water — [pointing to the water boiling
in a flask] — here vapor — you see it issuing out from the
top [of the flask]. Do not suppose that that ice and that
water are two entirely different things, or that the steam
rising in bubbles and ascending in vapor there is absolutely
different from the fluid water: it may be different in some
particulars, having reference to the amounts of power which
it contains; but it is the same, nevertheless, as the great
ocean of water around our globe, and I employ it here for
the sake of illustration, because if we look into it we shall
find that it supplies us with examples of all the powers to
THE FORCE OF GRAVITATION 9
which I shall have to refer. For instance, here is water —
it is heavy; but let us examine it with regard to the amount
of its heaviness or its gravity. I have before me a little
glass vessel and scales [nearly equipoised scales, one of
which contained a half-pint glass vessel], and the glass
vessel is at present the lighter of the two ; but if I now take
some water and pour it in, you see that that side of the
scales immediately goes down; that shows you (using com-
mon language, which I will not suppose for the present
you have hitherto applied very strictly) that it is heavy,
and if I put this additional weight into the opposite scale,
I should not wonder if this vessel would hold water enough
to weigh it down. [The lecturer poured more water into
the jar, which again went down.] Why do I hold the
bottle above the vessel to pour the water into it? You will
say, because experience has taught me that it is necessary.
I do it for a better reason — because it is a law of nature
that the water should fall toward the earth, and therefore
the very means which I use to cause the water to enter
the vessel are those which will carry the whole body of
water down. That power is what we call gravity, and you
see there [pointing to the scales] a good deal of water
gravitating toward the earth. Now here [exhibiting a small
piece of platinum ( x )] is another thing which gravitates
toward the earth as much as the whole of that water. See
what a little there is of it; that little thing is heavier than
so much water [placing the metal in opposite scales to the
water]. What a wonderful thing it is to see that it re-
quires so much water as that [a half-pint vessel full] to
fall toward the earth, compared with the little mass of
substance I have here! And again, if I take this metal
[a bar of aluminium ( 2 ) about eight times the bulk of the
platinum], we find the water will balance that as well as it
did the platinum; so that we get, even in the very outset,
an example of what we want to understand by the words
forces or powers.
I have spoken of water, and first of all of its property of
falling downward : you know very well how the oceans sur-
1 Platinum, with one exception the heaviest body known, is 21% times
heavier than water.
2 Aluminium is 2^ times heavier than water.
10 FARADAY
round the globe — how they fall round the surface, giving
roundness to it, clothing it like a garment ; but, besides that,
there are other properties of water. Here, for instance, is
some quicklime, and if I add some water to it, you will
find another power and property in the water. ( 3 ) It is now
very hot; it is steaming up; and I could perhaps light
phosphorus or a lucifer-match with it. Now that could not
happen without a force in the water to produce the result;
but that force is entirely distinct from its power of falling
to the earth. Again, here is another substance [some anhy-
drous sulphate of copper ( 4 )] which will illustrate another
kind of power. [The lecturer here poured some water over
the white sulphate of copper, which immediately became
blue, evolving considerable heat at the same time.] Here
is the same water with a substance which heats nearly as
much as the lime does, but see how differently. So great
indeed is this heat in the case of lime, that it is sufficient
sometimes (as you see here) to set wood on fire; and this
explains what we have sometimes heard, of barges laden
with quicklime taking fire in the middle of the river, in con-
sequence of this power of heat brought into play by a leakage
of the water into the barge. You see how strangely different
subjects for our consideration arise when we come to think
over these various matters — the power of heat evolved by
acting upon lime with water, and the power which water has
of turning this salt of copper from white to blue.
I want you now to understand the nature of the most sim-
ple exertion of this power of matter called weight or gravity.
Bodies are heavy; you saw that in the case of water when
I placed it in the balance. Here I have what we call a
weight [an iron half cwt.] — a thing called a weight because
in it the exercise of that power of pressing downward is es-
pecially used for the purposes of weighing; and I have also
one of these little inflated India-rubber bladders, which are
3 Power or property in water. This power — the heat by which the water
is kept in a fluid state — is said, under ordinary circumstances, to be latent
or insensible. When, however, the water changes its form, and, by uniting
with the lime or sulphate of copper, becomes solid, the heat which retained
it in a liquid state is evolved.
4 Anhydrous sulphate of copper: sulphate of copper deprived of its water
of crystallization. To obtain it the blue sulphate is calcined in an earthen
crucible.
THE FORCE OF GRAVITATION 11
very beautiful although very common (most beautiful things
are common), and I am- going to put the weight upon it, to
give you a sort of illustration of the downward pressure of
the iron, and of the power which the air possesses of re-
sisting that pressure ; it may burst, but we must try to avoid
that. [During the last few observations the lecturer had
succeeded in placing the half cwt. in a state of quiescence
upon the inflated India-rubber ball, which consequently as-
sumed a shape very much resembling a flat cheese with
round edges.] There you see a bubble of air bearing half
a hundred weight, and you must conceive for yourselves
what a wonderful power there must be to pull this weight
downward, to sink it thus in the ball of air.
Let me now give you another illustration of this power.
You know what a pendulum is. I have one here (Fig. i),
and if I set it swinging, it will continue to swing to and fro.
Now I wonder whether you can tell
me why that body oscillates to and
fro — that pendulum bob, as it is
sometimes called. Observe, if I hold
the straight stick horizontally, as high
as the position of the ball at the two
ends of its journey, you see that the
ball is in a higher position at the two
extremities than it is when in the
middle. Starting from one end of
the stick, the ball falls toward the FlG - x
centre, and then rising again to the opposite end, it con-
stantly tries to fall to the lowest point, swinging and vi-
brating most beautifully, and with wonderful properties in
other respects — the time of its vibration, and so on — but
concerning which we will not now trouble ourselves.
If a gold leaf, or piece of thread, or any other substance
were hung where this ball is, it would swing to and fro in
the same manner, and in the same time too. Do not be
startled at this statement; I repeat, in the same manner and
in the same time, and you will see by-and-by how this is.
Now that power which caused the water to descend in the
balance — which made the iron weight press upon and flatten
the bubble of air — which caused the swinging to and fro of
12 FARADAY
the pendulum, that power is entirely due to the attraction
which there is between the falling body and the earth. Let
us be slow and careful to comprehend this. It is not that
the earth has any particular attraction toward bodies which
fall to it, but, that all these bodies possess an attraction
every one toward the other. It is not that the earth has any
special power which these balls themselves have not; for just
as much power as the earth has to attract these two balls
[dropping two ivory balls], just so much power have they in
proportion to their bulks to draw themselves one to the
other; and the only reason why they fall so quickly to the
earth is owing to its greater size. Now if I were to place
these two balls near together, I should not be able, by the
most delicate arrangement of apparatus, to make you, or
myself, sensible that these balls did attract one another; and
yet we know that such is the case, because if, instead of
taking a small ivory ball, we take a mountain, and put a ball
like this near it, we find that, owing to the vast size of the
mountain as compared with the billiard ball, the latter is
drawn slightly toward it, showing clearly that an attraction
does exist, just as it did between the shell-lac which I rubbed
and the piece of paper which was overturned by it.
Now it is not very easy to make these things quite clear at
the outset and I must take care not to leave anything unex-
plained as I proceed, and, therefore, I must make you clearly
understand that all bodies are attracted to the earth, or,
to use a more learned term, gravitate. You will not
mind my using this word, for when I say that this penny-
piece gravitates, I mean nothing more nor less than that
it falls toward the earth, and, if not intercepted,, it would
go on falling, falling, until it arrived at what we call the
centre of gravity of the earth, which I will explain to you
by-and-by.
I want you to understand that this property of gravitation
is never lost; that every substance possesses it; that there is
never any change in the quantity of it; and, first of all, I
will take as illustration a piece of marble. Now this marble
has weight, as you will see if I put it in these scales; it
weighs the balance down, and if I take it off, the balance goes
back again and resumes its equilibrium. I can decompose
THE FORCE OF GRAVITATION
13
this marble and change it in the same manner as I can
change ice into water and water into steam. I can convert
a part of it into its own steam easily, and show you that
this steam from the marble has the property of remaining
in the same place at common temperatures, which mater-
steam has not. If I add a little liquid to the marble and
decompose it ( 5 ), I get that which you see — [the lecturer here
put several lumps of marble into a glass jar, and poured
water and then acid over them; the carbonic acid immedi-
ately commenced to escape with considerable effervescence]
— the appearance of boiling, which is only the separation of
one part of the marble from another. Now this [marble]
steam, and that [water] steam, and all other steams, gravitate
just like any other substance does; they all are attracted
the one toward the other, and all fall toward the earth, and
what I want you to see is that this steam gravitates. I have
here (Fig. 2) a large vessel placed upon a balance, and the
Fig. 2
moment I pour this steam into it you see that the steam
gravitates. Just watch the index, and see whether it tilts
over or not. [The lecturer here poured the carbonic acid
out of the glass in which it was being generated into the
vessel suspended on the balance, when the gravitation of the
carbonic acid was at once apparent.] Look how it is going
6 Add a little liquid to the marble and decompose it. Marble is composed
of carbonic acid and lime, and, in chemical language, is called carbonate
of lime. When sulphuric acid is added to it, the carbonic acid is set free,
and the sulphuric acid unites with the lime to form sulphate of lime.
Carbonic acid, under ordinary circumstances, is a colorless invisible gas,
about half as heavy again as air. Dr. Faraday first showed that under
great pressure it could be obtained in a liquid state. Thilorier, a French
chemist, afterward found that it could be solidified.
14 FARADAY
down. How pretty that is! I poured nothing in but the
invisible steam, or vapor, or gas which came from the mar-
ble, but you see that part of the marble, although it has
taken the shape of air, still gravitates as it did before. Now
will it weigh down that bit of paper? [placing a piece of
paper in the opposite scale]. Yes, more than that; it nearly
weighs down this bit of paper [placing another piece of
paper in]. And thus you see that other forms of matter
besides solids and liquids tend to fall to the earth ; and, there-
fore, you will accept from me the fact that all things gravi-
tate, whatever may be their form or condition. Now here is
another chemical test which is very readily applied. [Some
of the carbonic acid was poured from one vessel into another,
and its presence in the latter shown by introducing into it a
lighted taper, which was immediately extinguished.] You
see from this result also that it gravitates. All these experi-
ments show you that, tried by the balance, tried by pouring
like water from one vessel to another, this steam, or vapor,
or gas is, like all other things, attracted to the earth.
There is another point I want in the next place to draw
your attention to. I have here a quantity of shot; each of
these falls separately, and each has its own gravitating
power, as you perceive when I let them fall loosely on a
sheet of paper. If I put them into a bottle, I collect them
together as one mass, and philosophers have discovered that
there is a certain point in the middle of the whole collection
of shots that may be considered as the one point in which
all their gravitating power is centred, and that point they
call the centre of gravity; it is not at all a bad name, and
rather a short one — the centre of gravity. Now suppose I
take a sheet of pasteboard, or any other thing easily dealt
with, and run a bradawl through it at one corner, A
(Fig. 3), and Mr. Anderson holds that up in his hand before
us, and I then take a piece of thread and an ivory ball,
and hang that upon the awl, then the centre of gravity of both
the pasteboard and the ball and string are as near as they
can get to the centre of the earth; that is to say, the
whole of the attracting power of the earth is, as it were,
centred in a single point of the cardboard, and this point
is exactly below the point of suspension. All I have to do,
THE FORCE OF GRAVITATION
15
therefore, is to draw a line, A B, corresponding with the
string, and we shall find that the centre of gravity is some-
where in that line. But where? To find that out, all we
Fig. 4
have to do is to take another place for the awl (Fig. 4),
hang the plumb-line, and make the same experiment, and
there [at the point C] is the centre of gravity, — there where
the two lines which I have traced cross each other; and if
I take that pasteboard and make a hole with the bradawl
through it at that point, you will see that it will be supported
in any position in which it may be placed. Now, knowing
that, what do I do when I try to stand upon one leg? Do you
not see that I push myself over to the left side, and quietly
take up the right leg, and thus bring some central point
in my body over this left leg? What is that point which I
throw over? You will know at once that it is the centre of
gravity — that point in me where the whole gravitating force
of my body is centred, and which I thus bring in a line
over my foot.
Here is a toy I happened to see the other day, which will,
I think, serve to illustrate our subject very well. That toy
ought to lie something in this manner (Fig. 5), and would
do so if it were uniform in substance; but you see it does
not; it will get up again. And now philosophy comes to our
aid, and I am perfectly sure, without looking inside the
figure, that there is some arrangement by which the centre
16
FARADAY
of gravity is at the low ^st point when the image is standing
upright; and we may be certain, when I am tilting it over
Fig.
Fig. 6
(see Fig. 6), that I am lifting up the centre of gravity (a) 9
and raising it from the earth. All this is effected by put-
ting a piece of lead inside the lower part of the image,
and making the base of large curvature, and there you have
the whole secret. But what will happen if I try to make the
figure stand upon a sharp
point? You observe I must
get that point exactly under
the centre of gravity, or it
will fall over thus [endeavor-
ing unsuccessfully to balance
it] ; and this, you see, is a dif-
ficult matter; I can not make
it stand steadily; but if I em-
barrass this poor old lady with
a world of trouble, and hang
this wire with bullets at each
end about her neck, it is very
evident that, owing to there
being those balls of lead hang-
ing down on either side, in
addition to the lead inside, I
have lowered the centre of
gravity, and now she will stand upon this point (Fig. 7),
and, what is more, she proves the truth of our philosophy by
standing sideways.
I remember an experiment which puzzled me very much
Fig.
THE FORCE OF GRAVITATION
17
when a boy. I read it in a conjuring book, and this was how
the problem was put to us : " How/' as the book said, " how
to hang a pail of water, by means of a stick, upon the side of
a table" (Fig. 8). Now I have here a table, a piece of
stick, and a pail, and the proposition is, how can that pail
be hung to the edge of this table? It is to be done, and
can you at all anticipate what arrangement I shall make to
enable me to succeed? Why this. I take a stick, and put
it in the pail between the bottom and the horizontal piece
Fig. 8
Fig. 9
of wood, and thus give it a stiff handle, and there it is ; and,
what is more, the more water I put into the pail, the better
it will hang. It is very true that before I quite succeeded I
had the misfortune to push the bottoms of several pails out;
but here it is hanging firmly (Fig. 9), and you now see
how you can hang up the pail in the way which the conjuring
books require.
Again, if you are really so inclined (and I do hope all of
you are), you will find a great deal of philosophy in this
[holding up a cork and a pointed thin stick about a foot
long]. Do not refer to your toy-books, and say you have
seen that before. Answer me rather, if I ask you, have you
understood it before? It is an experiment which appeared
very wonderful to me when I was a boy. I used to take
a piece of cork (and I remember I thought at first that it was
very important that it should be cut out in the shape of a
man, but by degrees I got rid of that idea), and the problem
18
FARADAY
Fig. io
was to balance it on the point of a stick. Now you will see
I have only to place two sharp-pointed sticks one on each
side, and give it wings, thus, and
you will find this beautiful condi-
tion fulfilled.
We come now to another point.
All bodies, whether heavy or light,
fall to the earth by this force
which we call gravity. By ob-
servation, moreover, we see that
bodies do not occupy the same
time in falling ; I think you will be
able to see that this piece of paper
and that ivory ball fall with dif-
ferent velocities to the table [drop-
ping them] ; and if, again, I take
a feather and an ivory ball, and
let them fall, you see they reach
the table or earth at different times ; that is to say, the ball
faster than the feather. Now that should not be so, for all
bodies do fall equally fast to the earth. There are one or
two beautiful points included in that statement. First of all,
it is manifest that an ounce, or a pound, or a ton, or a thou-
sand tons, all fall equally fast, no one faster than another:
here are two balls of lead, a very light one and a very heavy
one, and you perceive they both fall to the earth in the same
time. Now if I were to put into a little bag a number of
these balls sufficient to make up a bulk equal to the large one,
they would also fall in the same time; for if an avalanche
fall from the mountains, the rocks, snow, and ice, together
falling toward the earth, fall with the same velocity, what-
ever be their size.
I can not take a better illustration of this than that of gold
leaf, because it brings before us the reason of this apparent
difference in the time of the fall. Here is a piece of gold
leaf. Now if I take a lump of gold and this gold leaf, and
let them fall through the air together, you see that the lump
of gold — the sovereign or coin — will fall much faster than
the gold leaf. But why? They are both gold, whether
sovereign or gold leaf. Why should they not fall to the
THE FORCE OF GRAVITATION 19
earth with the same quickness? They would do so, but
that the air around our globe interferes very much where
we have the piece of gold so extended and enlarged as to
offer much obstruction on falling through it. I will, how-
ever, show you that gold leaf does fall as fast when the
resistance of the air is excluded; for if I take a piece of
gold leaf and hang it in the centre of a bottle so that the
gold, and the bottle, and the air within shall all have an
equal chance of falling, then the gold leaf will fall as
fast as anything else. And if I suspend the bottle con-
taining the gold leaf to a string, and set it oscillating like a
pendulum, I may make it vibrate as hard as I please and
the gold leaf will not be disturbed, but will swing as steadily
as a piece of iron would do ; and I might even swing it
round my head with any degree of force, and it would re-
main undisturbed. Or I can try another kind of experiment :
if I raise the gold leaf in this way [pulling the bottle up to
the ceiling of the theatre by means of a cord and pulley,
and then suddenly letting it fall within a few inches of
the lecture table], and allow it then to fall from the ceiling
downward (I will put something beneath to catch it, sup-
posing I should be maladroit), you will perceive that the
gold leaf is not in the least disturbed. The resistance of
the air having been avoided, the glass bottle and gold
leaf all fall exactly in the same time.
Here is another illustration: I have hung a piece of gold
leaf in the upper part of this long glass vessel, and I have
the means by a little arrangement at the top, of letting
the gold leaf loose. Before we let it loose we will remove
the air by means of an air-pump, and, while that is being
done, let me show you another experiment of the same
kind. Take a penny-piece, or a half crown, and a round
piece of paper a trifle smaller in diameter than the coin,
and try them side by side to see whether they fall at the
same time [dropping them]. You see they do not — the
penny-piece goes down first. But, now place this paper
flat on the top of the coin, so that it shall not meet with
any resistance from the air, and upon then dropping them
you see they do both fall in the same time [exhibiting 1 the
effect]. I dare say, if I were to put this piece of gold
20 FARADAY
leaf, instead of the paper, on the coin, it would do as well.
It is very difficult to lay the gold leaf so flat that the air shall
not get under it and lift it up in falling, and I am rather
doubtful as to the success of this, because the gold leaf
is puckery, but will risk the experiment. There they go
together ! [letting them fall] and you see at once that they
both reach the table at the same moment.
We have now pumped the air out of the vessel, and you
will perceive that the gold leaf will fall as quickly in this
vacuum as the coin does in the air. I am now going to let
it loose, and you must watch to see how rapidly it falls.
There ! [letting the gold loose] there it is, falling as gold
should fall.
I am sorry to see our time for parting is drawing so near.
As we proceed, I intend to write upon the board behind me
certain words, so as to recall to your minds what we have
already examined; and I put the word Forces as a head-
ing, and I will then add beneath the names of the special
forces according to the order in which we consider them;
and although I fear that I have not sufficiently pointed out
to you the more important circumstances connected with the
force of Gravitation, especially the law which governs
its attraction (for which, I think, I must take up a little time
at our next meeting), still I will put that word on the
board, and hope you will now remember that we have in
some degree considered the force of gravitation — that force
which causes all bodies to attract each other when they
are at sensible distances apart, and tends to draw them
together.
LECTURE II
GRAVITATION— COHESION
DO me the favor to pay me as much attention as you
did at our last meeting, and I shall not repent of
that which I have proposed to undertake. It will be
impossible for us to consider the Laws of Nature, and what
they effect, unless we now and then give our sole attention,
so as to obtain a clear idea upon the subject. Give me now
that attention, and then I trust we shall not part without
our knowing something about those laws, and the manner in
which they act. You recollect, upon the last occasion, I
explained that all bodies attracted each other, and that this
power we called gravitation. I told you that when we
brought these two bodies [two equal-sized ivory balls sus-
pended by threads] near together, they attracted each other,
and that we might suppose that the whole power of this
attraction was exerted between their respective centres of
gravity; and, furthermore, you learned from me that if,
instead of a small ball I took a larger one, like that [chang-
ing one of the balls for a much larger one], there was much
more of this attraction exerted ; or, if I made this ball larger
and larger, until, if it were possible, it became as large as the
Earth itself — or I might take the Earth itself as the large ball
— that then the attraction would become so powerful
as to cause them to rush together in this manner [dropping
the ivory ball]. You sit there upright, and I stand upright
here, because we keep our centres of gravity properly bal-
anced with respect to the earth ; and I need not tell you that
on the other side of this world the people are standing and
moving about with their feet toward our feet, in a reversed
position as compared with us, and all by means of this power
of gravitation to the centre of the earth.
I must not, however, leave the subject of gravitation with-
21
22 FARADAY
out telling you something about its laws and regularity;
and, first as regards its power with respect to the distance
that bodies are apart. If I take one of these balls and place
it within an inch of the other, they attract each other with
a certain power. If I hold it at a greater distance off, they
attract with less power ; and if I hold it at a greater distance
still, their attraction is still less. Now this fact is of the
greatest consequence; for, knowing this law, philosophers
have discovered most wonderful things. You know that
there is a planet, Uranus, revolving round the sun with us,
but eighteen hundred millions of miles off, and because there
is another planet as far off as three thousand millions of
miles, this law of attraction, or gravitation, still holds good,
and philosophers actually discovered this latter planet, Nep-
tune, by reason of the effects of its attraction at this over-
whelming distance. Now I want you clearly to understand
what this law is. They say (and they are right) that two
bodies attract each other inversely as the square of the
distance — a sad jumble of words until you understand them ;
but I think we shall soon comprehend what this law is, and
what is the meaning of the " inverse square of the distance. "
I have here (Fig. ii) a lamp, A, shining most intensely
upon this disc, B, C, D, and this light acts as a sun by which
Fig. ii
I can get a shadow from this little screen B F (merely a
square piece of card), which, as you know, when I place it
close to the large screen, just shadows as much of it as is
GRAVITY 23
exactly equal to its own size; but now let me take this
card, E, which is equal to the other one in size, and place
it midway between the lamp and the screen; now look at
the size of the shadow B D — it is four times the original
size. Here, then, comes the " inverse square of the distanced
This distance, A E, is one, and that distance, A B, is two,
but that size E being one, this size B D of shadow is four
instead of two, which is the square of the distance, and, if I
put the screen at one-third of the distance from the lamp,
the shadow on the large screen would be nine times the size.
Again, if I hold this screen here, at B F, a certain amount
of light falls on it; and if I hold it nearer the lamp at E,
more light shines upon it. And you see at once how much
— exactly the quantity which I have shut off from the part
of this screen, B D, now in shadow; moreover, you see that
if I put a single screen here, at G, by the side of the shadow,
it can only receive one-fourth of the proportion of light
which is obstructed. That, then, is what is meant by the
inverse of the square of the distance. This screen E is the
brightest because it is the nearest, and there is the whole
secret of this curious expression, inversely as the square of
the distance. Now if you can not perfectly recollect this
when you go home, get a candle and throw a shadow of
something — your profile, if you like — on the wall and then
recede or advance, and you will find that your shadow is
exactly in proportion to the square of the distance you are
off the wall ; and then, if you consider how much light shines
on you at one distance, and how much at another, you get
the inverse accordingly. So it is as regards the attraction
of these two balls; they attract according to the square of
the distance, inversely. I want you to try and remember
these words, and then you will be able to go into all the
calculations of astronomers as to the planets and other
bodies, and tell why they move so fast, and why they go
round the sun without falling into it and be prepared to
enter upon many other interesting inquiries of the like
nature.
Let us now leave this subject which I have written upon
the board under the word Force — Gravitation — and
go a step farther. All bodies attract each other at sensible
24
FARADAY
distances. I showed you the electric attraction on the last
occasion (though I did not call it so) ; that attracts
at a distance; and in order to make our progress a
little more gradual, suppose I take a few iron particles [drop-
ping some small fragments of iron on the table]. There!
I have already told you that in all cases where bodies
fall it is the particles that are attracted. You may con-
sider these, then, as separate particles magnified, so as
to be evident to your sight ; they are loose from each other —
they all gravitate — they all fall to the earth — for the force
of gravitation never fails. Now I have here a centre of
power which I will not name at present, and when these
particles are placed upon it, see what an attraction they have
for each other.
Here I have an arch of iron filings (Fig. 12) regularly
built up like an iron bridge, because I have put them within
a sphere of action which will cause
them to attract each other. See ! I
could let a mouse run through it; and
yet, if I try to do the same thing
with them here [on the table], they
do not attract each other at all. It
is that [the magnet] which makes
them hold together. Now just as
these iron particles hold together in
the form of an elliptical bridge, so
do the different particles of iron
which constitute this nail hold to-
gether and make it one. And here is a bar of iron;
why, it is only because the different parts of this iron
are so wrought as to keep close together by the attrac-
tion between the particles that it is held together in one
mass. It is kept together, in fact, merely by the attraction
of one particle to another, and that is the point I want
now to illustrate. If I take a piece of flint, and strike it
with a hammer, and break it thus [breaking off a piece of
the flint], I have done nothing more than separate the par-
ticles which compose these two pieces so far apart that their
attraction is too weak to cause them to hold together, and
it is only for that reason that there are now two pieces in
Fig.
COHESION 25
the place of one. I will show you an experiment to prove
that this attraction does still exist in those particles; for
here is a piece of glass (for what was true of the flint and
the bar of iron is true of the piece of glass, and is true of
every other solid — they are all held together in the lump
by the attraction between their parts), and I can show you
the attraction between its separate particles; for if I take
these portions of glass which I have reduced to very fine
powder, you see that I can actually build them up into a
solid wall by pressure between two flat surfaces. The
power which I thus have of building up this wall is due to
the attraction of the particles forming, as it were, the cement
which holds them together; and so in this case, where I
have taken no very great pains to bring the particles to-
gether, you see perhaps a couple of ounces of finely pounded
glass standing as an upright wall: is not this attraction
most wonderful? That bar of iron one inch square has
such power of attraction in its particles — giving to it such
strength — that it will hold up twenty tons' weight before
the little set of particles in the small space equal to one
division across which it can be pulled apart will separate.
In this manner suspension bridges and chains are held
together by the attraction of their particles, and I am going
to make an experiment which will show how strong is this
attraction of the particles. [The lecturer here placed his
foot on a loop of wire fastened to a support above, and
swung with his whole weight resting upon it for some
moments.] You see, while hanging here, all my weight is
supported by these little particles of the wire, just as in
pantomimes they sometimes suspend gentlemen and damsels.
How can we make this attraction of the particles a little
more simple? There are many things which, if brought
together properly, will show this attraction. Here is a
boy's experiment (and I like a boy's experiment). Get a
tobacco-pipe, fill it with lead, melt it, and then pour it out
upon a stone, and thus get a clean piece of lead (this is a
better plan than scraping it; scraping alters the condition
of the surface of the lead). I have here some pieces of
lead which I melted this morning for the sake of making
them clean. Now these pieces of lead hang together by the
26 FARADAY
attraction of their particles, and if I press these two sepa-
rate pieces close together, so as to bring their particles
within the sphere of attraction, you will see how soon they
become one. I have merely to give them a good squeeze,
and draw the upper piece slightly round at the same time,
and here they are as one, and all the bending and twisting I
can give them will not separate them again; I have joined
the lead together, not with solder, but simply by means of
the attraction of the particles.
This, however, is not the best way of bringing those par-
ticles together; we have many better plans than that; and
I will show you one that will do very well for juvenile
experiments. There is some alum crystallized very beauti-
fully by nature (for all things are far more beautiful in their
natural than their artificial form), and here I have some of
the same alum broken into fine powder. In it I have
destroyed that force of which I have placed the name on this
board — Cohesion, or the attraction exerted between the
particles of bodies to hold them together. Now I am going
to show you that if we take this powdered alum and some
hot water, and mix them together, I shall dissolve the alum ;
all the particles will be separated by the water far more
completely than they are here in the powder; but then,
being in the water, they will have the opportunity as it cools
(for that is the condition which favors their coalescence)
of uniting together again and forming one mass ( 6 ).
Now, having brought the alum into solution, I will pour
it into this glass basin, and you will, to-morrow, find that
these particles of alum which I have put into the water, and
so separated that they are no longer solid, will, as the water
cools, come together and cohere, and by to-morrow morning
we shall have a great deal of the alum crystallized out — that
is to say, come back to the solid form. [The lecturer here
poured a little of the hot solution of alum into the glass
dish, and when the latter had thus been made warm, the
remainder of the solution was added.] I am now doing that
6 Crystallisation of alum. The solution must be saturated — that is, it
must contain as much alum as can possibly be dissolved. In making the
solution, it is best to add powdered alum to hot water as long as it dis-
solves; and when no more is taken up, allow the solution to stand a few
minutes, and then pour it off from the dirt and undissolved alum.
COHESION 27
which I advise you to do if you use a glass vessel, namely,
warming it slowly and gradually; and in repeating this ex-
periment, do as I do — pour the liquid out gently, leaving all
the dirt behind in the basin; and remember that the more
carefully and quietly you make this experiment at home, the
better the crystals. To-morrow you will see the particles
of alum drawn together; and if I put two pieces of coke
in some part of the solution (the coke ought first to be
washed very clean, and dried), you will find to-morrow that
we shall have a beautiful crystallization over the coke, mak-
ing it exactly resemble a natural mineral.
Now how curiously our ideas expand by watching these
conditions of the attraction of cohesion ! how many new
phenomena it gives us beyond those of the attraction of
gravitation ! See how it gives us great strength. The
things we deal with in building up the structures on the
earth are of strength — we use iron, stone, and other things
of great strength; and only think that all those structures
you have about you — think of the Great Eastern, if you
please, which is of such size and power as to be almost
more than man can manage — are the result of this power
of cohesion and attraction.
I have here a body in which I believe you will see a
change taking place in its condition of cohesion at the mo-
ment i 4 : is made. It is at first yellow; it then becomes a
fine crimson red. Just watch when I pour these two liquids
together — both colorless as water. [The lecturer here mixed
together solutions of perchloride of mercury and iodide of
potassium, when a yellow precipitate of biniodide of mercury
fell down, which almost immediately became crimson red.]
Now there is a substance which is very beautiful, but see
how it is changing color. It was reddish-yellow at first,
but it has now become red ( 7 ). I have previously pre-
pared a little of this red substance, which you see formed
1 Red precipitate of biniodide of mercury. A little care is necessary to
obtain this precipitate. The solution of iodide of potassium should be
added to the solution of perchloride of mercury (corrosive sublimate) very
gradually. The red precipitate which first falls is redissolved when the
liquid is stirred: when a little more of the iodide of potassium is added a
pale red precipitate is formed, which, on the farther addition of the iodide,
changes into the brilliant scarlet biniodide of mercury. If too much iodide
of potassium is added, the scarlet precipitate disappears, and a colorless
solution is left.
28 FARADAY
in the liquid, and have put some of it upon paper [exhibiting
several sheets of paper coated with scarlet biniodide o£
mercury] ( 8 ). There it is — the same substance spread upon
paper; and there, too, is the same substance; and here is
some more of it [exhibiting a piece of paper as large as
the other sheets, but having only very little red color on it,
the greater part being yellow] — a little more of it, you will
say. Do not be mistaken; there is as much upon the sur-
face of one of these pieces of paper as upon the other.
What you see yellow is the same thing as the red body, only
the attraction of cohesion is in a certain degree changed,
for I will take this red body, and apply heat to it (you
may perhaps see a little smoke arise, but that is of no
consequence) ; and if you look at it it will first of
all darken — but see how it is becoming yellow. I have now
made it all yellow, and, what is more, it will remain
so; but if I take any hard substance, and rub the yel-
low part with it, it will immediately go back again to the
red condition [exhibiting the experiment]. There it is.
You see the red is not put back, but brought back by the
change in the substance. Now [warming it over the spirit
lamp] here it is becoming yellow again, and that is all
because its attraction of cohesion is changed. And what
will you say to me when I tell you that this piece of common
charcoal is just the same thing, only differently coalesced,
as the diamonds which you wear? (I have put a specimen
outside of a piece of straw which was charred in a par-
ticular way — it is just like black lead.) Now this charred
straw, this charcoal, and these diamonds, are all of them the
same substance, changed but in their properties as respects
the force of cohesion.
Here is a piece of glass [producing a piece of plate-
glass about two inches square]. (I shall want this after-
ward to look to and examine its internal condition), and
here is some of the same sort of glass differing only in its
power of cohesion, because while yet melted it had been
8 Paper coated with scarlet biniodide of mercury. In order to fix the
biniodide on paper, it must be mixed with a little weak gum water, and
then spread over the paper, which must be dried without heat.
Biniodide of mercury is said to be dimorphous; that is, is able to assume
two different forms.
COHESION 29
dropped into cold water [exhibiting a " Prince Rupert's
drop,"( 9 ) (Fig. 13)], and if I take one of these little tear-
like pieces and break off ever so little from the point, the
whole will at once burst and fall to pieces. I will now
break off a piece of this. [The lecturer nipped off a small
piece from the end of one of Rupert's drops, whereupon the
Fig. 14
whole immediately fell to pieces.] There ! you see the solid
glass has suddenly become powder, and more than that, it has
knocked a hole in the glass vessel in which it was held. I
can show the effect better in this bottle of water, and it is
very likely the whole bottle will go. [A 6-oz. vial was filled
with water, and a Rupert's drop placed in it with the point
of the tail just projecting out; upon breaking the tip off,
the drop burst, and the shock, being transmitted through
the water to the sides of the bottle, shattered the latter
to pieces.]
Here is another form of the same kind of experiment.
I have here some more glass which has not been annealed
[showing some thick glass vessels( 10 ) (Fig. 14)], and if I
take one of these glass vessels and drop a piece of pounded
glass into it (or I will take some of these small pieces of rock
crystal; they have the advantage of being harder than
glass), and so make the least scratch upon the inside, the
whole bottle will break to pieces — it can not hold together.
[The lecturer here dropped a small fragment of rock
crystal into one of these glass vessels, when the bottom im-
9 " Prince Rupert's Drops.'* These are made by pouring drops of melted
green glass into cold water. They were not, as is commonly supposed, in-
vented by Prince Rupert, but were first brought to England by him in 1660.
They excited a great deal of curiosity, and were considered " a kind of
miracle in nature."
10 Thick glass vessels. They are called Proofs or Bologna phials.
30
FARADAY
mediately came out and fell upon the plate.] There ! it
goes through, just as it would through a sieve.
Now I have shown you these things for the purpose of
bringing your minds to see that bodies are not merely
held together by this power of cohesion, but that they are
held together in very curious ways. And suppose I take
some things that are held together by this force, and ex-
amine them more minutely. I will first take a bit of glass,
and if I give it a blow with a hammer I shall just break
it to pieces. You saw how it was in the case of the flint
when I broke the piece off; a piece of a similar kind would
come off, just as you would expect; and if I were to break
it up still more, it would be, as you have seen, simply
a collection of small particles of no definite shape or form.
But supposing I take some other thing — this stone, for
instance (Fig. 15) [taking a piece of mica( u )], and if I
Fig. 15
Fig. 16
Fig. 17
hammer this stone I may batter it a great deal before I can
break it up. I may even bend it without breaking it — that
is to say, I may bend it in one particular direction without
breaking it much, although I feel in my hands that I am
doing it some injury. But now, if I take it by the edges,
I find that it breaks up into leaf after leaf in a most ex-
traordinary manner. Why should it break up like that?
Not because all stones do, or all crystals; for there is some
salt (Fig. 16) — you know what common salt is( 12 ) ; here
is a piece of this salt, which by natural circumstances has
had its particles so brought together that they have been
11 Mica. A silicate of alumina and magnesia. It has a bright metallic
lustre; hence its name, from mico, to shine.
12 Common salt or chloride of sodium crystallizes in the form of solid
cubes, which, aggregated together, form a mass, which may be broken up
into the separate cubes.
COHESION 31
allowed free opportunity of combining or coalescing, and
you shall see what happens if I take this piece of salt
and break it. It does not break as flint did, or as the mica
did, but with a clean sharp angle and exact surfaces, beauti-
ful and glittering as diamonds [breaking it by gentle blows
with a hammer] ; there is a square prism which I may
break up into a square cube. You see these fragments are all
square; one side may be longer than the other, but they
will only split up so as to form square or oblong pieces with
cubical sides. Now I go a little farther, and I find another
stone (Fig. 17) [Iceland or calc-spar] ( 1S ) which I may
break in a similar way, but not with the same result. Here
is a piece which I have broken off, and you see there are
plain surfaces perfectly regular with respect to each other,
but it is not cubical — it is what we call a rhomboid. It
still breaks in three directions most beautifully and regularly
with polished surfaces, but with sloping sides, not like the
salt. Why not? It is very manifest that this is owing to
the attraction of the particles one for the other being less
in the direction in which they give way than in other direc-
tions. I have on the table before me a number of little
bits of calcareous spar, and I recommend each of you to
take a piece home, and then you can take a knife and
try to divide it in the direction of any of the surfaces al-
ready existing. You will be able to do it at once; but if you
try to cut it across the crystals, you can not; by hammering
you may bruise and break it up, but you can only divide it
into these beautiful little rhomboids.
Now I want you to understand a little more how this is,
and for this purpose I am going to use the electric light
again. You see we can not look into the middle of a body
like this piece of glass. We perceive the outside form and
the inside form, and we look through it, but we can not well
find out how these forms become so, and I want you, there-
fore, to take a lesson in the way in Which we use a ray of light
for the purpose of seeing what is in the interior of bodies.
Light is a thing which is, so to say, attracted by every sub-
stance that gravitates (and we do not know anything that
13 Iceland or cole spar. Native carbonate of lime in its primitive crystal-
line form.
32 FARADAY
does not) . All matter affects light more or less by what we
may consider as a kind of attraction, and I have arranged
(Fig. 18) a very simple experiment upon the floor of the
room for the purpose of illustrating this. I have put into
Fig. i 8
that basin a few things which those who are in the body of
the theatre will not be able to see, and I am going to make
use of this power which matter possesses of attracting a
ray of light. If Mr. Anderson pours some water, gently
and steadily, into the basin, the water will attract the rays
of light downward, and the piece of silver and the sealing-
wax will appear to rise up into the sight of those who were
before not high enough to see over the side of the basin
to its bottom. [Mr. Anderson here poured water into the
basin, and upon the lecturer asking whether any body could
see the silver and sealing-wax, he was answered by a general
affirmative.] Now I suppose that every body can see that
they are not at all disturbed, while from the way they appear
to have risen up you would imagine the bottom of the basin
and the articles in it were two inches thick, although they
are only one of our small silver dishes and a piece of
sealing-wax which I have put there. The light which now
goes to you from that piece of silver was obstructed by the
edge of the basin when there was no water there, and you
were unable to see any thing of it; but when we poured
in water the rays were attracted down by it over the edge
of the basin, and you were thus enabled to see the articles
at the bottom.
I have shown you this experiment first, so that you might
understand how glass attracts light, and might then see how
other substances like rock-salt and calcareous spar, mica,
hc vol. xxx — A
COHESION
33
and other stones, would affect the light; and, if Dr. Tyn-
dall will be good enough to let us use his light again, we will
first of all show you how it may be bent by a piece of glass
(Fig. 19). [The electric lamp was again lit, and the beam
Fig. 19
of parallel rays of light which it emitted was bent about
and decomposed by means of the prism.] Now, here you
see, if I send the light through this piece of plain glass,
A, it goes straight through without being bent (unless the
glass be held obliquely, and then the phenomenon becomes
more complicated) ; but if I take this piece of glass, B [a
prism], you see it will show a very different effect. It no
longer goes to that wall, but it is bent to this screen, C,
and how much more beautiful it is now [throwing the
prismatic spectrum on the screen]. This ray of light is
bent out of its course by the attraction of the glass upon it;
and you see I can turn and twist the rays to and fro in
different parts of the room, just as I please. Now it goes
there, now here. [The lecturer projected the prismatic spec-
trum about the theatre.] Here I have the rays once more
bent on to the screen, and you see how wonderfully and
beautifully that piece of glass not only bends the light by
virtue of its attraction, but actually splits it up into dif-
ferent colors. Now I want you to understand that this
piece of glass [the prism], being perfectly uniform in its
internal structure, tells us about the action of these other
bodies which are not uniform — which do not merely cohere,
but also have within them, in different parts, different de-
grees of cohesion, and thus attract and bend the light with
varying powers. We will now let the light pass through
one or two of these things which I just now showed you
broke so curiously: and, first of all, I will take a piece of
HC VOL. XXX — B
34 FARADAY
mica. Here, you see, is our ray of light: we have first to
make it what we call polarized; but about that you need
not trouble yourselves; it is only to make our illustration
more clear. Here, then, we have our polarized ray of light,
and I can so adjust it as to make the screen upon which it is
shining either light or dark, although I have nothing in the
course of this ray of light but what is perfectly transparent
[turning the analyzer round]. I will now make it so that it
is quite dark, and we will, in the first instance, put a piece
of common glass into the polarized ray so as to show you
that it does not enable the light to get through. You see the
screen remains dark. The glass, then, internally, has no
effect upon light. [The glass was removed and a piece of
mica introduced.] Now there is the mica which we split
up so curiously into leaf after leaf, and see how that enables
the light to pass through to the screen, and how, as Dr.
Tyndall turns it round in his hand, you have those different
colors, pink, and purple, and green, coming and going most
beautifully; not that the mica is more transparent than the
glass, but because of the different manner in which its
particles are arranged by the force of cohesion.
Now we will see how calcareous spar acts upon this light
— that stone which split up into rhombs, and of which you
are each of you going to take a little piece home. [The
mica was removed, and a piece of calc-spar introduced at
A.] See how that turns the light round and round, and
V-'\\
NSSc=
*^A\\\\^s*.v*-"
4?
I
Fig. 20
produces these rings and that black cross (Fig. 20). Look
at those colors: are they not most beautiful for you and for
me? (for I enjoy these things as much as you do). In what
COHESION 35
a wonderful manner they open out to us the internal arrange-
ment of the particles of this calcareous spar by the force
of cohesion.
And now I will show you another experiment. Here is
that piece of glass which before had no action upon the light.
You shall see what it will do when we apply pressure to it.
Here, then, we have our ray of polarized light, and I will
first of all show you that the glass has no effect upon it
in its ordinary state; when I place it in the course of the
light, the screen still remains dark. Now Dr. Tyndall will
press that bit of glass between three little points, one point
against two, so as to bring a strain upon the parts, and you
will see what a curious effect that has. [Upon the screen
two white dots gradually appeared.] Ah ! these points show
the position of the strain ; in these parts the force of cohesion
is being exerted in a different degree to what it is in the
other parts, and hence it allows the light to pass through.
How beautiful that is ! how it makes the light come through
some parts and leaves it dark in others, and all because
we weaken the force of cohesion between particle and par-
ticle. Whether you have this mechanical power of strain-
ing, or whether we take other means, we get the same
result; and, indeed, I will show you by another experiment
that if we heat the glass in one part, it will alter its internal
structure and produce a similar effect. Here is a piece of
common glass, and if I insert this in the path of
the polarized ray, I believe it will do nothing. There is
the common glass [introducing it]. No light passes
through; the screen remains quite dark; but I am going
to warm this glass in the lamp, and you know yourselves
that when you pour warm water upon glass you put a
strain upon it sufficient to break it sometimes — something
like there was in the case of the Prince Rupert's drops. [The
glass was warmed in the spirit lamp, and again placed across
the ray of light.] Now you see how beautifully the light goes
through those parts which are hot, making dark and light
lines just as the crystal did, and all because of the altera-
tion I have effected in its internal condition; for these dark
and light parts are a proof of the presence of forces acting
and dragging in different directions within the solid mass.
LECTURE III
COHESION— CHEMICAL AFFINITY
WE will first return for a few minutes to one of the
experiments made yesterday. You remember what
we put together on that occasion — powdered alum
and warm water. Here is one of the basins then used.
Nothing has been done to it since; but you will find, on
examining it, that it no longer contains any powder, but
a number of beautiful crystals. Here also are the pieces of
coke which I put into the other basin; they have a fine
mass of crystals about them. That other basin I will leave
as it is. I will not pour the water from it, because it will
show you that the particles of alum have done something
more than merely crystallize together. They have pushed
the dirty matter from them, laying it around the outside or
outer edge of the lower crystals — squeezed out, as it were,
by the strong attraction which the particles of alum have
for each other.
And now for another experiment. We have already
gained a knowledge of the manner in which the particles
of bodies — of solid bodies — attract each other, and we have
learned that it makes calcareous spar, and so forth, crystal-
lize in these regular forms. Now let me gradually lead
your minds to a knowledge of the means we possess of
making this attraction alter a little in its force; either of
increasing, or diminishing, or, apparently, of destroying it
altogether. I will take this piece of iron [a rod of iron about
two feet long and a quarter of an inch in diameter]. It
has at present a great deal of strength, due to its attraction
of cohesion; but if Mr. Anderson will make part of this
red-hot in the fire, we shall then find that it will become soft,
just as sealing-wax will when heated, and we shall also
find that the more it is heated the softer it becomes. Ah!
36
COHESION 37
but what does soft mean? Why, that the attraction be-
tween the particles is so weakened that it is no longer suf-
ficient to resist the power we bring to bear upon it. [Mr.
Anderson handed to the lecturer the iron rod, with one end
red-hot, which he showed could be easily twisted about with
a pair of pliers.] You see I now find no difficulty in bending
this end about as I like, whereas I can not bend the cold
part at all. And you know how the smith takes a piece
of iron and heats it in order to render it soft for his
purpose: he acts upon our principle of lessening the adhesion
of the particles, although he is not exactly acquainted with
the terms by which we express it.
And now we have another point to examine, and this
water is again a very good substance to take as an illustra-
tion (as philosophers we call it all water, even though it be
in the form of ice or steam). Why is this water hard?
[pointing to a block of ice] ; because the attraction of the
particles to each other is sufficient to make them retain
their places in opposition to force applied to it. But what
happens when we make the ice warm? Why, in that case
we diminish to such a large extent the power of attraction
that the solid substance is destroyed altogether. Let me
illustrate this: I will take a red-hot ball of iron [Mr. Ander-
son, by means of a pair of tongs, handed to the lecturer a
red-hot ball of iron, about two inches in diameter], because
it will serve as a convenient source of heat [placing the red-
hot iron in the centre of the block of ice]. You see I am
now melting the ice where the iron touches it. You see the
iron sinking into it; and while part of the solid water is
becoming liquid, the heat of the ball is rapidly going off.
A certain part of the water is actually rising in steam, the
attraction of some of the particles is so much diminished
that they can not even hold together in the liquid form,
but escape as vapor. At the same time, you see I can not
melt all this ice by the heat contained in this ball. In the
course of a very short time I shall find it will have become
quite cold.
Here is the water which we have produced by destroying
some of the attraction which existed between the particles
of the ice, for below a certain temperature the particles o£
38 FARADAY
water increase in their mutual attraction and become ice ; and
above a certain temperature the attraction decreases and
the water becomes steam. And exactly the same thing hap-
pens with platinum, and nearly every substance in nature;
if the temperature is increased to a certain point it becomes
liquid and a farther increase converts it into a gas. Is it not
a glorious thing for us to look at the sea, the rivers, and
so forth, and to know that this same body in the northern
regions is all solid ice and icebergs, while here, in a warmer
climate, it has its attraction of cohesion so much diminished
as to be liquid water? Well, in diminishing this force of
attraction between the particles of ice, we made use of
another force, namely that of heat; and I want you now
to understand that this force of heat is always concerned
when water passes from the solid to the liquid state. If
I melt ice in other ways I can not do without heat (for
we have the means of making ice liquid without heat — that
is to say, without using heat as a direct cause). Suppose, for
illustration, I make a vessel out of this piece of tinfoil [bend-
ing the foil up into the shape of a dish]. I am making
it metallic, because I want the heat which I am about to
deal with to pass readily through it; and I am going to
pour a little water on this board, and then place the tin
vessel on it. Now if I put some of this ice into the metal
dish, and then proceed to make it liquid by any of the
various means we have at our command, it still must take
the necessary quantity of heat from something, and in this
case it will take the heat from the tray, and from the water
underneath, and from the other things round about. Well, a
little salt added to the ice has the power of causing it to
melt, and we shall very shortly see the mixture become quite
fluid, and you will then find that the water beneath will
be frozen — frozen because it has been forced to give up
that heat which is necessary to keep it in the liquid state
to the ice on becoming liquid. I remember once, when I
was a boy, hearing of a trick in a country ale-house: the
point was how to melt ice in a quart pot by the fire and
freeze it to the stool. Well, the way they did it was this:
they put some pounded ice in a pewter pot, and added some
salt to it, and the consequence was that when the salt was
COHESION 39
mixed with it, the ice in the pot melted (they did not tell me
any thing about the salt and they set the pot by the fire, just
to make the result more mysterious), and in a short time the
pot and the stool were frozen together, as we shall very
shortly find it to be the case here, and all because salt has the
power of lessening the attraction between the particles of ice.
Here you see the tin dish is frozen to the board ; I can even
lift the little stool up by it.
This experiment can not, I think, fail to impress upon your
minds the fact that whenever a solid body loses some of
that force of attraction by means of which it remains solid,
heat is absorbed; and if on the other hand we convert a
liquid into a solid, e. g., water into ice, a corresponding
amount of heat is given out. I have an experiment showing
this to be the case. Here (Fig. 21) is a bulb, A, filled with
Fig. 21
air, the tube from which dips into some colored liquid in
the vessel B. And I dare say you know that if I put my
hand on the bulb A, and warm it, the colored liquid which
is now standing in the tube at C will travel forward. Now
we have discovered a means, by great care and research into
the properties of various bodies, of preparing a solution of a
salt( 14 ) which, if shaken or disturbed, will at once become
a solid; and as I explained to you just now (for what is
true of water is true of every other liquid), by reason of its
becoming solid heat is evolved, and I can make this evident
14 Solution of a salt. Acetate of soda. A solution saturated, or nearly
so, at the boiling point, is necessary, and it must be allowed to cool, and
remain at rest until the experiment is made.
40 FARADAY
to you by pouring it over this bulb; there! it is becoming
solid; and look at the colored liquid, how it is being driven
down the tube, and how it is bubbling out through the water
at the end; and so we learn this beautiful law of our
philosophy, that whenever we diminish the attraction of
cohesion we absorb heat, and whenever we increase that
attraction heat is evolved. This, then, is a great step in
advance, for you have learned a great deal in addition to
the mere circumstance that particles attract each other. But
you must not now suppose that because they are liquid they
have lost their attraction of cohesion; for here is the fluid
mercury, and if I pour it from one vessel into another, I find
that it will form a stream from the bottle down to the
glass — a continuous rod of fluid mercury, the particles of
which have attraction sufficient to make them hold together
all the way through the air down to the glass itself; and if
I pour water quietly from a jug, I can cause it to run in
a continuous stream in the same manner. Again: let me
put a little water on this piece of plate glass, and then
take another plate of glass and put it on the water,
there! the upper plate is quite free to move, gliding about
on the lower one from side to side; and yet, if I take
hold of the upper plate and lift it up straight, the cohesion
is so great that the lower one is held up by it. See how
it runs about as I move the upper one, and this is all owing
to the strong attraction of the particles of the water. Let
me show you another experiment. If I take a little soap
and water — not that the soap makes the particles of the
water more adhesive one for the other, but it certainly has
the power of continuing in a better manner the attraction
of the particles (and let me advise you, when about to
experiment with soap-bubbles, to take care to have every
thing clean and soapy). I will now blow a bubble, and that
I may be able to talk and blow a bubble too, I will take a plate
with a little of the soapsuds in it, and will just soap the
edges of the pipe and blow a bubble on to the plate. Now
there is our bubble. Why does it hold together in this
manner? Why, because the water of which it is composed
has an attraction of particle for particle — so great, indeed,
that it gives to this bubble the very power of an India-rubber
COHESION
41
ball; for you see, if I introduce one end of this glass tube
into the bubble, that it has the power of contracting so
powerfully as to force enough air through the tube to
blow out a light (Fig. 22) ; the light is blown out. And look !
see how the bubble is disappearing — see how it is getting
smaller and smaller.
There are twenty other experiments I might show you to
illustrate this power of cohesion of the particles of liquids.
For instance, what would you propose to me if, having
lost the stopper out of this alcohol bottle, I should want
to close it speedily with something near at hand. Well, a
bit of paper would not do, but a piece of linen cloth would,
or some of this cotton wool which I have here. I will put a
tuft of it into the neck of the alcohol bottle, and you see,
Fig. 22
Fig. 23
when I turn it upside down, that it is perfectly well stop-
pered so far as the alcohol is concerned; the air can pass
through, but the alcohol can not. And if I were to take
an oil vessel this plan would do equally well, for in former
times they used to send us oil from Italy in flasks stoppered
only with cotton wool (at the present time the cotton is put
in after the oil has arrived here, but formerly it used to be
sent so stoppered). Now if it were not for the particles
of liquid cohering together, this alcohol would run out;
and if I had time I could have shown you a vessel with
the top, bottom, and sides altogether formed like a sieve, and
yet it would hold water, owing to the cohesion.
You have now seen that the solid water can become fluid
by the addition of heat, owing to this lessening the attractive
force between its particles, and yet you see that there is a
good deal of attractive force remaining behind. I want now
to take you another step beyond. We saw that if we con-
42 FARADAY
tinued applying heat to the water (as indeed happened with
our piece of ice here), that we did at last break up that
attraction which holds the liquid together, and I am about
to take some other (any other liquid would do, but ether
makes a better experiment for my purpose) in order to illus-
trate what will happen when this cohesion is broken up.
Now this liquid ether, if exposed to a very low temperature,
will become a solid; but if we apply heat to it, it becomes
vapor ; and I want to show you the enormous bulk of the sub-
stance in this new form: when we make ice into water, we
lessen its bulk ; but when we convert water into steam, we in-
crease it to an enormous extent. You see it is very clear that
as I apply heat to the liquid I diminish its attraction of co-
hesion ; it is now boiling, and I will set fire to the vapor, so
that you may be enabled to judge of the space occupied by the
ether in this form by the size of its flame ; and you now see
what an enormously bulky flame I get from that small volume
of ether below. The heat from the spirit lamp is now being
consumed, not in making the ether any warmer, but in con-
verting it into vapor; and if I desired to catch this vapor and
condense it (as I could without much difficulty), I should
have to do the same as if I wished to convert steam into
water and water into ice: in either case it would be neces-
sary to increase the attraction of the particles by cold or
otherwise. So largely is the bulk occupied by the particles
increased by giving them this diminished attraction, that if
I were to take a portion of water a cubic inch in bulk (A,
Fig. 23), I should produce a volume of steam of that size,
B [1,700 cubic inches; nearly a cubic foot], so greatly is
the attraction of cohesion diminished by heat; and yet it
still remains water. You can easily imagine the conse-
quences which are due to this change in volume by heat —
the mighty powers of steam and the tremendous explosions
which are sometimes produced by this force of water. I
want you now to see another experiment, which will perhaps
give you a better illustration of the bulk occupied by a body
when in the state of vapor. Here is a substance which we
call iodine, and I am about to submit this solid body to the
same kind of condition as regards heat that I did the water
and the other [putting a few grains of iodine into a hot
COHESION 43
glass globe, which immediately became filled with the violet
vapor], and you see the same kind of change produced.
Moreover, it gives us the opportunity of observing how beau-
tiful is the violet-colored vapor from this black substance, or
rather the mixture of the vapor with air (for I would not
wish you to understand that this globe is entirely filled with
the vapor of iodine).
If I had taken mercury and converted it into vapor (as
I could easily do), I should have a perfectly colorless vapor;
for you must understand this about vapors, that bodies in
what we call the vaporous or the gaseous state are always
perfectly transparent, never cloudy or smoky; they are,
however, often colored, and we can frequently have colored
vapors or gases produced by colorless particles themselves
mixing together, as in this case [the lecturer here inverted
a glass cylinder full of binoxide of nitrogen ( 15 ) over a
cylinder of oxygen, when the dark red vapor of hyponitrous
acid was produced]. Here also you see a very excellent
illustration of the effect of a power of nature which we
have not as yet come to, but which stands next on our list —
Chemical Affinity. And thus you see we can have a
violet vapor or an orange vapor, and different other kinds
of vapor, but fhey are always perfectly transparent, or else
they would cease to be vapors.
I am now going to lead you a step beyond this considera-
tion of the attraction of the particles for each other. You
see we have come to understand that, if we take water as
an illustration, whether it be ice, or water, or steam, it
is always to be considered by us as water. Well, now pre-
pare your minds to go a little deeper into the subject. We
have means of searching into the constitution of water
beyond any that are afforded us by the action of heat, and
among these one of the most important is that force which
we call voltaic electricity, which we used at our last meeting
for the purpose of obtaining light, and which we carried
15 Binoxide of nitrogen and hyponitrous acid. Binoxide of nitrogen is
formed when nitric? acid and a little water are added to some copper
turnings. It produces deep red fumes as soon as it comes in contact with
the air, by combining with the oxygen of the latter to form hyponitrous
acid. Binoxide of nitrogen is composed of two parts of oxygen and one
part of nitrogen: hyponitrous acid is composed of one part of nitrogen and
three parts of oxygen.
44
FARADAY
about the room by means of these wires. This force is
produced by the battery behind me, to which, however, I
will not now refer more particularly; before we have done
we shall know more about this battery, but it must grow up
in our knowledge as we proceed. Now here (Fig. 24) is
a portion of water in this little vessel, C, and besides the
water there are two plates of the metal platinum, which
are connected with the wires (A and B) coming outside,
Fig. 24
and I want to examine that water, and the state and the
condition in which its particles are arranged. If I were to
apply heat to it you know what we should get; it would
assume the state of vapor, but it would nevertheless remain
water, and would return to the liquid state as soon as the
heat was removed. Now by means of these wires (which
are connected with the battery behind me, and come under
the floor and up through the table) we shall have a certain
amount of this new power at our disposal. Here you see
it is [causing the ends of the wires to touch] — that is the
electric light we used yesterday, and by means of these wires
we can cause water to submit itself to this power; for the
moment I put them into metallic connection (at A and B)
you see the water boiling in that little vessel (C), and you
hear the bubbling of the gas that is going through the tube
(D). See how I am converting the water into vapor; and
if I take a little vessel (E), and fill it with water, and put
it into the trough over the end of the tube (D), there goes
CHEMICAL AFFINITY 45
the vapor ascending into the vessel. And yet that is not
steam, for you know that if steam is brought near cold water,
it would at once condense, arid return back again to water;
this, then, can not be steam, for it is bubbling through the
cold water in this trough ; but it is a vaporous substance, and
we must therefore examine it carefully, to see in what way
the water has been changed. And now, in order to give you
a proof that it is not steam, I am going to show you that it
is combustible; for if I take this small vessel to a light,
the vapor inside explodes in a manner that steam could
never do.
I will now fill this large bell-jar (F) with water; and I
propose letting the gas ascend into it, and I will then show
you that we can reproduce the water back again from the
vapor or air that is there. Here is a strong glass vessel (G),
and into it we will let the gas (from F) pass. We will
there fire it by the electric spark, and then, after the ex-
plosion, you will find that we have got the water back
again; it will not be much, however, for you will recollect
that I showed you how small a portion of water produced
a very large volume of vapor. Mr. Anderson will now
pump all the air out of this vessel (G), and when I have
screwed it on to the top of our jar of gas (F), you will
see, upon opening the stop-cocks (H H H), the water will
jump up, showing that some of the gas has passed into the
glass vessel. I will now shut these stop-cocks, and we shall
be able to send the electric spark through the gas by means
of the wires (I, K) in the upper part of the vessel, and you
will see it burn with a most intense flash. [Mr. Anderson
here brought a Ley den jar, which he discharged through
the confined gas by means of the wires (I, K).] You saw
the flash, and now that you may see that there is no longer
any gas remaining, if I place it over the jar and open the
stop-cocks again, up will go the gas, and we can have a
second combustion; and so I might go on again and again,
and I should continue to accumulate more and more of the
water to which the gas has returned. Now is not this
curious? In this vessel (C) we can go on making from
water a large bulk of permanent gas, as we call it, and
then we can reconvert it into water in this way. [Mr.
46 FARADAY
Anderson brought in another Ley den jar, which, however,
from some cause, would not ignite the gas. It was therefore
recharged, when the explosion took place in the desired
manner.] How beautifully we get our results when we are
right in our proceedings! It is not that Nature is wrong
when we make a mistake. Now I will lay this vessel (G)
down by my right hand, and you can examine it by-and-
by; there is not very much water flowing down, but there
is quite sufficient for you to see.
Another wonderful thing about this mode of changing
the condition of the water is this: that we are able to get
the separate parts of which it is composed at a distance
the one from the other, and to examine them, and see what
they are like, and how many of them there are; and
for this purpose I have here some more water in a
slightly different apparatus to the former one (Fig. 25)
and if I place this in connection
with the wires of the battery (at
A, B), I shall get a similar decom-
position of the water at the two
platinum plates. Now I will put
^7| this little tube (O) over there, and
l||i| that will collect the gas together
i piP that comes from this side (A),
JjjW and this tube (H) will collect the
V ^y gas that comes from the other side
FlG 25 (B), and I think we shall soon
be able to see a difference. In this
apparatus the wires are a good way apart from each other,
and it now seems that each of them is capable of drawing off
particles from the water and sending them off, and you see
that one set of particles (H) is coming off twice as fast as
those collected in the other tube (O). Something is coming
out of the water there (at H) which burns [setting fire to the
gas]; but what comes out of the water here (at O), al-
though it will not burn, will support combustion very vigor-
ously. [The lecturer here placed a match with a glowing
tip in the gas, when it immediately rekindled.]
Here, then, we have two things, neither of them being
water alone, but which we get out of the water. Water is
CHEMICAL AFFINITY 47
therefore composed of two substances different to itself,
which appear at separate places when it is made to submit
to the force which I have in these wires; and if I take an in-
verted tube of water and collect this gas (H), you will see
that it is by no means the same as the one we collected in
the former apparatus (Fig. 24). That exploded with a
loud noise when it was lighted, but this will burn quite
noiselessly: it is called hydrogen; and the other we call
oxygen — that gas which so beautifully brightens up all com-
bustion, but does not burn of itself. So now we see that
water consists of two kinds of particles attracting each other
in a very different manner to the attraction of gravitation
or cohesion, and this new attraction we call chemical affinity,
or the force of chemical action between different bodies;
we are now no longer concerned with the attraction of iron
for iron, water for water, wood for wood, or like bodies
for each other, as we were when dealing with the
force of cohesion; we are dealing with another kind of
attraction — the attraction between particles of a different
nature one to the other. Chemical affinity depends entirely
upon the energy with which particles of different kinds
attract each other. Oxygen and hydrogen are particles of
different kinds, and it is their attraction to each other which
makes them chemically combine and produce water.
I must now show you a little more at large what chemical
affinity is. I can prepare these gases from other substances
as well as from water; and we will now prepare some oxy-
gen : here is another substance which contains oxygen — chlo-
rate of potash; I will put some of it into this glass retort,
and Mr. Anderson will apply heat to it: we have here dif-
ferent jars filled with water, and when, by the application
of heat, the chlorate of potash is decomposed, we will
displace the water, and fill the jars with gas.
Now, when water is opened out in this way by means of
the battery, which adds nothing to it materially, which takes
nothing from it materially (I mean no matter; I am not
speaking of force), which adds no matter to the water, it
is changed in this way — the gas which you saw burning a
little while ago, called hydrogen, is evolved in large quantity,
and the other gas, oxygen, is evolved in only half the
48
FARADAY
quantity; so that these two areas represent water, and these
are always the proportions between the two gases.
Oxygen 88.9
Hydrogen . . . . 11.1
1
Hydrogen
8
Oxygen
9
Water 100.0
But oxygen is sixteen times the weight of the other — eight
times as heavy as the particles of hydrogen in the water ; and
you therefore know that water is composed of nine parts
by weight — one of hydrogen and eight of oxygen ; thus :
Hydrogen 46.2 cubic inches
Oxygen 23.1 " "
Water (steam) 69.3
= 1 grain
= 8 grains
= 9 grains
Now Mr. Anderson has prepared some oxygen, and we
will proceed to examine what is the character of this gas.
First of all, you remember I told you that it does not burn,
but that it affects the burning of other bodies. I will just
set fire to the point of this little bit of wood, and then
plunge it into the jar of oxygen, and you will see what this
gas does in increasing the brilliancy of the combustion. It
does not burn, it does not take fire, as the hydrogen would;
but how vividly the combustion of the match goes on!
Again, if I were to take this wax taper and light it, and
turn it upside down in the air, it would in all probability
put itself out, owing to the wax running down into the
wick. [The lecturer here turned the lighted taper upside
down, when in a few seconds it went out.] Now that will
not happen in oxygen gas; you will see how differently it
acts (Fig. 26). [The taper was again lighted, turned up-
side down, and then introduced into a jar of oxygen.] Look
at that ! See how the very wax itself burns, and falls down
in a dazzling stream of fire, so powerfully does the oxygen
CHEMICAL AFFINITY
49
support combustion. Again, here is another experiment
which will serve to illustrate the force, if I may so call
it, of oxygen. I have here a circular flame
of spirit of wine, and with it I am about
to show you the way in which iron burns,
because it will serve very well as a com-
parison between the effect produced by air
and oxygen. If I take this ring flame, I
can shake, by means of a sieve, the fine
particles of iron filings through it, and you
will see the way in which they burn.
[The lecturer here shook through the flame
some iron filings, which took fire and fell
through with beautiful scintillations.] But if I now hold the
flame over a jar of oxygen [the experiment was repeated over
a jar of oxygen, when the combustion of the filings as they
fell into the oxygen became almost insupportably brilliant],
you see how wonderfully different the effect is in the jar,
because there we have oxygen instead of common air.
Fig. 26
LECTURE IV
CHEMICAL AFFINITY— HEAT
WE shall have to pay a little more attention to the
forces existing in water before we can have a
clear idea on the subject. Besides the attraction
which there is between its particles to make it hold to-
gether as a liquid or a solid, there is also another force,
different from the former — one which, yesterday, by means
of the voltaic battery, we overcame, drawing from the water
two different substances, which, when heated by means of
the electric spark, attracted each other, and rushed into
combination to reproduce water. Now I propose to-day to
continue this subject, and trace the various phenomena of
chemical affinity ; and for this purpose, as we yesterday con-
sidered the character of oxygen, of which I have here two
jars (oxygen being those particles derived from the water
which enable other bodies to burn), we will now consider
the other constituent of water, and, without embarrassing
you too much with the way in which these things are made,
I will proceed now to show you our common way of making
hydrogen. (I called it hydrogen yesterday: it is so called
because it helps to generate water.)* I put into this retort
some zinc, water and oil of vitriol, and immediately an
action takes place, which produces an abundant evolution of
gas, now coming over into this jar, and bubbling up in
appearance exactly like the oxygen we obtained yesterday.
The processes, you see, are very different, though the
result is the same, in so far as it gives us certain gaseous
particles. Here, then, is the hydrogen. I showed you yester-
day certain qualities of this gas; now let me exhibit you
some other properties. Unlike oxygen, which is a supporter
of combustion and wiil not burn, hydrogen itself is com-
* vSwp, M water," and ytvvau, " I generate."
50
CHEMICAL AFFINITY
51
bustible. There is a jar full of it; and if I carry it along
in this manner and put a light to it, I think you will see it
take fire — not with a bright light; you will, at all events,
Fig. 27
hear it if you do not see it. Now that is a body entirely
different from oxygen; it is extremely light; for, although
yesterday you saw twice as much of this hydrogen produced
on the one side as on the other by the voltaic battery, it was
only one-eighth the weight of the oxygen. I carry this jar
upside down. Why? Because I know that it is a very light
body, and that it will continue in this jar upside down quite
as effectually as the water will in that jar which is not upside
down; and just as I can pour water from one vessel into
another in the right position to receive it, so can I pour this
gas from one jar into another when they are upside down.
See what I am about to do. There is no hydrogen in this
jar at present, but I will gently turn this jar of hydrogen
up under this other jar (Fig. 28), and then we will examine
the two. We shall see, on applying
a light, that the hydrogen has left the
jar in which it was at first, and has
poured upward into the other, and
there we shall find it.
You now understand that we can
have particles of very different kinds,
and that they can have different bulks
and weights; and there are two or
three very interesting experiments which serve to illustrate
this. For instance, if I blow soap bubbles with the breath
from my mouth, you will see them fall, because I fill them
Fig. 28
52 FARADAY
with common air, and the water which forms the bubble car-
ries it down. But now, if I inhale hydrogen gas into my lungs
(it does no harm to the lungs, although it does no good to
them), see what happens. [The lecturer inhaled some hydro-
gen, and, after one or two ineffectual attempts, succeeded in
blowing a splendid bubble, which rose majestically and slowly
to the ceiling of the theatre, where it burst.] That shows you
very well how light a substance this is ; for, notwithstanding
all the heavy bad air from my lungs, and the weight of the
bubble, you saw how it was carried up. I want you now to
consider this phenomenon of weight as indicating how exceed-
ingly different particles are one from the other; and I will
take as illustrations these very common things, air, water, the
heaviest body, platinum, and this gas, and observe how they
differ in this respect; for if I take a piece of platinum of
that size (Fig. 29), it is equal to the weight of portions of
Hydrogen.
Platinum.
Water.
O
Fig. 29
water, air, and hydrogen of the bulks I have represented in
these spheres; and this illustration gives you a very good
idea of the extraordinary difference with regard to the
gravity of the articles having this enormous difference in
bulk. [The following tabular statement having reference to
this illustration appeared on the diagram-board.]
Hydrogen .... 1
Air 14.4 1
Water "943 829 x
Platinum .... 256774 17831 21.5
CHEMICAL AFFINITY
53
Whenever oxygen and hydrogen unite together they pro-
duce water, and you have seen the extraordinary difference
between the bulk and appearance of the water so produced
and the particles of which it consists chemically. Now we
have never yet been able to reduce either oxygen or hydro-
gen to the liquid state; and yet their first impulse, when
chemically combined, is to take up first this liquid condition
and then the solid condition. We never combine these dif-
ferent particles together without producing water; and it is
curious to think how often you must have made the experi-
ment of combining oxygen and hydrogen to form water
without knowing it. Take a candle, for instance, and a clean
silver spoon (or a piece of clean tin will do), and, if you
hold it over the flame, you immediately cover it with dew —
not a smoke — which presently evaporates. This, perhaps,
will serve to show it better. Mr. Anderson will put a candle
under that jar, and you will see how soon the water is pro-
duced (Fig. 30). Look at that dimness on the sides of the
glass, which will soon produce drops,
and trickle down into the plate.
Well, that dimness and these drops
are water, formed by the union of
the oxygen of the air with the hydro-
gen existing in the wax of which
that candle is formed.
And now, having brought you, in
the first place, to the consideration of
chemical attraction, I must enlarge
your ideas so as to include all sub-
stances which have this attraction
it changes the character of bodies,
this way and that way in the most
ner, and produces other phenomena
about. Here is some chlorate of
some sulphuret of antimony ( 16 ). We
different sets of particles together and I want to show
you, in a general sort of way, some of the phenomena which
for each other; for
and alters them in
extraordinary man-
wonderful to think
potash, and there
will mix these two
16 Chlorate of potash and sulphuret of antimony. Great care must be
taken in mixing these substances, as the mixture is dangerously explosive.
They must be powdered separately and mixed together v/ith a feather on a
sheet of paper, or by passing them several times through a small sieve.
54 FARADAY
take place when we make different particles act together.
Now I can make these bodies act upon each other in several
ways. In this case I am going to apply heat to the mixture ;
but if I were to give a blow with a hammer, the same result
would follow. [A lighted match was brought to the mixture,
which immediately exploded with a sudden flash, evolving a
dense white smoke.] There you see the result of the action
of chemical affinity overcoming the attraction of cohesion of
the particles. Again, here is a little sugar ( 1T ), quite a
different substance from the black sulphuret of antimony,
and you shall see what takes place when we put the two to-
gether. [The mixture was touched with sulphuric acid,
when it took fire, and burnt gradually and with a brighter
flame than in the former instance.] Observe this chemical
affinity traveling about the mass, and setting it on fire, and
throwing it into such wonderful agitation !
I must now come to a few circumstances which require
careful consideration. We have already examined one of
the effects of this chemical affinity, but, to make the matter
more clear, we must point out some others. And here are
two salts dissolved in water ( 18 ). They are both colorless
solutions, and in these glasses you can not see any difference
between them. But if I mix them, I shall have chemical
attraction take place. I will pour the two together into this
glass, and you will at once see, I have no doubt, a certain
amount of change. Look, they are already becoming milky,
but they are sluggish in their action — not quick as the others
were — for we have endless varieties of rapidity in chemical
action. Now, if I mix them together, and stir them so as
to bring them properly together, you will soon see what a
different result is produced. As I mix them they get thicker
and thicker, and you see the liquid is hardening and stiff-
ening, and before long I shall have it quite hard; and be-
fore the end of the lecture it will be a solid stone — a wet
stone, no doubt, but more or less solid — in consequence
of the chemical affinity. Is not this changing two liquids
17 The mixture of chlorate of potash and sugar does not require the same
precautions. They may be rubbed together in a pestle and mortar without
fear. One part of chlorate of potash and three parts of sugar will answer.
The mixture need only be toucned with a glass rod dipped in oil of vitriol.
18 Two salts dissolved in water. Sulphate of soda and chloride of calcium.
The solutions must be saturated for the experiment to succeed well.
CHEMICAL AFFINITY 55
into a solid body a wonderful manifestation of chemical
affinity ?
There is another remarkable circumstance in chemical
affinity, which is, that it is capable of either waiting or acting
at once. And this is very singular, because we know of noth-
ing of the kind in the forces either of gravitation or co-
hesion. For instance: here are some oxygen particles, and
here is a lump of carbon particles. I am going to put the
carbon particles into the oxygen; they can act, but they do
not — they are just like this unlighted candle. It stands here
quietly on the table, waiting until we want to light it. But it
is not so in this other case : here is a substance, gaseous like
the oxygen, and if I put these particles of metal into it the
two combine at once. The copper and the chlorine unite by
their power of chemical affinity, and produce a body entirely
unlike either of the substances used. And in this other case,
it is not that there is any deficiency of affinity between the
carbon and oxygen, for the moment I choose to put them in
a condition to exert their affinity, you will see the difference.
[The piece of charcoal was ignited, and introduced into the
jar of oxygen, when the combustion proceeded with vivid
scintillations.]
Now this chemical action is set going exactly as it would
be if I had lighted the candle, or as it is when the servant
puts coals on and lights the fire : the substances wait until we
do something which is able to start the action. Can any
thing be more beautiful than this combustion of charcoal in
oxygen? You must understand that each of these little
sparks is a portion of the charcoal, or the bark of the char-
coal thrown off white hot into the oxygen, and burning in it
most brilliantly, as you see. And now let me tell you another
thing, or you will go away with a very imperfect notion of
the powers and effects of this affinity. There you see some
charcoal burning in oxygen. Well, a piece of lead will burn
in oxygen just as well as the charcoal does, or indeed better,
for absolutely that piece of lead will act at once upon the
oxygen as the copper did in the other vessel with regard to
the chlorine. And here, also, a piece of iron — if I light it
and put it into the oxygen, it will burn away just as the
carbon did. And I will take some lead, and show you that it
56 FARADAY
will burn in the common atmospheric oxygen at the ordinary
temperature. These are the lumps of lead which you remem-
ber we had the other day — the two pieces which clung to-
gether. Now these pieces, if I take them to-day and press
them together, will not stick, and the reason is that they have
attracted from the atmosphere a part of the oxygen there
present, and have become coated as with a varnish by the
oxide of lead, which is formed on the surface by a real
process of combustion or combination. There you see the
iron burning very well in oxygen, and I will tell you the
.reason why those scissors and that lead do not take fire while
they are lying on the table. Here the lead is in a lump, and
the coating of oxide remains on its surface, while there you
see the melted oxide is clearing itself off from the iron, and
allowing more and more to go on burning. In this case,
however, [holding up a small glass tube containing lead
pyrophorus( 19 )], the lead has been very carefully produced
in fine powder, and put into a glass tube, and hermetically
sealed so as to preserve it, and I expect you will see it take
fire at once. This has been made about a month ago, and
has thus had time to sink down to its normal temperature;
what you see, therefore, is the result of chemical affinity
alone. [The tube was broken at the end, and the lead poured
out on to a piece of paper, whereupon it immediately took
fire.] Look! look at the lead burning! Why, it has set fire
to the paper ! Now that is nothing more than the common
affinity always existing between very clean lead and the at-
mospheric oxygen ; and the reason why this iron does not
burn until it is made red hot is because it has got a coating
of oxide about it, which stops the action of the oxygen —
putting a varnish, as it were, upon its surface, as we varnish
a picture — absolutely forming a substance which prevents
the natural chemical affinity between the bodies from acting.
I must now take you a little farther in this kind of illus-
tration, or consideration I would rather call it, of chemical
affinity. This attraction between different particles exists
also most curiously in cases where they are previously com-
19 Lead pyrophorus. This is tartrate of lead which has been heated in
a glass tube to dull redness as long as vapors are emitted. As soon as
they cease to be evolved the end of the tube is sealed, and it is allowed
to cool.
CHEMICAL AFFINITY 57
bined with other substances. Here is a little chlorate of
potash containing the oxygen which we found yesterday
could be procured from it ; it contains the oxygen there com-
bined and held down by its chemical affinity with other
things, but still it can combine with sugar, as you saw. This
affinity can thus act across substances, and I want you to see
how curiously what we call combustion acts with respect to
this force of chemical affinity. If I take a piece of phos-
phorus and set fire to it, and then place a jar of air over the
phosphorus, you see the combustion which we are having
there on account of chemical affinity (combustion being in
all cases the result of chemical affinity). The phosphorus
is escaping in that vapor, which will condense into a snow-
like mass at the close of the lecture. But suppose I limit the
atmosphere, what then? why, even the phosphorus will go
out. Here is a piece of camphor, which will burn very well
in the atmosphere, and even on water it will float and burn,
by reason of some of its particles gaining access to the air.
But if I limit the quantity of air by placing a jar over it, as
I am now doing, you will soon find the camphor will go out.
Well, why does it go out? not for want of air, for there is
plenty of air remaining in the jar. Perhaps you will be
shrewd enough to say for want of oxygen.
This, therefore, leads us to the inquiry as to whether oxy-
gen can do more than a certain amount of work. The oxygen
there (Fig. 30) can not go on burning an unlimited quantity
of candle, for that has gone out, as you see; and its amount
of chemical attraction or affinity is just as strikingly limited:
it can no more be fallen short of or exceeded than can the
attraction of gravitation. You might as soon attempt to
destroy gravitation, or weight, or all things that exist,
as to destroy the exact amount of force exerted by this
oxygen. And when I pointed out to you that eight by weight
of oxygen to one by weight of hydrogen went to form
water, I meant this, that neither of them would combine in
different proportions with the other, for you can not get ten
of hydrogen to combine with six of oxygen, or ten of oxygen
to combine with six of hydrogen; it must be eight of oxygen
and one of hydrogen. Now suppose I limit the action in this
way : this piece of cotton wool burns, as you see, very well in
58 FARADAY
the atmosphere; and I have known of cases of cotton-mills
being fired as if with gunpowder through the very finely
divided particles of cotton being diffused through the atmos-
phere in the mill, when it has sometimes happened that a
flame has caught these raised particles, and it has run from
one end of the mill to the other and blown it up. That, then,
is on account of the affinity which the cotton has for the
oxygen; but suppose I set fire to this piece of cotton which
is rolled up tightly; it does not go on burning, because I
have limited the supply of oxygen, and the inside is pre-
vented from having access to the oxygen, just as it was in
the case of the lead by the oxide. But here is some cotton
which has been imbued with oxygen in a certain manner.
I need not trouble you now with the way it is prepared;
it is called gun-cotton ( 20 ). See how that burns [setting
fire to a piece] ; it is very different from the other, because
the oxygen which must be present in its proper amount is
put there beforehand. And I have here some pieces of
paper which are prepared like the gun-cotton ( 21 ), and im-
bued with bodies containing oxygen. Here is some which
has been soaked in nitrate of strontia : you will see the beau-
tiful red color of its flame; and here is another which I
think contains baryta, which gives that fine green light; and
I have here some more which has been soaked in nitrate of
copper: it does not burn quite so brightly, but still very
beautifully. In all these cases the combustion goes on inde-
pendent of the oxygen of the atmosphere. And here we have
some gunpowder put into a case, in order to show that it is
capable of burning under water. You know that we put it
into a gun, shutting off the atmosphere with shot, and yet the
oxygen which it contains supplies the particles with that
without which chemical action could not proceed. Now I
have a vessel of water here, and am going to make the
experiment of putting this fuse under the water, and you
20 Gun-cotton is madeby immersing cotton-wool in a mixture of sulphuric
acid and the strongest nitric acid or of sulphuric acid and nitrate of potash.
21 Paper prepared like gun-cotton. It should be bibulous paper, and must
be soaked for ten minutes in a mixture of ten parts, by measure, of oil of
vitriol with five parts of strong fuming nitric acid. The paper must after-
ward be thoroughly washed with warm distilled water, and then carefully
dried at a gentle heat. The paper is then saturated with chlorate of
strontia, or chlorate of baryta, or nitrate of copper, by immersion in a
warm solution of these salts. (See Chemical News, vol. i., p. 36.)
CHEMICAL AFFINITY 59
will see whether that water can extinguish it; here it is
burning out of the water, and there it is burning under the
water; and so it will continue until exhausted, and all by
reason of the requisite amount of oxygen being contained
within the substance. It is by this kind of attraction of the
different particles one to the other that we are enabled to
trace the laws of chemical affinity, and the wonderful variety
of the exertions of these laws.
Now I want you to observe that one great exertion of this
power which is known as chemical affinity is to produce
HEAT and light; you know, as a matter of fact, no doubt,
that when bodies burn they give out heat, but it is a curious
thing that this heat does not continue; the heat goes away
as soon as the action stops, and you see, thereby, that it
depends upon the action during the time it is going on. It
is not so with gravitation; this force is continuous, and is
just as effective in making that lead press on the table as
it was when it first fell there. Nothing occurs there which
disappears when the action of falling is over; the pressure
is upon the table, and will remain there until the lead is re-
moved; whereas, in the action of chemical affinity to give
light and heat, they go away immediately the action is over.
This lamp seems to evolve heat and light continuously, but it
is owing to a constant stream of air coming into it on all
sides, and this work of producing light and heat by chemical
affinity will subside as soon as the stream of air is inter-
rupted. What, then, is this curious condition of heat ? Why,
it is the evolution of another power of matter — of a power
new to us, and which we must consider as if it were now
for the very first time brought under our notice. What is
heat? We recognize heat by its power of liquefying solid
bodies and vaporizing liquid bodies ; by its power of setting
in action, and very often overcoming, chemical affinity. Then
how do we obtain heat? We obtain it in various ways; most
abundantly by means of the chemical affinity we have just
before been speaking about, but we can also obtain it in
many other ways. Friction will produce heat. The Indians
rub pieces of wood together until they make them hot enough
to take fire; and such things have been known as two
branches of a tree rubbing together so hard as to set the tree
60 FARADAY
on fire. I do not suppose I shall set these two pieces of wood
on fire by friction, but I can readily produce heat enough to
ignite some phosphorus. [The lecturer here rubbed two
pieces of cedar wood strongly against each other for a min-
ute, and then placed on them a piece of phosphorus, which
immediately took fire.] And if you take a smooth metal
button stuck on a cork, and rub it on a piece of soft deal
wood, you will make it so hot as to scorch wood and paper,
and burn a match.
I am now going to show you that we can obtain heat, not
by chemical affinity alone, but by the pressure of air. Sup-
pose I take a pellet of cotton and moisten it with a
little ether, and put it into a glass tube (Fig. 31),
and then take a piston and press it down suddenly,
I expect I shall be able to burn a little of that
ether in the vessel. It wants a suddenness of pres-
sure, or we shall not do what we require. [The
piston was forcibly pressed down, when a flame, due
to the combustion of the ether, was visible in the
lower part of the syringe.] All we want is to get
a little ether in vapor, and give fresh air each
time, and so we may go on again and again, getting
heat enough by the compression of air to fire
the ether-vapor.
ig. 31 This, then, I think will be sufficient, accompanied
with all you have previously seen, to show you how we pro-
cure heat. And now for the effects of this power. We need
not consider many of them on the present occasion, because,
when you have seen its power of changing ice into water and
water into steam, you have seen the two principal results
of the application of heat. I want you now to see how it
expands all bodies — all bodies but one, and that under limited
circumstances. Mr. Anderson will hold a lamp under that
retort, and you will see, the moment he does so, that the
air will issue abundantly from the neck which is under
water, because the heat which he applies to the air causes
it to expand. And here is a brass rod (Fig. 32) which goes
through that hole, and fits also accurately into this gauge;
but if I make it warm with this spirit lamp, it will only
go in the gauge or through the hole with difficulty; and if
CHEMICAL AFFINITY 61
I were to put it into boiling water it would not go through at
all. Again, as soon as the heat escapes from bodies, they
Fig. 32
collapse: see how the air is contracting in the vessel now
that Mr. Anderson has taken away his lamp; the stem of it
is filling with water. Notice too, now, that although I can
not get the tube through this hole or into the gauge, the
moment I cool it, by dipping it into water, it goes through
with perfect facility, so that we have a perfect proof of
this power of heat to contract and expand bodies.
LECTURE V
MAGNETISM— ELECTRICITY
I WONDER whether we shall be too deep to-day or not.
Remember that we spoke of the attraction by gravitation
of all bodies to all bodies by their simple approach. Re-
member that we spoke of the attraction of particles of the
same kind to each other — that power which keeps them to-
gether in masses — iron attracted to iron, brass to brass, or
water to water. Remember that we found, on looking into
water, that there were particles of two different kinds at-
tracted to each other; and this was a great step beyond the
first simple attraction of gravitation, because here we deal
with attraction between different kinds of matter. The hy-
drogen could attract the oxygen and reduce it to water, but
it could not attract any of its own particles, so that there
we obtained a first indication of the existence of two
attractions.
To-day we come to a kind of attraction even more curious
than the last, namely, the attraction which we find to be of a
double nature — of a curious and dual nature. And I want,
first of all, to make the nature of this doubleness clear to
you. Bodies are sometimes endowed with a wonderful at-
traction, which is not found in them in their ordinary state.
For instance, here is a piece of shellac, having the attraction
of gravitation, having the attraction of cohesion, and if I
set fire to it, it would have the attraction of chemical affinity
to the oxygen in the atmosphere. Now all these powers we
find in it as if they were parts of its substance; but there is
another property which I will try and make evident by means
of this ball, this bubble of air [a light India-rubber ball,
inflated and suspended by a thread]. There is no attraction
between this ball and this shellac at present; there may be a
little wind in the rooms slightly moving the ball about, but
62
ELECTRICITY 63
there is no attraction. But if I rub the shellac with a piece
of flannel [rubbing the shellac, and then holding it near the
ball], look at the attraction which has arisen out of the shellac
simply by this friction, and which I may take away as easily
by drawing it gently through my hand. [The lecturer re-
peated the experiment of exciting the shellac, and then
removing the attractive power by drawing it through his
hand.] Again, you will see I can repeat this experiment
with another substance; for if I take a glass rod, and rub it
with a piece of silk covered with what we call amalgam, look
at the attraction which it has ; how it draws the ball toward
it; and then, as before, by quietly rubbing it through the
hand, the attraction will be all removed again, to come back
by friction with this silk.
But now we come to another fact. I will take this piece
of shellac, and make it attractive by friction; and remember
that, whenever we get an attraction of gravity, chemical
affinity, adhesion, or electricity (as in this case), the body
which attracts is attracted also, and just as much as that
ball was attracted by the shellac, the shellac was attracted
by the ball. Now I will suspend this piece of excited shellac
in a little paper stirrup, in this way (Fig. 33), in order to
make it move easily, and I will take another piece of shellac,
and, after rubbing it with flannel, will bring them near to-
gether : you will think that they ought to attract each other ;
but now what happens? It does not attract; on the con-
trary, it very strongly repels, and I can thus drive it round
64
FARADAY
to any extent. These, therefore, repel each other, although
they are so strongly attractive — repel each other to the ex-
tent of driving this heavy piece of shellac round and round
in this way. But if I excite this piece of shellac as before,
and take this piece of glass and rub it with silk, and then
bring them near, what think you will happen? [The lec-
turer held the excited glass near the excited shellac, when
they attracted each other strongly.] You see, therefore,
what a difference there is between these two attractions;
they are actually two kinds of attraction concerned in this
case, quite different to any thing we have met with before,
but the force is the same. We have here, then, a double
attraction — a dual attraction or force — one attracting and
the other repelling.
Again, to show you another experiment which will help to
make this clear to you : Suppose I set up this rough indicator
again [the excited shellac suspended in the stirrup] : it is
rough, but delicate enough for my purpose; and suppose I
take this other piece of shellac, and take away the power,
which I can do by drawing it gently through the hand; and
suppose I take a piece of flannel (Fig. 34), which I have
\*
■
Fig. 34
shaped into a cap for it and made dry. I will put this shellac
into the flannel, and here comes out a very beautiful result.
I will rub this shellac and the flannel together (which I can
do by twisting the shellac round), and leave them in contact;
ELECTRICITY 65
and then if I ask, by bringing them near our indicator, what
is the attractive force? it is nothing; but if I take them apart,
and then ask what will they do when they are separated?
why, the shellac is strongly repelled, as it was before, but
the cap is strongly attractive; and yet, if I bring them both
together again, there is no attraction; it has all disappeared
[the experiment was repeated]. Those two bodies, therefore,
still contain this attractive power ; when they were parted, it
was evident to your senses that they had it, though they do
not attract when they are together.
This, then, is sufficient, in the outset, to give you an idea of
the nature of the force which we call ELECTRICITY.
There is no end to the things from which you can evolve this
power. When you go home, take a stick of sealing-wax — I
have rather a large stick, but a smaller one will do — and
make an indicator of this sort (Fig. 35). Take a watch-
glass (or your watch itself will do; you only want some-
FlG. 35
thing which shall have a round face) ; and now, if you
place a piece of flat glass upon that, you have a very easily
moved centre; and if I take this lath and put it on the flat
glass (you see I am searching for the centre of gravity of
this lath; I want to balance it upon the watch-glass), it is
very easily moved round; and if I take this piece of sealing-
wax and rub it against my coat, and then try whether it is
attractive [holding it near the lath], you see how strong the
attraction is ; I can even draw it about. Here, then, you have
a very beautiful indicator, for I have, with a small piece of
sealing-wax and my coat, pulled round a plank of that kind,
so you need be in no want of indicators to discover the
presence of this attraction. There is scarcely a substance
which we may not use. Here are some indicators (Fig. 36).
I bend round a strip of paper into a hoop, and we have as
hc vol. xxx — c
66 FARADAY
good an indicator as can be required. See how it rolls along,
traveling after the sealing-wax! If I make them smaller, of
course we have them running faster, and sometimes they are
actually attracted up into the air. Here, also, is a little
collodion balloon. It is so electrical that it will scarcely leave
my hand unless to go to the
other. See how curiously elec-
trical it is ; it is hardly possible
for me to touch it without
making it electrical; and here is
a piece which clings to any
thing it is brought near, and
which it is not easy to lay down.
And here is another substance, gutta-percha, in thin strips:
it is astonishing how, by rubbing this in your hands, you
make it electrical ; but our time forbids us to go farther into
this subject at present; you see clearly there are two kinds of
electricities which may be obtained by rubbing shellac with
flannel or glass with silk.
Now there are some curious bodies in nature (of which I
have two specimens on the table) which are called magnets
or loadstones; ores of iron, of which there is a great deal
sent from Sweden. They have the attraction of gravitation,
and attraction of cohesion, and certain chemical attraction;
but they also have a great attractive power, for this little key
is held up by this stone. Now that is not chemical attraction ;
it is not the attraction of chemical affinity, or of aggregation
of particles, or of cohesion, or of electricity (for it will not
attract this ball if I bring it near it), but it is a separate and
dual attraction, and, what is more, one which is not readily
removed from the substance, for it has existed in it for ages
and ages in the bowels of the earth. Now we can make
artificial magnets (you will see me to-morrow make artificial
magnets of extraordinary power). And let us take one of
these artificial magnets and examine it, and see where the
power is in the mass, and whether it is a dual power. You
see it attracts these keys, two or three in succession, and it
will attract a very large piece of iron. That, then, is a very
different thing indeed to what you saw in the case of the
shellac, for that only attracted a light ball, but here I have
ELECTRICITY
67
several ounces of iron held up. And if we come to examine
this attraction a little more closely, we shall find it presents
some other remarkable differences; first of all, one end of
this bar (Fig. 37) attracts this key, but the middle does not
attract. It is not, then, the whole of the substance which
attracts. If I place this little key in the middle it does not
adhere; but if I place it there, a little nearer the end, it does,
though feebly. Is it not, then, very curious to find that
there is an attractive power at the extremities which is not
in the middle — to have thus in one bar two places in which
this force of attraction resides? If I take this bar and
balance it carefully on a point, so that it will be free to
move round, I can try what action this piece of iron has
on it. Well, it attracts one end, and it also attracts the
other end, just as you saw the shellac and the glass did, with
the exception of its not attracting in the middle. But if now,
Fig. 37
Fig. 38
instead of a piece of iron, I take a magnet, and examine it
in a similar way, you see that one of its ends repels the
suspended magnet; the force, then, is no longer attraction,
but repulsion; but, if I take the other end of the magnet
and bring it near, it shows attraction again.
You will see this better, perhaps, by another kind of ex-
periment. Here (Fig. 38) is a little magnet, and I have
colored the ends differently, so that you may distinguish one
from the other. Now this end (S) of the magnet (Fig. 37)
attracts the uncolored end of the little magnet. You see it
pulls toward it with great power ; and, as I carry it round,
the uncolored end still follows. But now, if I gradually bring
the middle of the bar magnet opposite the uncolored end of
the needle, it has no effect upon it, either of attraction or
repulsion, until, as I come to the opposite extremity (N), you
68 FARADAY
see that it is the colored end of the needle which is pulled
toward it. We are now, therefore, dealing with two kinds of
power, attracting different ends of the magnet — a double
power, already existing in these bodies, which takes up the
form of attraction and repulsion. And now, when I put up
this label with the word MAGNETISM, you will understand
that it is to express this double power.
Now with this loadstone you may make magnets arti-
ficially. Here is an artificial magnet (Fig. 39) in which both
ends have been brought together in order to increase the
attraction. This mass will lift that lump of iron,
and, what is more, by placing this keeper, as it is
called, on the top of the magnet, and taking hold
of the handle, it will adhere sufficiently strongly
to allow itself to be lifted up, so wonderful is its
power of attraction. If you take a needle, and
just draw one of its ends along one extremity of
the magnet, and then draw the other end along
the other extremity, and then gently place it on
the surface of some water (the needle will gen-
pp" - " erally float on the surface, owing to the slight
greasiness communicated to it by the fingers), you
will be able to get all the phenomena of attraction and
repulsion by bringing another magnetized needle near to it.
I want you now to observe that, although I have shown
you in these magnets that this double power becomes evident
principally at the extremities, yet the whole of the magnet is
concerned in giving the power. That will at first seem rather
strange; and I must therefore show you an experiment to
prove that this is not an accidental matter, but that the whole
of the mass is really concerned in this force, just as in fall-
ing the whole of the mass is acted upon by the force of
gravitation. I have here (Fig. 40) a steel bar, and I am
going to make it a magnet by rubbing ^
it on the large magnet (Fig. 39). I f »» »,»
have now made the two ends magnetic ~ ""'"" Fjg
in opposite ways. I do not at present
know one from the other, but we can soon find out. You see,
when I bring it near our magnetic needle (Fig. 38), one end
repels and the other attracts; and the middle will neither
ELECTRICITY 69
attract nor repel — it can not, because it is half way between
the two ends. But now, if I break out that piece (n, s), and
then examine it, see how strongly one end (n) pulls at this
end (S, Fig. 38), and how it repels the other end (N). And
so it can be shown that every part of the magnet contains
this power of attraction and repulsion, but that the power is
only rendered evident at the end of the mass. You will un-
derstand all this in a little while ; but what you have now to
consider is that every part of this steel is in itself a magnet.
Here is a little fragment which I have broken out of the very
centre of the bar, and you will still see that one end is at-
tractive and the other is repulsive. Now is not this power a
most wonderful thing? And very strange, the means of tak-
ing it from one substance and bringing it to other matters.
I can not make a piece of iron or any thing else heavier or
lighter than it is ; its cohesive power it must and does have ;
but, as you have seen by these experiments, we can add or
subtract this power of magnetism, and almost do as we like
with it.
And now we will return for a short time to the subject
treated of at the commencement of this lecture. You see
Fig. 41
here (Fig. 41) a large machine arranged for the purpose of
rubbing glass with silk, and for obtaining the power called
electricity; and the moment the handle of the machine is
turned a certain amount of electricity is evolved, as you will
see by the rise of the little straw indicator (at A). Now I
70 FARADAY
know, from the appearance of repulsion of the pith ball at
the end of the straw, that electricity is present in those brass
conductors (BB), and I want you to see the manner in
which that electricity can pass away [touching the conductor
(B) with his finger, the lecturer drew a spark from it, and
the straw electrometer immediately fell]. There, it has all
gone; and that I have really taken it away you shall see by
an experiment of this sort. If I hold this cylinder of brass
by the glass handle, and touch the conductor with it, I take
away a little of the electricity. You see the spark in which
it passes, and observe that the pith-ball indicator has fallen
a little, which seems to imply that so much electricity is lost ;
but it is not lost; it is here in this brass, and I can take it
away and carry it about, not because it has any substance of
its own, but by some strange property which we have not
before met with as belonging to any other force. Let us
see whether we have it here or not. [The lecturer brought
the charged cylinder to a jet from which gas was issuing;
the spark was seen to pass from the cylinder to the jet, but
the gas did not light.] Ah ! the gas did not light, but you
saw the spark; there is, perhaps, some draught in the room
which blew the gas on one side, or else it would light; we
will try this experiment afterward. You see from the spark
that I can transfer the power from the machine to this
cylinder, and then carry it away and give it to some other
body.
You know very well, as a matter of experiment, that we can
transfer the power of heat from one thing to another ; for if
I put my hand near the fire it becomes hot. I can show you
this by placing before us this ball, which has just been
brought red-hot from the fire. If I press this wire to it
some of the heat will be transferred from the ball, and I
have only now to touch this piece of gun-cotton with the
hot wire, and you see how I can transfer the heat from the
ball to the wire, and from the wire to the cotton. So you
see that some powers are transferable, and others are not.
Observe how long the heat stops in this ball. I might touch
it with the wire or with my finger, and if I did so quickly I
should merely burn the surface of the skin; whereas, if I
touch that cylinder, however rapidly, with my finger, the
ELECTRICITY 71
electricity is gone at once — dispersed on the instant, in a
manner wonderful to think of.
I must now take up a little of your time in showing you
the manner in which these powers are transferred from one
thing to another; for the manner in which force may be
conducted or transmitted is extraordinary, and most essential
for us to understand. Let us see in what manner these pow-
ers travel from place to place. Both heat and electricity
can be conducted; and here is an arrangement I have made
to show how the former can travel. It consists of a bar of
copper (Fig. 42) ; and if I take a spirit lamp (this is one way
Fig. 42
of obtaining the power of heat) and place it under that little
chimney, the flame will strike against the bar of copper
and keep it hot. Now you are aware that power is being
transferred .from the flame of that lamp to the copper, and
you will see by-and-by that it is being conducted along the
copper from particle to particle; for, inasmuch as I have
fastened these wooden balls by a little wax at particular dis-
tances from the point where the copper is first heated, first
one ball will fall and then the more distant ones, as the heat
travels along, and thus you will learn that the heat travels
gradually through the copper. You will see that this is a
very slow conduction of power as compared with electricity.
If I take cylinders of wood and metal, joined together at the
ends, and wrap a piece of paper round, and then apply the
heat of this lamp to the place where the metal and wood
join, you will see how the heat will accumulate where the
wood is, and burn the paper with which I have covered it;
but where the metal is beneath, the heat is conducted away
too fast for the paper to be burned. And so, if I take a
72 FARADAY
piece of wood and a piece of metal joined together, and put
it so that the flame shall play equally both upon one and the
other, we shall soon find that the metal will become hot
before the wood; for if I put a piece of phosphorus on the
wood and another piece on the copper, you will find that the
phosphorus on the copper will take fire before that on the
wood is melted; and this shows you how badly the wood
conducts heat. But with regard to the traveling of elec-
tricity from place to place, its rapidity is astonishing. I will,
first of all, take these pieces of glass and metal, and you will
soon understand how it is that the glass does not lose the
power which it acquired when it is rubbed by the silk; by
one or two experiments I will show you. If I take this
piece of brass and bring it near the machine, you see how
the electricity leaves the latter and passes to the brass cyl-
inder. And again: if I take a rod of metal and touch the
machine with it, I lower the indicator; but when I touch it
with a rod of glass, no power is drawn away, showing you
that the electricity is conducted by the glass and the metal
in a manner entirely different; and, to make you see that
more clearly, we will take one of our Leyden jars. Now I
must not embarrass your minds with this subject too much,
but if I take a piece of metal and bring it against the knob
at the top and the metallic coating at the bottom, you will
see the electricity passing through the air as a brilliant spark.
It takes no sensible time to pass through this ; and if I were
to take a long metallic wire, no matter what the length, at
least as far as we are concerned, and if I make one end of it
touch the outside, and the other touch the knob at the top,
see how the electricity passes ! It has flashed instantaneously
through the whole length of this wire. Is not this different
from the transmission of heat through this copper bar
(Fig. 42) which has taken a quarter of an hour or more to
reach the first ball?
Here is another experiment for the purpose of showing
the conductibility of this power through some bodies and not
through others. Why do I have this arrangement made of
brass ? [pointing to the brass work of the electrical machine,
Fig. 41.] Because it conducts electricity. And why do I
have these columns made of glass? Because they obstruct the
ELECTRICITY
73
passage of electricity. And why do I put that paper tassel
(Fig. 43) at the top of the pole, upon a glass rod, and con-
nect it with this machine by means of a wire? You see at
once that as soon as the handle
of the machine is turned, the
electricity which is evolved
travels along this wire and up
the wooden rod, and goes to
the tassel at the top, and you
see the power of repulsion with
which it has endowed these
strips of paper, each spreading
outward to the ceiling and sides
of the room. The outside of
that wire is covered with gutta-
percha; it would not serve to
keep the force from you when
touching it with your hands,
because it would burst through;
but it answers our purpose for
the present. And so you per-
ceive how easily I can manage
to send this power of electricity
from place to place by choosing
the materials which can conduct
the power. Suppose I want to
fire a portion of gunpowder, I
can readily do it by this trans-
ferable power of electricity. I IG " 43
will take a Leyden jar, or any other arrangement which
gives us this power, and arrange wires so that they may
carry the power to the place I wish ; and then placing a little
gunpowder on the extremities of the wires, the moment I
make the connection by this discharging rod I shall fire
the gunpowder [the connection was made and the gun-
powder ignited]. And if I were to show you a stool like
this, and were to explain to you its construction, you
could easily understand that we use glass legs because
these are capable of preventing the electricity from going
away to the earth. If, therefore, I were to stand on
74 FARADAY
this stool, and receive the electricity through this conductor,
I could give it to anything that I touched. [The lecturer
stood upon the insulating stool, and placed himself in con-
nection with the conductor of the machine.] Now I am
electrified; I can feel my hair rising up, as the paper tassel
did just now. Let us see whether I can succeed in lighting
gas by touching the jet with my finger. [The lecturer
brought his finger near a jet from which gas was issuing,
when, after one or two attempts, the spark which came from
his finger to the jet set fire to the gas.] You now see how
it is that this power of electricity can be transferred from
the matter in which it is generated, and conducted along
wires and other bodies, and thus be made to serve new pur-
poses, utterly unattainable by the powers we have spoken
of on previous days; and you will not now be at a loss to
bring this power of electricity into comparison with those
which we have previously examined, and to-morrow we
shall be able to go farther into the consideration of these
transferable powers.
LECTURE VI
THE CORRELATION OF THE PHYSICAL FORCES
WE have frequently seen, during the course of
these lectures, that one of those powers or forces
of matter, of which I have written the names on
that board, has produced results which are due to the action
of some other force. Thus you have seen the force of
electricity acting in other ways than in attracting; you have
also seen it combine matters together or disunite them by
means of its action on the chemical force; and in this case,
therefore, you have an instance in which these two powers
are related. But we have other and deeper relations than
these; we have not merely to see how it is that one power
affects another — how the force of heat affects chem-
ical affinity, and so forth, but we must try and compre-
hend what relation they bear to each other, and how these
powers may be changed one into the other; and it will
to-day require all my care, and your care too, to make this
clear to your minds. I shall be obliged to confine myself to
one or two instances, because to take in the whole extent
of this mutual relation and conversion of forces would sur-
pass the human intellect.
In the first place, then, here is a piece of fine zinc foil,
and if I cut it into narrow strips and apply to it the power
of heat, admitting the contact of air at the same time, you
will find that it burns; and then, seeing that it burns, you
will be prepared to say that there is chemical action taking
place. You see all I have to do is to hold the piece of zinc
at the side of the flame, so as to let it get heated, and yet
to allow the air which is flowing into the flame from all
sides to have access to it; there is the piece of zinc burning
just like a piece of wood, only brighter. A part of the zinc
is going up into the air in the form of that white smoke, and
75
76 FARADAY
part is falling down on to the table. This, then, is the
action of chemical affinity exerted between the zinc and the
oxygen of the air. I will show you what a curious kind of
affinity this is by an experiment which is rather striking
when seen for the first time. I have here some iron filings
and gunpowder, and will mix them carefully together, with
as little rough handling as possible; now we will compare
the combustibility, so to speak, of the two. I will pour some
spirit of wine into a basin and set it on fire; and, having
our flame, I will drop this mixture of iron filings and gun-
powder through it, so that both sets of particles will have
an equal chance of burning. And now tell me which of
them it is that burns? You see a plentiful combustion of
the iron filings ; but I want you to observe that, though they
have equal chances of burning, we shall find that by far the
greater part of the gunpowder remains untouched; I have
only to drain off this spirit of wine, and let the powder
which has gone through the flame dry, which it will do in
a few minutes, and I will then test it with a lighted match.
So ready is the iron to burn, that it takes, under certain
circumstances, even less time to catch fire than gunpowder.
[As soon as the gunpowder was dry, Mr. Anderson handed
it to the lecturer, who applied a lighted match to it, when
a sudden flash showed how large a proportion of gunpowder
had escaped combustion when falling through the flame of
alcohol.]
These are all cases of chemical affinity, and I show them
to make you understand that we are about to enter upon
the consideration of a strange kind of chemical affinity, and
then to see how far we are enabled to convert this force of
affinity into electricity or magnetism, or any other of the
forces which we have discussed. Here is some zinc (I keep
to the metal zinc, as it is very useful for our purpose), and
I can produce hydrogen gas by putting the zinc and sulphuric
acid together, as they are in that retort; there you see the
mixture which gives us hydrogen — the zinc is pulling the
water to pieces and setting free hydrogen gas. Now we
have learned by experience that if a little mercury is spread
over that zinc, it does not take away its power of decom-
posing the water, but modifies it most curiously. See how
THE CORRELATION OF THE PHYSICAL FORCES 77
that mixture is now boiling; but when I add a little mercury
to it the gas ceases to come off. We have now scarcely a
bubble of hydrogen set free, so that the action is suspended
for the time. We have not destroyed the power of chemical
affinity, but modified it in a wonderful and beautiful manner.
Here are some pieces of zinc covered with mercury exactly
in the same way as the zinc in that retort is covered ; and if
I put this plate into sulphuric acid I get no gas, but this
most extraordinary thing occurs, that if I introduce along
with the zinc another metal which is not so combustible, then
I reproduce all the action. I am now going to put to the
amalgamated zinc in this retort some portions of copper wire
(copper not being so combustible a metal as the zinc), and
observe how I get hydrogen again, as in the first instance;
there, the bubbles are coming over through the pneumatic
trough, and ascending faster and faster in the jar; the zinc
now is acting by reason of its contact with the copper.
Every step we are now taking brings us to a knowledge of
new phenomena. That hydrogen which you now see coming
off so abundantly does not come from the zinc, as it did be-
fore, but from the copper. Here is a jar containing a solu-
tion of copper. If I put a piece of this amalgamated zinc
into it, and leave it there, it has scarcely any action ; and here
is a plate of platinum which I will immerse in the same
solution, and might leave it there for hours, days, months,
or even years, and no action would take place; but, by
putting them both together, and allowing them to touch
(Fig. 44), you see what a coating of copper there is im-
mediately thrown down on the platinum. Why is this ? The
platinum has no power of itself to reduce that metal from
that fluid, but it has, in some mysterious way, received this
power by its contact with the metal zinc. Here, then, you see
a strange transfer of chemical force from one metal to an-
other; the chemical force from the zinc is transferred and
made over to the platinum by the mere association of the
two metals. I might take, instead of the platinum, a piece
of copper or of silver, and it would have no action of its
own on this solution, but the moment the zinc was introduced
and touched the other metal, then the action would take
place, and it would become covered with copper. Now is
78
FARADAY
not this most wonderful and beautiful to see? We still have
the identical chemical force of the particles of zinc acting,
and yet, in some strange manner, we have power to make
that chemical force, or something it produces, travel from
Fig. 44
Fig. 45
one place to another; for we do make the chemical force
travel from the zinc to the platinum by this very curious
experiment of using the two metals in the same fluid in
contact with each other.
Let us now examine these phenomena a little more closely.
Here is a drawing (Fig. 45) in which I have represented a
vessel containing the acid liquid and the slips of zinc and
platinum or copper, and I have shown them touching each
other outside by means of a wire coming from each of
them (for it matters not whether they touch in the fluid or
outside; by pieces of metal attached, they still, by that
communication between them, have this power transferred
from one to the other). Now if, instead of only using one
vessel, as I have shown there, I take another, and another,
and put in zinc and platinum, zinc and platinum, zinc and
platinum, and connect the platinum of one vessel with
the zinc of another, the platinum of this vessel with the zinc
of that, and so on, we should only be using a series of these
vessels instead of one. This we have done in that arrange-
ment which you see behind me. I am using what we call a
Grove's voltaic battery, in which one metal is zinc and the
other platinum; and I have as many as forty pairs of these
plates all exercising their force at once in sending the whole
amount of chemical power there evolved through these wires
under the floor and up to these two rods coming through the
THE CORRELATION OF THE PHYSICAL FORCES 79
table. We need do no more than just bring these two ends
in contact, when the spark shows us what power is present;
and what a strange thing it is to see that this force is
brought away from the battery behind me, and carried along
through these wires ! I have here an apparatus (Fig. 46)
which Sir Humphrey Davy constructed many years ago, in
order to see whether this power from
the voltaic battery caused bodies to
attract each other in the same manner
as the ordinary electricity did. He
made it in order to experiment with
his large voltaic battery, which was
the most powerful then in existence.
You see there are in this glass jar
two leaves of gold, which I can cause
to move to and fro by this rack-work.
I will connect each of these gold
leaves with separate ends of this
battery, and if I have a sufficient
number of plates in the battery, I
shall be able to show you that there IG * 4
will be some attraction between those leaves even before,
they come in contact; if I bring them sufficiently near
when they are in communication with the ends of the battery,
they will be drawn gently together ; and you will know when
this takes place, because the power will cause the gold leaves
to burn away, which they could only do when they touched
each other. Now I am going to cause these two leaves of
gold to approach gradually, and I have no doubt that some
of you will see that they approach before they burn, and
those who are too far off to see them approach will see by
their burning that they have come together. Now they are
attracting each other, long before the connection is complete,
and there they go ! burnt up in that brilliant flash, so strong
is the force. You thus see, from the attractive force at the
two ends of this battery that these are really and truly
electrical phenomena.
Now let us consider what is this spark. I take these
two ends and bring them together, and there I get this
glorious spark like the sunlight in the heavens above us.
80 FARADAY
What is this? It is the same thing which you saw when
I discharged the large electrical machine, when you saw one
single bright flash; it is the same thing, only continued, be-
cause here we have a more effective arrangement. Instead
of having a machine which we are obliged to turn for a long
time together, we have here a chemical power which sends
forth the spark; and it is wonderful and beautiful to see
how this spark is carried about through these wires. I want
you to perceive, if possible, that this very spark and the heat
it produces (for there is heat) is neither more nor less than
the chemical force of the zinc — its very force carried along
wires and conveyed to this place. I am about to take a
portion of the zinc and burn it in oxygen gas for the sake
of showing you the kind of light produced by the actual
combustion in oxygen gas of some of this metal. [A
tassel of zinc-foil was ignited at a spirit lamp and intro-
duced into a jar of oxygen, when it burnt with a brilliant
light.] That shows you what the affinity is when we come
to consider it in its energy and power. And the zinc is
being burned in the battery behind me at a much more
rapid rate than you see in that jar, because the zinc is there
dissolving and burning, and produces here this great electric
light. That very same power which in that jar you saw
evolved from the actual combustion of the zinc in oxygen, is
carried along these wires and made evident here; and you
may, if you please, consider that the zinc is burning in those
cells, and that this is the light of that burning [bringing
the two poles in contact and showing the electric light] ;
and we might so arrange our apparatus as to show that
the amounts of power evolved in either case are identical.
Having thus obtained power over the chemical force, how
wonderfully we are able to convey it from place to place !
When we use gunpowder for explosive purposes, we can
send into the mine chemical affinity by means of this elec-
tricity; not having provided fire beforehand, we can send
it in at the moment we require it. Now here (Fig. 47) is
a vessel containing two charcoal points, and I bring it
forward as an illustration of the wonderful power of con-
veying this force from place to place. I have merely to
connect these by means of wires to the opposite ends of the
THE CORRELATION OF THE PHYSICAL FORCES 81
battery, and bring the points in contact. See what an
exhibition of force we have ! We have exhausted the air
so that the charcoal can not burn, and therefore the light
you see is really the burning of the zinc in the cells behind
me; there is no disappearance of the
carbon, although we have that glori-
ous electric light; and the moment
I cut off the connection it stops.
Here is a better instance to enable
some of you to see the certainty
with which we can convey this
force, where under ordinary circum-
stances, chemical affinity would not
act. We may absolutely take these
two charcoal poles down under
water, and get our electric light
there. There they are in the wa-
ter, and you observe, when I bring
them into connection, we have the
same light as we had in that glass
vessel.
Now besides this production of
light, we have all the other effects and powers of burning zinc.
I. have a few wires here which are not combustible, and I am
going to take one of them, a small platinum wire, and suspend
it between these two rods which are connected with the
Fig. 47
Fig. 48
battery, and when contact is made at the battery see what
heat we get (Fig. 48). Is not that beautiful? It is a
complete bridge of power. There is metallic connection
82 FARADAY
all the way round in this arrangement, and where I have
inserted the platinum, which offers some resistance to the
passage of the force, you see what an amount of heat is
evolved; this is the heat which the zinc would give if
burnt in oxygen; but, as it is being burnt in the voltaic
battery, it is giving it out at this spot. I will now shorten
this wire for the sake of showing you that, the shorter the
obstructing wire is, the more and more intense is the heat,
until at last our platinum is fused and falls down, breaking
off the circuit.
Here is another instance. I will take a piece of the
metal silver, and place it on charcoal connected with one
end of the battery, and lower the other charcoal pole on
to it. See how brilliantly it burns! (Fig. 49.) Here is a
Fig. 49
piece of iron on the charcoal: see what a combustion is
going on ; and we might go on in this way, burning almost
every thing we place between the poles. Now I want to
show you that this power is still chemical affinity ; that if
we call the power which is evolved at this point heat, or
electricity, or any other name referring to its source, or
the way in which it travels, we still shall find it to be
chemical action. Here is a colored liquid which can show
by its change of color the effects of chemical action ; I
will pour part of it into this glass, and you will find that
these wires have a very strong action. I am not going
to show you any effects of combustion or heat, but I will
take these two platinum plates, and fasten one to the one
pole and the other to the other end, and place them in this
solution, and in a very short time you will see the blue
color will be entirely destroyed. See, it is colorless now !
I have merely brought the end of the wires into the solu-
THE CORRELATION OF THE PHYSICAL FORCES 83
tion of indigo, and the power of electricity has come
through these wires and made itself evident by its chem-
ical action. There is also another curious thing to be
noticed now we are dealing with the chemistry of electricity,
which is, that the chemical power which destroys the color
is only due to the action on one side. I will pour some
more of this sulphindigotic acid( 22 ) into a flat dish, and
will then make a porous dike of sand separating the two
portions of fluid into two parts (Fig. 50), and now we shall
be able to see whether there is any difference in the two
ends of the battery, and which it is that possesses this
peculiar action. You see it is the one on my right hand which
has the power of destroying the
blue, for the portion on that side is
thoroughly bleached, while nothing
has apparently occurred on the
other side. I say apparently, for
you must not imagine that because
you can not perceive any action
none has taken place. IG * 5 °
Here we have another instance of chemical action. I
take these platinum plates again and immerse them in
this solution of copper, from which we formerly precipi-
tated some of the metal, when the platinum and zinc were
both put in it together. You see that these two platinum
plates have no chemical action of any kind; they might
remain in the solution as long as I liked, without having
any power of themselves to reduce the copper; but the
moment I bring the two poles of the battery in contact
with them, the chemical action which is there transformed
into electricity and carried along the wires again becomes
chemical action at the two platinum poles, and now we
shall have the power appearing on the left-hand side, and
throwing down the copper in the metallic state on the
platinum plate; and in this way I might give you many
instances of the extraordinary way in which this chemical
22 Sulphindigotic acid. A mixture of one part of indigo and fifteen parts
of concentrated oil of vitriol. It is bleached on the side at which hydrogen
gas is evolved in consequence of the liberated hydrogen withdrawing oxygen
from the indigo, thereby forming a colorless deoxidized indigo. In making
the experiment, only enough of the sulphindigotic acid must be added to
give the water a decided blue color.
84 FARADAY
action or electricity may be carried about. That strange
nugget of gold, of which there is a model in the other room,
and which has an interest of its own in the natural history
of gold, and which came from Ballarat, and was worth
£8,000 or £9,000 when it was melted down last November,
was brought together in the bowels of the earth, perhaps
ages and ages ago, by some such power as this. And
there is also another beautiful result dependent upon chem-
ical affinity in that fine lead-tree ( 23 ), the lead growing
and growing by virtue of this power. The lead and the
zinc are combined together in a little voltaic arrangement
in a manner far more important than the powerful one
you see here, because in nature these minute actions are
going on forever, and are of great and wonderful im-
portance in the precipitation of metals and formation of
mineral veins, and so forth. These actions are not for a
limited time, like my battery here, but they act forever in
small degrees, accumulating more and more of the results.
I have here given you all .the illustrations that time will
permit me to show you of chemical affinity producing
electricity, and electricity again becoming chemical affinity.
Let that suffice for the present; and now let us go a little
deeper into the subject of this chemical force, or this elec-
tricity — which shall I name first? — the one producing the
other in a variety of ways. These forces are also won-
derful in their power of producing another of the forces
we have been considering, namely, that of magnetism; and
you know that it is only of late years, and long since I was
born, that the discovery of the relations of these two forces
of electricity and chemical affinity to produce magnetism
have become known. Philosophers had been suspecting this
affinity for a long time, and had long had great hopes of
success; for in the pursuit of science we first start with
hopes and expectations ; these we realize and establish,
never again to be lost, and upon them we found new ex-
23 Lead tree. To make a lead tree, pass a bundle of brass wires through
the cork of a bottle, and fasten a plate of zinc round them just as they
issue from the cork, so that the zinc may be in contact with every one of
the wires. Make the wires to diverge so as to form a sort of cone, and,
having filled the bottle quite full of a solution of sugar of lead, insert the
wires and cork, and seal it down, so as to perfectly exclude the air. In
a short time the metallic lead will begin to crystallize around the divergent
wires, and form a beautiful object.
THE CORRELATION OF THE PHYSICAL FORCES 85
pectations of farther discoveries, and so go on pursuing, real-
izing, establishing, and founding new hopes again and
again.
Now observe this : here is a piece of wire which I am
about to make into a bridge of force, that is to say, a com-
municator between the two ends of the battery. It is
copper wire only, and is therefore not magnetic of itself.
We will examine this wire with our magnetic needle (Fig.
51), and, though connected with one extreme end of the
battery, you see that before the circuit is completed it has
Fig. 51
no power over the magnet. But observe it when I make
contact; watch the needle; see how it is swung round; and
notice how indifferent it becomes if I break contact again;
so, you see, we have this wire evidently affecting the
magnetic needle under these circumstances. Let me show
you that a little more strongly. I have here a quantity of
wire which has been wound into a spiral, and this will affect
the magnetic needle in a very curious manner, because,
owing to its shape, it will act very like a real magnet. The
copper spiral has no power over that magnetic needle at
present ; but if I cause the electric current to circulate through
it, by bringing the two ends of the battery in contact with
the ends of the wire which forms the spiral, what will
happen? Why, one end of the needle is most powerfully
drawn to it; and if I take the other end of the needle, it
is repelled; so, you see, I have produced exactly the same
phenomena as I had with the bar magnet, one end attracting
and the other repelling. Is not this, then, curious to
see, that we can construct a magnet of copper? Further-
more, if I take an iron bar, and put it inside the coil, so
long as there is no electric current circulating round, it has
86
FARADAY
no attraction, as you will observe if I bring a little iron
filings or nails near the iron. But now, if I make contact
with the battery, they are attracted at once. It becomes
at once a powerful magnet, so much so that I should not
wonder if these mag-
netic needles on dif-
ferent parts of the
table pointed to it.
And I will show you,
by another experi-
ment, what an attrac-
tion it has. This piece and that piece of iron,
and many other pieces, are now strongly attracted
(Fig. 52) ; but, as soon as I break contact, the
power is all gone, and they fall. What, then, can
IG ' 52 be a better or a stronger proof than this of the re-
lation of the powers of magnetism and electricity? Again:
here is a little piece of iron which is not magnetized. It will
not, at present, take up any one of these nails ; but I will take
a piece of wire and coil it round the iron (the wire being
covered with cotton in every part
it does not touch the iron), so that
the current must go round in this
spiral coil; I am, in fact, prepar-
ing an electro-magnet (we are
obliged to use such terms to ex-
press our meaning, because it is a
magnet made by electricity — be-
cause we produce by the force of
electricity a magnet of far greater
power than a permanent steel one).
It is now completed, and I will re-
peat the experiment which you saw
the other day, of building up
a bridge of iron nails: the con- FlG# S3
tact is now made and the current is going through ; it is now
a powerful magnet ; here are the iron nails which we had the
other day, and now I have brought this magnet near them
they are clinging so hard that I can scarcely move them with
my hand (Fig. 53). But when the contact is broken, see how
THE CORRELATION OF THE PHYSICAL FORCES 87
they fall. What can show you better than such an ex-
periment as this the magnetic attraction with which we have
endowed these portions of iron? Here, again, is a fine
illustration of this strong power of magnetism. It is a mag-
net of the same sort as the one you have just seen. I am
about to make the current of electricity pass through the wires
which are round this iron for the purpose of showing you
what powerful effects we get. Here are the poles of the
magnet; and let us place on one of them this long bar
of iron. You see, as soon as contact is made, how it rises in
position (Fig. 54) ; and if I take such a piece as this cylinder,
Fig. 54
and place it on, woe be to me if I get my finger between; I
can roll it over, but if I try to pull it off, I might lift up the
whole magnet, but I have no power to overcome the magnetic
power which is here evident. I might give you an infinity of
illustrations of this high magnetic power. There is that long
bar of iron held out, and I have no doubt that if I were to ex-
amine the other end I should find that it was a magnet. See
what power it must have to support not only these nails, but
all those lumps of iron hanging on to the end. What, then,
can surpass these evidences of the change of chemical force
into electricity, and electricity into magnetism ? I might show
you many other experiments whereby I could obtain elec-
tricity and chemical action, heat and light from a magnet, but
what more need I show you to prove the universal cor-
relation of the physical forces of matter, and their mutual
conversion one into another?
88 FARADAY
And now let us give place as juveniles to the respect we
owe to our elders, and for a time let me address myself to
those of our seniors who have honored me with their pres-
ence during these lectures. I wish to claim this moment
for the purpose of tendering our thanks to them, and my
thanks to you all for the way in which you have borne the
inconvenience that I at first subjected you to. I hope that
the insight which you have here gained into some of the
laws by which the universe is governed, may be the occasion
of some among you turning your attention to these subjects;
for what study is there more fitted to the mind of man than
that of the physical sciences? And what is there more capa-
ble of giving him an insight into the actions of those laws, a
knowledge of which gives interest to the most trifling phe-
nomenon of nature, and makes the observing student find
" Tongues in trees, books in the running brooks,
Sermons in stones, and good in every thing?"
THE CHEMICAL HISTORY
OF A CANDLE
LECTURE I
A CANDLE: THE FLAME— ITS SOURCES— STRUC-
TURE— MOBILITY— BRIGHTNESS
I PURPOSE, in return for the honor you do us by coming
to see what are our proceedings here, to bring before
you, in the course of these lectures, the Chemical His-
tory of a Candle. I have taken this subject on a former
occasion, and, were it left to my own will, I should prefer
to repeat it almost every year, so abundant is the interest that
attaches itself to the subject, so wonderful are the varieties
of outlet which it offers into the various departments of phi-
losophy. There is not a law under which any part of this
universe is governed which does not come into play and is
touched upon in these phenomena. There is no better, there
is no more open door by which you can enter into the study
of natural philosophy than by considering the physical phe-
nomena of a candle. I trust, therefore, I shall not disappoint
you in choosing this for my subject rather than any newer
topic, which could not be better, were it even so good.
And, before proceeding, let me say this also : that, though
our subject be so great, and our intention that of treating it
honestly, seriously, and philosophically, yet I mean to pass
away from all those who are seniors among us. I claim the
privilege of speaking to juveniles as a juvenile myself. I
have done so on former occasions, and, if you please, I shall
do so again. And, though I stand here with the knowledge
of having the words I utter given to the world, yet that shall
not deter me from speaking in the same familiar way to
those whom I esteem nearest to me on this occasion.
89
90 FARADAY
And now, my boys and girls, I must first tell you of what
candles are made. Some are great curiosities. I have here
some bits of timber, branches of trees particularly famous
for their burning. And here you see a piece of that very
curious substance, taken out of some of the bogs in Ireland,
called candle-wood; a hard, strong, excellent wood, evidently
fitted for good work as a register of force, and yet, withal,
burning so well that where it is found they make splinters
of it, and torches, since it burns like a candle, and gives a
very good light indeed. And in this wood we have one of
the most beautiful illustrations of the general nature of a
candle that I can possibly give. The fuel provided, the
means of bringing that fuel to the place of chemical action,
the regular and gradual supply of air to that place of action
— heat and light — all produced by a little piece of wood of
this kind, forming, in fact, a natural candle.
But we must speak of candles as they are in commerce.
Here are a couple of candles commonly called dips. They
are made of lengths of cotton cut off, hung up by a loop,
dipped into melted tallow, taken out again and cooled, then
redipped, until there is an accumulation of tallow round the
cotton. In order that you may have an idea of the various
characters of these candles, you see these which I hold in
my hand — they are very small and very curious. They are,
or were, the candles used by the miners in coal mines. In
olden times the miner had to find his own candles, and it
was supposed that a small candle would not so soon set fire
to the fire-damp in the coal mines as a large one; and for
that reason, as well as for economy's sake, he had candles
made of this sort — 20, 30, 40, or 60 to the pound. They have
been replaced since then by the steel-mill, and then by the
Davy lamp, and other safety lamps of various kinds. I have
here a candle that was taken out of the Royal George, (*)
it is said, by Colonel Pasley. It has been sunk in the sea
for many years, subject to the action of salt water. It shows
you how well candles may be preserved; for, though it is
1 The Royal George sunk at Spithead on the 29th of August, 1782. Colonel
Pasley commenced operations for the removal of the wreck by the explosion
of gunpowder, in August, 1839. The candle which Professor Faraday ex-
hibited must therefore have been exposed to the action of salt water for
upward of fifty-seven years.
CHEMICAL HISTORY OF A CANDLE 91
cracked about and broken a great deal, yet when lighted it
goes on burning regularly, and the tallow resumes its natural
condition as soon as it is fused.
Mr. Field, of Lambeth, has supplied me abundantly with
beautiful illustrations of the candle and its materials; I
shall therefore now refer to them. And, first, there is the
suet — the fat of the ox — Russian tallow, I believe, employed
in the manufacture of these dips, which Gay-Lussac, or some
one who intrusted him with his knowledge, converted into
that beautiful substance, stearin, which you see lying beside
it. A candle, you know, is not now a greasy thing like an
ordinary tallow candle, but a clean thing, and you may almost
scrape off and pulverize the drops which fall from it with-
out soiling any thing. This is the process he adopted :( 2 )
The fat or tallow is first boiled with quick-lime, and made
into a soap, and then the soap is decomposed by sulphuric
acid, which takes away the lime, and leaves the fat rear-
ranged as stearic acid, while a quantity of glycerin is pro-
duced at the same time. Glycerin — absolutely a sugar, or a
substance similar to sugar — comes out of the tallow in this
chemical change. The oil is then pressed out of it; and you
see here this series of pressed cakes, showing how beautifully
the impurities are carried out by the oil w part as the pressure
goes on increasing, and at last you have left that substance,
which is melted, and cast into candles as here represented.
The candle I have in my hand is a stearin candle, made of
stearin from tallow in the way I have told you. Then here
is a sperm candle, which comes from the purified oil of the
spermaceti whale. Here, also, are yellow bee's-wax and re-
fined bee's-wax, from which candles are made. Here, too, is
that curious substance called paraffine, and some paraffine
candles, made of paraffine obtained from the bogs of Ireland.
2 The fat or tallow consists of a chemical combination of fatty acids with
glycerin. The lime unites with the palmitic, oleic, and stearic acids, and
separates the glycerin. After washing, the insoluble lime soap is decom-
posed with hot dilute sulphuric acid. The melted fatty acids thus rise as
an oil to the surface, when they are decanted. They are again washed and
cast into thin plates, which, when cold, are placed between layers of cocoa-
nut matting and submitted to intense hydraulic pressure. In this way the
soft oleic acid is squeezed out, while the hard palmitic and stearic acids
remain. These are farther purified by pressure at a higher temperature and
washing in warm dilute sulphuric acid, when they are ready to be made into
candles. These acids are harder and whiter than the fats from which they
were obtained, while at the same time they are cleaner and more combustible.
92 FARADAY
I have here also a substance brought from Japan since we
have forced an entrance into that out-of-the-way place — a
sort of wax which a kind friend has sent me, and which
forms a new material for the manufacture of candles.
And how are these candles made? I have told you about
dips, and I will show you how moulds are made. Let us
imagine any of these candles to be made of materials which
can be cast. " Cast ! " you say. " Why, a candle is a thing
that melts, and surely if you can melt it you can cast it."
Not so. It is wonderful, in the progress of manufacture,
and in the consideration of the means best fitted to produce
the required result, how things turn up which one would
not expect beforehand. Candles can not always be cast. A
wax candle can never be cast. It is made by a particular
process which I can illustrate in a minute or two, but I must
not spend much time on it. Wax is a thing which, burning
so well, and melting so easily in a candle, can not be cast.
However, let us take a material that can be cast. Here is
a frame, with a number of moulds fastened in it. The first
thing to be done is to put a wick through them. Here is
one — a plaited wick, which does not require snuffing ( 3 ) —
supported by a little wire. It goes to the bottom, where it is
pegged in ; the little peg holding the cotton tight, and stopping
the aperture so that nothing fluid shall run out. At the
upper part there is a little bar placed across, which stretches
the cotton and holds it in the mould. The tallow is then
melted, and the moulds are filled. After a certain time, when
the moulds are cool, the excess of tallow is poured off at one
corner, and then cleaned off altogether, and the ends of the
wick cut away. The candles alone then remain in the mould,
and you have only to upset them, as I am doing, when out
they tumble, for the candles are made in the form of cones,
being narrower at the top than at the bottom ; so that, what
with their form and their own shrinking, they only need a
little shaking and out they fall. In the same way are made
these candles of stearin and of paraffine. It is a curious
thing to see how wax candles are made. A lot of cottons are
hung upon frames, as you see here, and covered with metal
3 A little borax or phosphorus salt is sometimes added in order to make
the ash fusible.
CHEMICAL HISTORY OF A CANDLE 93
tags at the ends to keep the wax from covering the cotton
in those places. These are carried to a heater, where the
wax is melted. As you see, the frames can turn round ; and,
as they turn, a man takes a vessel of wax and pours it first
down one, and then the next, and the next, and so on. When
he has gone once round, if it is sufficiently cool, he gives the
first a second coat, and so on until they are all of the required
thickness. When they have been thus clothed, or fed, or
made up to that thickness, they are taken off and placed else-
where. I have here, by the kindness of Mr. Field, several
specimens of these candles. Here is one only half finished.
They are then taken down and well rolled upon a fine stone
slab, and the conical top is moulded by properly shaped tubes,
and the bottoms cut off and trimmed. This is done so beau-
tifully that they can make candles in this way weighing
exactly four or six to the pound, or any number they please.
We must not, however, take up more time about the mere
manufacture, but go a little farther into the matter. I have
not yet referred you to luxuries in candles (for there is such
a thing as luxury in candles). See how beautifully these
are colored ; you see here mauve, magenta, and all the chemi-
cal colors recently introduced, applied to candles. You
observe, also, different forms employed. Here is a fluted
pillar most beautifully shaped; and I have also here some
candles sent me by Mr. Pearsall, which are ornamented with
designs upon them, so that, as they burn, you have, as it
were, a glowing sun above, and a bouquet of flowers be-
neath. All, however, that is fine and beautiful is not useful.
These fluted candles, pretty as they are, are bad candles;
they are bad because of their external shape. Nevertheless,
I show you these specimens, sent to me from kind friends on
all sides, that you may see what is done and what may be
done in this or that direction ; although, as I have said, when
we come to these refinements, we are obliged to sacrifice a
little in utility.
Now as to the light of the candle. We will light one or
two, and set them at work in the performance of their proper
functions. You observe a candle is a very different thing
from a lamp. With a lamp you take a little oil, fill your ves-
sel, put in a little moss or some cotton prepared by artificial
94 FARADAY
means, and then light the top of the wick. When the flame
runs down the cotton to the oil, it gets extinguished, but it
goes on burning in the part above. Now I have no doubt
you will ask how it is that the oil which will not burn of
itself gets up to the top of the cotton, where it will burn. We
shall presently examine that; but there is a much more
wonderful thing about the burning of a candle than this.
You have here a solid substance with no vessel to contain
it; and how is it that this solid substance can get up to the
place where the flame is? How is it that this solid gets
there, it not being a fluid? or, when it is made a fluid, then
how is it that it keeps together? This is a wonderful thing
about a candle.
We have here a good deal of wind, which will help us in
some of our illustrations, but tease us in others ; for the sake,
therefore, of a little regularity, and to simplify the matter, I
shall make a quiet flame, for who can study a subject when
there are difficulties in the way not belonging to it? Here
is a clever invention of some costermonger or street-stander
in the market-place for the shading of their candles on Sat-
urday nights, when they are selling their greens, or potatoes,
or fish. I have very often admired it. They put a lamp-
glass round the candle, supported on a kind of gallery, which
clasps it, and it can be slipped up and down as required. By
the use of this lamp-glass, employed in the same way, you
have a steady flame, which you can look at, and carefully
examine, as I hope you will do, at home.
You see then, in the first instance, that a beautiful cup is
formed. As the air comes to the candle, it moves upward
by the force of the current which the heat of the candle
produces, and it so cools all the sides of the wax, tallow, or
fuel as to keep the edge much cooler than the part within ;
the part within melts by the flame that runs down the wick
as far as it can go before it is extinguished, but the part on
the outside does not melt. If I made a current in one direc-
tion, my cup would be lop-sided, and the fluid would con-
sequently run over; for the same force of gravity which
holds worlds together holds this fluid in a horizontal position,
and if the cup be not horizontal, of course the fluid will run
away in guttering. You see, therefore,, that the cup is
CHEMICAL HISTORY OF A CANDLE 9S
formed by this beautifully regular ascending current of air
playing upon all sides, which keeps the exterior of the candle
cool. No fuel would serve for a candle which has not the
property of giving this cup, except such fuel as the Irish
bog-wood, where the material itself is like a sponge and
holds its own fuel. You see now why you would have had
such a bad result if you were to burn these beautiful candles
that I have shown you, which are irregular, intermittent in
their shape, and can not, therefore, have that nicely-formed
edge to the cup which is the great beauty in a candle. I hope
you will now see that the perfection of a process — that is, its
utility — is the better point of beauty about it. It is not the
best looking thing, but the best acting thing, which is the
most advantageous to us. This good-looking candle is a bad-
burning one. There will be a guttering round about it be-
cause of the irregularity of the stream of air and the badness
of the cup which is formed thereby. You may see some
pretty examples (and I trust you will notice these instances)
of the action of the ascending current when you have a little
gutter run down the side of a candle, making it thicker there
than it is elsewhere. As the candle goes on burning, that
keeps its place and forms a little pillar sticking up by the
side, because, as it rises higher above the rest of the wax or
fuel, the air gets better round it, and it is more cooled and
better able to resist the action of the heat at a little distance.
Now the greatest mistakes and faults with regard to candles,
as in many other things, often bring with them instruction
which we should not receive if they had not occurred. We
come here to be philosophers, and I hope you will always
remember that whenever a result happens, especially if it
be new, you should say, "What is the cause? Why does it
occur ?" and you will, in the course of time, find out the
reason.
Then there is another point about these candles which will
answer a question — that is, as to the way in which this fluid
gets out of the cup, up the wick, and into the place of com-
bustion. You know that the flames on these burning wicks
in candles made of bees -wax, stearin, or spermaceti, do not
run down to the wax or other matter, and melt it all away, but
keep to their own right place. They are fenced off from the
96 FARADAY
fluid below, and do not encroach on the cup at the sides. I
can not imagine a more beautiful example than the condition
of adjustment under which a candle makes one part subserve
to the other to the very end of its action. A combustible
thing like that, burning away gradually, never being intruded
upon by the flame, is a very beautiful sight, especially when
you come to learn what a vigorous thing flame is — what
power it has of destroying the wax itself when it gets hold
of it, and of disturbing its proper form if it come only too
near.
But how does the flame get hold of the fuel? There is a
beautiful point about that — capillary attraction.^) "Capil-
lary attraction ! " you say — " the attraction of hairs." Well,
never mind the name; it was given in old times, before we
had a good understanding of what the real power was. It is
by what is called capillary attraction that the fuel is conveyed
to the part where combustion goes on, and is deposited there,
not in a careless way, but very beautifully in the very midst of
the centre of action, which takes place around it. Now I am
going to give you one or two instances of capillary attrac-
tion. It is that kind of action or attraction which makes two
things that do not dissolve in each other still hold together.
When you wash your hands, you wet them thoroughly; you
take a little soap to make the adhesion better, and you find
your hands remain wet. This is by that kind of attraction
of which I am about to speak. And, what is more, if your
hands are not soiled (as they almost always are by the usages
of life), if you put your finger into a little warm water, the
water will creep a little way up the finger, though you may
not stop to examine it. I have here a substance which is
rather porous — a column of salt — and I will pour into the
plate at the bottom, not water, as it appears, but a saturated
solution of salt which can not absorb more, so that the action
which you see will not be due to its dissolving any thing.
We may consider the plate to be the candle, and the salt the
wick, and this solution the melted tallow. (I have colored
* Capillary attraction or repulsion is the cause which determines the ascent
or descent of a fluid in a capillary tube. If a piece of thermometer tubing,
open at each end, be plunged into water, the latter will instantly rise in the
tube considerably above its external level. If, on the other hand, the tube
be plunged into mercury, a repulsion instead of attraction will be exhibited,
and the level of the mercury will be lower in the tube than it is outside.
CHEMICAL HISTORY OF A CANDLE 97
the fluid, that you may see the action better.) You observe
that, now I pour in the fluid, it rises and gradually creeps up
the salt higher and higher; and provided the column does
not tumble over, it will go to the top. If this blue solution
were combustible, and we were
to place a wick at the top of
the salt, it would burn as it
entered into the wick. It is
a most curious thing to see
this kind of action taking place,
and to observe how singular
some of the circumstances are
about it. When you wash your
hands, you take a towel to
wipe off the water; and it is FlG - 55
by that kind of wetting, or that kind of attraction which
makes the towel become wet with water, that the wick
is made wet with the tallow. I have known some care-
less boys and girls (indeed, I have known it happen to
careful people as well) who, having washed their hands
and wiped them with a towel, have thrown the towel over
the side of the basin, and before long it has drawn all the
water out of the basin and conveyed it to the floor, because it
happened to be thrown over the side in such a way as to
serve the purpose of a siphon. ( 5 ) That you may the better
see the way in which the substances act one upon another, I
have here a vessel made of wire gauze filled with water, and
you may compare it in its action to the cotton in one respect,
or to a piece of calico in the other. In fact, wicks are some-
times made of a kind of wire gauze. You will observe that
this vessel is a porous thing; for if I pour a little water on
to the top, it will run out at the bottom. You would be puz-
zled for a good while if I asked you what the state of this
vessel is, what is inside it, and why it is there? The vessel
is full of water, and yet you see the water goes in and runs
out as if it were empty. In order to prove this to you I have
5 The late Duke of Sussex was, we believe, the first to show that a prawn
might be washed upon this principle. If the tail, after pulling off the fan
part, be placed in a tumbler of water, and the head be allowed to hang over
the outside, the water will be sucked up the tail by capillary attraction, and
will continue to run out through the head until the water in the glass has
sunk so low that the tail ceases to dip into it.
HC VOL. XXX — D
98
FARADAY
only to empty it. The reason is this: the wire, being once
wetted, remains wet ; the meshes are so small that the fluid is
attracted so strongly from the one side to the other as to
remain in the vessel, although it is porous. In like manner,
the particles of melted tallow ascend the cotton and get to
the top; other particles then follow by their mutual attrac-
tion for each other, and as they reach the flame they are
gradually burned.
Here is another application of the same principle. You
see this bit of cane. I have seen boys about the streets, who
are very anxious to appear like men, take a piece of cane,
and light it, and smoke it, as an
imitation of a cigar. They are
enabled to do so by the perme-
ability of the cane in one direc-
tion, and by its capillarity. If I
place this piece of cane on a plate containing
some camphene (which is very much like paraf-
fine in its general character), exactly in the
same manner as the blue fluid rose through
the salt will this fluid rise through the piece
of cane. There being no pores at the side,
the fluid can not go in that direction, but must
pass through its length. Already the fluid is
at the top of the cane; now I can light it and
make it serve as a candle. The fluid has risen
by the capillary attraction of the piece of cane,
just as it does through the cotton in the candle.
Now the only reason why the candle does not burn all
down the side of the wick is that the melted tallow extin-
guishes the flame. You know that a candle, if turned upside
down, so as to allow the fuel to run upon the wick, will be put
out. The reason is, that the flame has not had time to make
the fuel hot enough to burn, as it does above, where it is
carried in small quantities into the wick, and has all the
effect of the heat exercised upon it.
There is another condition which you must learn as regards
the candle, without which you would not be able fully to
understand the philosophy of it, and that is the vaporous con-
dition of the fuel. In order that you may understand that.
"Fig. 56
CHEMICAL HISTORY OF A CANDLE 99
let me show you a very pretty but very commonplace experi-
ment. If you blow a candle out cleverly, you will see the
vapor rise from it. You have, I know, often smelt the vapor
of a blown-out candle, and a very bad smell it is ; but if you
blow it out cleverly you will be able to see pretty well the
vapor into which this solid matter is transformed. I will
blow out one of these candles in such a way as not to disturb
the air around it by the continuing action of my breath ; and
now, if I hold a lighted taper two or three inches from the
wick, you will observe a train of fire going through the air
till it reaches the candle. I am obliged to be quick and ready,
because if I allow the vapor time to cool, it becomes con-
densed into a liquid or solid, or the stream of combustible
matter gets disturbed.
Now as to the shape or form of the flame. It concerns us
much to know about the condition which the matter of the
candle finally assumes at the top of the wick, where you have
such beauty and brightness as nothing but combustion or
flame can produce. You have the glittering beauty of gold
and silver, and the still higher lustre of jewels like the ruby
and diamond ; but none of these rival the brilliancy
and beauty of flame. What diamond can shine /},
like flame? It owes its lustre at night-time to /Aj
the very flame shining upon it. The flame shines
in darkness, but the light which the diamond has
is as nothing until the flame shines upon it, when
it is brilliant again. The candle alone shines by
itself and for itself, or for those who have ar-
ranged the materials. Now let us look a little at
the form of the flame as you see it under the
glass shade. It is steady and equal, and its gen-
eral form is that which is represented in the
diagram, varying with atmospheric disturbances,
and also varying according to the size of the
candle. It is a bright oblong, brighter at the top
than toward the bottom, with the wick in the mid-
dle, and, besides the wick in the middle, certain darker parts
towards the bottom, where the ignition is not so perfect as in
the part above. I have a drawing here, sketched many years
ago by Hooker, when he made his investigations. It is the draw-
100
FARADAY
ing of the flame of a lamp, but it will apply to the flame of a
candle. The cup of the candle is the vessel or lamp ; the melted
spermaceti is the oil ; and the wick is common to both. Upon
that he sets this little flame, and then he represents what is
true, a certain quantity of matter rising about it which you
do not see, and which, if you have not been here before, or
are not familiar with the subject, you will not know of. He
has here represented the parts of the surrounding atmosphere
that are very essential to the flame, and that are always pres-
ent with it. There is a current formed, which draws the
flame out; for the flame which you see is really drawn out
by the current, and drawn upward to a great height, just as
Hooker has here shown you by that prolongation of the cur-
rent in the diagram. You may see this by taking a lighted
candle, and putting it in the sun so as to get its shadow
thrown on a piece of paper. How remarkable
it is that that thing which is light enough to
produce shadows of other objects can be made
to throw its own shadow on a piece of white
paper or card, so that you can actually see
streaming round the flame something which
is not part of the flame, but is ascending and
drawing the flame upward. Now I am going
to imitate the sunlight by applying the voltaic
battery to the electric lamp. You now see our
sun and its great luminosity; and by placing
a candle between it and the screen, we get the
shadow of the flame. You observe the shadow
of the candle and of the wick; then there is
a darkish part, as represented in the diagram,
and then a part which is more distinct. Curi-
ously enough, however, what we see in the
shadow as the darkest part of the flame is, in reality, the
brightest part; and here you see streaming upward the
ascending current of hot air, as shown by Hooker, which
draws out the flame, supplies it with air, and cools the sides
of the cup of melted fuel.
I can give you here a little farther illustration, for the pur-
pose of showing you how flame goes up or down according
to the current. I have here a flame — it is not a candle flame
'l« '• Ml i Jill' I
I 'll I III I 'lll»ll''
| HI II l >ll|l H •
! ill IS Ik*!
i!»i!i !
Fig. 58
CHEMICAL HISTORY OF A CANDLE
101
— but you can, no doubt, by this time generalize enough to
be able to compare one thing with another : what I am about
to do is to change the ascending current that takes the flame
upward into a descending current. This I can easily do by the
little apparatus you see before me. The flame, as I have said,
is not a candle flame, but it is produced by alcohol, so that it
shall not smoke too much. I will also color the flame with
another substance, ( 6 ) so that you may trace its course; for,
with the spirit alone, you could hardly see well enough to
have the opportunity of tracing its
direction. By lighting this spirit
of wine we have then a flame pro-
duced, and you observe that when
held in the air it naturally goes up-
ward. You understand now, easily
enough, why flames go up under
ordinary circumstances: it is be-
cause of the draught of air by
which the combustion is formed.
But now, by blowing the flame
down, you see I am enabled to
make it go downward into this
little chimney, the direction of the
current being changed. Before we have concluded this course
of lectures we shall show you a lamp in which the flame goes
up and the smoke goes down, or the flame goes down and
the smoke goes up. You see, then, that we have the power
in this way of varying the flame in different directions.
There are now some other points that I must bring before
you. Many of the flames you see here vary very much in
their shape by the currents of air blowing around them in
different directions; but we can, if we like, make flames so
that they will look like fixtures, and we can photograph them
— indeed, we have to photograph them — so that they become
fixed to us, if we wish to find out every thing concerning
them. That, however, is not the only thing I wish to men-
tion. If I take a flame sufficiently large, it does not keep that
homogeneous, that uniform condition of shape, but it breaks
6 The alcohol had chloride of copper dissolved in it: this produces a beau-
tiful green flame.
102 FARADAY
out with a power of life which is quite wonderful. I am
about to use another kind of fuel, but one which is truly and
fairly a representative of the wax or tallow of a candle. I
have here a large ball of cotton, which will serve as a wick.
And, now that I have immersed it in spirit and applied a light
to it, in what way does it differ from an ordinary candle?
Why, it differs very much in one respect, that we have a
vivacity and power about it, a beauty and a life entirely dif-
ferent from the light presented by a candle. You see those
fine tongues of flame rising up. You have the same general
disposition of the mass of the flame from below upward, but, in
addition to that, you have this remarkable breaking out into
tongues which you do not perceive in the case of a candle.
Now, why is this? I must explain it to you, because, when
you understand that perfectly, you will be able to follow me
better in what I have to say hereafter. I suppose some here
will have made for themselves the experiment I am going to
show you. Am I right in supposing that any body here has
played at snapdragon? I do not know a more beautiful illus-
tration of the philosophy of flame, as to a certain part of its
history, than the game of snapdragon. First, here is the
dish ; and let me say, that when you play snapdragon properly
you ought to have the dish well warmed; you ought also to
have warm plums, and warm brandy, which, however, I have
not got. When you have put the spirit into the dish, you
have the cup and the fuel; and are not the raisins acting like
the wicks? I now throw the plums into the dish, and light
the spirit, and you see those beautiful tongues of flame that
I refer to. You have the air creeping in over the edge of
the dish forming these tongues. Why? Because, through
the force of the current and the irregularity of the action
of the flame, it can not flow in one uniform stream. The air
flows in so irregularly that you have what would otherwise
be a single image broken up into a variety of forms, and
each of these little tongues has an independent existence of
its own. Indeed, I might say, you have here a multitude of
independent candles. You must not imagine, because you see
these tongues all at once, that the flame is of this particular
shape. A flame of that shape is never so at any one time.
Never is a body of flame, like that which you just saw rising
CHEMICAL HISTORY OF A CANDLE
103
from the ball, of the shape it appears to you. It consists of
a multitude of different shapes, succeeding each other so fast
that the eye is only able to take cognizance of them all at
once. In former times I purposely analyzed a flame of that
general character, and the diagram shows you the different
parts of which it is composed. They do not occur all at once;
Fig. 60
it is only because we see these shapes in such rapid succes-
sion that they seem to us to exist all at one time.
It is too bad that we have not got farther than my game
of snapdragon; but we must not, under any circumstances,
keep you beyond your time. It will be a lesson to me in
future to hold you more strictly to the philosophy of the
thing than to take up your time so much with these
illustrations.
LECTURE II
A CANDLE: BRIGHTNESS OF THE FLAME— AIR
NECESSARY FOR COMBUSTION— PRO-
DUCTION OF WATER
WE were occupied the last time we met in con-
sidering the general character and arrangement as
regards the fluid portion of a candle, and the way
in which that fluid got into the place of combustion. You
see, when we have a candle burning fairly in a regular,
steady atmosphere, it will have a shape something like the
one shown in the diagram, and will look pretty uniform,
although very curious in its character. And now I have
to ask your attention to the means by which we are enabled
to ascertain what happens in any particular part of the
flame; why it happens; what it does in happening; and
where, after all, the whole candle goes to; because, as you
know very well, a candle being brought before us and
burned, disappears, if burned properly, without the least
trace of dirt in the candle stick; and this is a very curious
circumstance. In order, then, to examine this candle care-
fully, I have arranged certain apparatus, the use of which
you will see as I go on. Here is a candle; I am about to
put the end of this glass tube into the middle of the flame —
into that part which old Hooker has represented in the
diagram as being rather dark, and which you can see at any
time if you will look at a candle carefully, without blowing
it about. We will examine this dark part first.
Now I take this bent glass tube, and introduce one end
into that part of the flame, and you see at once that some-
thing is coming from the flame, out at the other end of the
tube; and if I put a flask there, and leave it for a little while,
you will see that something from the middle part of the
flame is gradually drawn out, and goes through the tube,
104
CHEMICAL HISTORY OF A CANDLE
105
and into that flask, and there behaves very differently from
what it does in the open air. It not only escapes from the
end of the tube, but falls down to the bottom of the flask
like a heavy substance, as indeed it is. We find that this is
Fig. 6i
the wax of the candle made into a vaporous fluid — not a gas.
(You must learn the difference between a gas and a vapor:
a gas remains permanent ; a vapor is something that will con-
dense.) If you blow out a candle, you perceive a very nasty
smell, resulting from the condensation of this vapor. That
is very different from what you have outside the flame ; and,
in order to make that more clear to you, I am about to pro-
duce and set fire to a larger portion of this vapor ; for what
we have in the small way in a candle, to understand thor-
oughly, we must, as philosophers, produce in a larger way,
if needful, that we may examine the different parts. And
now Mr. Anderson will give me a source of heat, and I am
about to show you what that vapor is. Here is some wax
in a glass flask, and I am going to make it hot, as the inside
of that candle-flame is hot, and the matter about the wick is
hot. [The lecturer placed some wax in a glass flask, and
heated it over a lamp.] Now I dare say that is hot enough
for me. You see that the wax I put in it has become fluid.
■tf/
106 FARADAY
and there is a little smoke coming from it. We shall very
soon have the vapor rising up. I will make it still hotter,
and now we get more of it, so that I can actually pour the
vapor out of the flask into that basin, and set it on fire
there. This, then, is exactly the same kind of vapor as we
have in the middle of the candle ; and that you may be sure
this is the case, let us try whether
we have not got here, in this
flask, a real combustible vapor
out of the middle of the candle. [Taking the flask
into which the tube from the candle proceeded, and
introducing a lighted taper.] See how it burns.
Now this is the vapor from the middle of the candle,
produced by its own heat; and that is one of the first
things you have to consider with respect to the
progress of the wax in the course of its combustion,
and as regards the changes it undergoes. I will ar-
range another tube carefully in the flame, and I
Fig. 62 should not wonder if we were able, by a little care, to
get that vapor to pass through the tube to the other ex-
tremity, where we will light it, and obtain absolutely the
flame of the candle at a place distant from it. Now, look at
that. Is not that a very pretty experiment? Talk about
laying on gas — why, we can actually lay on a candle 1 And
you see from this that there are clearly two different kinds
of action — one the production of the vapor, and the other
the combustion of it — both of which take place in particular
parts of the candle.
I shall get no vapor from that part which is already burnt.
If I raise the tube (Fig. 61) to the upper part of the flame,
so soon as the vapor has been swept out what comes away
will be no longer combustible; it is already burned. How
burned? Why, burned thus: In the middle of the flame,
where the wick is, there is this combustible vapor; on the
outside of the flame is the air which we shall find necessary
for the burning of the candle; between the two, intense
chemical action takes place, whereby the air and the fuel act
upon each other, and at the very same time that we obtain
light the vapor inside is destroyed. If you examine where
the heat of a candle is, you will find it very curiously ar-
CHEMICAL HISTORY OF A CANDLE 107
ranged. Suppose I take this candle, and hold a piece of
paper close upon the flame, where is the heat of that flame?
Do you not see that it is not in the inside? It is in a ring,
exactly in the place where I told you the chemical action
was; and even in my irregular mode of making the experi-
ment, if there is not too much disturbance, there will always
be a ring. This is a good experiment for you to make at
home. Take a strip of paper, have the air in the room quiet,
and put the piece of paper right across the middle of the
flame — (I must not talk while I make the experiment) — and
you will find that it is burnt in two places, and that it is not
burnt, or very little so, in the middle; and when you have
tried the experiment once or twice, so as to make it nicely,
you will be very interested to see where the heat is, and to
find that it is where the air and the fuel come together.
This is most important for us as we proceed with our
subject. Air is absolutely necessary for combustion; and,
what is more, I must have you understand that fresh air is
necessary, or else we should be imperfect in our reasoning
and our experiments. Here is a jar of air; I place it over
a candle, and it burns very nicely in it at first, showing that
what I have said about it is true; but there will soon
be a change. See how the flame is drawing upward, pres-
ently fading, and at last going out. And going out, why?
Not because it wants air merely, for the jar is as full now
as it was before; but it wan's pure, fresh air. The jar is
full of air, partly changed, partly not changed; but it does
not contain sufficient of the fresh air which is necessary for
the combustion of a candle. These are all points which we,
as young chemists, have to gather up; and if we look a little
more closely into this kind of action, we shall find certain
steps of reasoning extremely interesting. For instance, here
is the oil-lamp I showed you — an excellent lamp for our
experiments — the old Argand lamp. I now make it like a
candle [obstructing the passage of air into the centre of the
flame] ; there is the cotton ; there is the oil rising up in
it, and there is the conical flame. It burns poorly because
there is a partial restraint of air. I have allowed no air to
get to it save around the outside of the flame, and it does
not burn well. I can not admit more air from the outside,
108 FARADAY
because the wick is large; but if, as Argand did so cleverly,
I open a passage to the middle of the flame, and so let air
come in there, you will see how much more beautifully it
burns. If I shut the air off, look how it smokes; and why?
We have now some very interesting points to study: we
have the case of the combustion of a candle ; we have the case
of a candle being put out by the want of air; and we have
now the case of imperfect combustion, and this is to us so
interesting that I want you to understand it as thoroughly
as you do the case of a candle burning in its best possible
manner. I will now make a great flame, because we need
the largest possible illustrations. Here is a larger wick
[burning turpentine on a ball of cotton]. All these things
are the same as candles, after all. If we have larger wicks,
we must have a larger supply of air, or we shall have less
perfect combustion. Look, now, at this black substance
going up into the atmosphere; there is a regular stream of
it. I have provided means to carry off the imperfectly-
burned part, lest it should annoy you. Look at the soots
that fly off from the flame; see what an imperfect combus-
tion it is, because it can not get enough air. What, then, is
happening? Why, certain things which are necessary to
the combustion of a candle are absent, and very bad results
are accordingly produced; but we see what happens to a
candle when it is burnt in a pure and proper state of air.
At the time when I showed you this charring by the ring of
flame on the one side of the paper, I might have also shown
you, by turning to the other side, that the burning of a
candle produces the same kind of soot — charcoal, or carbon.
But, before I show that, let me explain to you, as it is
quite necessary for our purpose, that, though I take a candle,
and give you, as the general result, its combustion in the
form of a flame, we must see whether combustion is always
in this condition, or whether there are other conditions of
flame; and we shall soon discover that there are, and that
they are most important to us. I think, perhaps, the best
illustration of such a point to us, as juveniles, is to show the
result of strong contrast. Here is a little gunpowder. You
know that gunpowder burns with flame; we may fairly call
it flame. It contains carbon and other materials, which
CHEMICAL HISTORY OF A CANDLE 109
altogether cause it to burn with a flame. And here is some
pulverized iron, or iron filings. Now I purpose burning
these two things together. I have a little mortar in which
I will mix them. (Before I go into these experiments, let
me hope that none of you, by trying to repeat them for fun's
sake, will do any harm. These things may all be very
properly used i.f you take care, but without that much mis-
chief will be done.) Well, then, here is a little gunpowder,
which I put at the bottom of that little wooden vessel, and
mix the iron filings up with it, my object being to make the
gunpowder set fire to the filings and burn them in the air,
and thereby show the difference between substances burning
with flame and not with flame. Here is the mixture; and
when I set fire to it you must watch the combustion, and you
will see that it is of two kinds. You will see the gunpowder
burning with a flame and the filings thrown up. You will
see them burning, too, but without the production of flame.
They will each burn separately. [The lecturer then ignited
the mixture.] There is the gunpowder, which burns with a
flame, and there are the filings: they burn with a different
kind of combustion. You see, then, these two great dis-
tinctions; and upon these differences depend all the utility
and all the beauty of flame which we use for the purpose
of giving out light. When we use oil, or gas, or candle for
the purpose of illumination, their fitness all depends upon
these different kinds of combustion.
There are such curious conditions of flame that it requires
some cleverness and nicety of discrimination to distinguish
the kinds of combustion one from another. For instance,
here is a powder which is very combustible, consisting, as
you see, of separate little particles. It is called lycopodium, ( 7 )
and each of these particles can produce a vapor, and pro-
duce its own flame; but, to see them burning, you would
imagine it was all one flame. I will now set fire to a quan-
tity, and you will see the effect. We saw a cloud of flame,
apparently in one body; but that rushing noise [referring
to the sound produced by the burning] was a proof that the
combustion was not a continuous or regular one. This is
7 Lycopodium is a yellowish powder found in the fruit of the club moss
(Lycopodium clavatum). It is used in fire-works.
110 FARADAY
the lightning of the pantomimes, and a very good imitation.
[The experiment was twice repeated by blowing lycopodium
from a glass tube through a spirit flame.] This is not an
example of combustion like that of the filings I have been
speaking of, to which we must now return.
Suppose I take a candle and examine that part of it
which appears brightest to our eyes. Why, there I get these
black particles, which already you have seen many times
evolved from the flame, and which I am now about to evolve
in a different way. I will take this candle and clear away
the gutterage, which occurs by reason of the currents of
air; and if I now arrange the glass tube so as just to dip
into this luminous part, as in our first experiment, only
higher, you see the result. In place of having the same
white vapor that you had before, you will now have a black
vapor. There it goes, as black as ink. It is certainly very
different from the white vapor; and when we put a light
to it we shall find that it does not burn, but that it puts
the light out. Well, these particles, as I said before, are
just the smoke of the candle; and this brings to mind that
old employment which Dean Swift recommended to servants
for their amusement, namely, writing on the ceiling of a
room with a candle. But what is that black substance? Why,
it is the same carbon which exists in the candle. How
comes it out of the candle? It evidently existed in the
candle, or else we should not have had it here. And now
I want you to follow me in this explanation. You would
hardly think that all those substances which fly about Lon-
don, in the form of soots and blacks, are the very beauty and
life of the flame, and which are burned in it as those iron
filings were burned here. Here is a piece of wire gauze,
which will not let the flame go through it, and I think you
will see, almost immediately, that when I bring it low
enough to touch that part of the flame which is otherwise
so bright, it quells and quenches it at once, and allows a
volume of smoke to rise up.
I want you now to follow me in this point — that whenever
a substance burns, as the iron filings burnt in the flame of
gunpowder, without assuming the vaporous state (whether
it becomes liquid or remains solid), it becomes exceedingly
CHEMICAL HISTORY OF A CANDLE 111
luminous. 1 have here taken three or four examples apart
from the candle on purpose to illustrate this point to you,
because what I have to say is applicable to all substances,
whether they burn or whether they do not burn — that they
are exceedingly bright if they retain their solid state, and
that it is to this presence of solid particles in the candle-
flame that it owes its brilliancy.
Here is a platinum wire, a body which does not change
by heat. If I heat it in this flame, see how exceedingly
luminous it becomes. I will make the flame dim for the
purpose of giving a little light only, and yet you will see
that the heat which it can give to that platinum wire, though
far less than the heat it has itself, is able to raise the plati-
num wire to a far higher state of effulgence. This flame
has carbon in it; but I will take one that has no carbon in
it. There is a material, a kind of fuel — a vapor or gas,
whichever you like to call it — in that vessel, and it has no
solid particles in it; so I take that because it is an example
of flame itself burning without any solid matter whatever;
and if I now put this solid substance in it, you see what an
intense heat it has, and how brightly it causes the solid body
to glow. This is the pipe through which we convey this par-
ticular gas, which we call hydrogen, and which you shall
know all about the next time we meet. And here is a sub-
stance called oxygen, by means of which this hydrogen can
burn; and although we produce, by their mixture, far
greater heat( 8 ) than you can obtain from the candle, yet
there is very little light. If, however, I take a solid sub-
stance, and put that into it, we produce an intense light. If
I take a piece of lime, a substance which will not burn, and
which will not vaporize by the heat (and because it does not
vaporize remains solid, and remains heated), you will soon
observe what happens as to its glowing. I have here a most
intense heat produced by the burning of hydrogen in con-
tact with the oxygen; but there is as yet very little light —
not for want of heat, but for want of particles which can
retain their solid state; but when I hold this piece of lime
in the flame of the hydrogen as it burns in the oxygen, see
1 8 Bunsen has calculated that the temperature of the oxyhydrogen blow-
pipe is 8061 ° Centigrade. Hydrogen burning in air has a temperature of
3259 C, and coal-gas in air, 2350 C
112 FARADAY
how it glows ! This is the glorious lime light, which rivals
the voltaic light, and which is almost equal to sunlight I
have here a piece of carbon or charcoal, which will burn
and give us light exactly in the same manner as if it were
burnt as part of a candle. The heat that is in the flame of a
candle decomposes the vapor of the wax, and sets free the
carbon particles; they rise up heated and glowing as this
now glows, and then enter into the air. But the particles,
when burnt, never pass off from a candle in the form of car-
bon. They go off into the air as a perfectly invisible sub-
stance, about which we shall know hereafter.
Is it not beautiful to think that such a process is going on,
and that such a dirty thing as charcoal can become so incan-
descent? You see it comes to this — that all bright flames
contain these solid particles; all things that burn and pro-
duce solid particles, either during the time they are burning,
as in the candle, or immediately after being burnt, as in the
case of the gunpowder and iron filings — all these things give
us this glorious and beautiful light.
I will give you a few illustrations. Here is a piece of
phosphorus, which burns with a bright flame. Very well;
we may now conclude that phospho-
rus will produce, either at the mo-
ment that it is burning or after-
wards, these solid particles. Here
is the phosphorus lighted, and I
cover it over with this glass for the
purpose of keeping in what is pro-
duced. What is all that smoke?
That smoke consists of those very
particles which are produced by the
combustion of the phosphorus. Here again are two sub-
stances. This is chlorate of potassa, and this other sulphuret
of antimony. I shall mix these together a little, and then
they may be burnt in many ways. I shall touch them with a
drop of sulphuric acid, for the purpose of giving you an illus-
tration of chemical action, and they will instantly burn.( 9 )
9 The folfowing is the action of the sulphuric in inflaming the mixture of
sulphuret of antimony and chlorate of potassa. A portion of the latter is
decomposed by the sulphuric acid into oxide of chlorine, bisulphate of potassa,
CHEMICAL HISTORY OF A CANDLE 113
[The lecturer then ignited the mixture by means of sul-
phuric acid.] Now, from the appearance of things, you
can judge for yourselves whether they produce solid matter
in burning. I have given you the train of reasoning which
will enable you to say whether they do or do not; for what
is this bright flame but the solid particles passing off?
Mr. Anderson has in the furnace a very hot crucible. I
am about to throw into it some zinc filings, and they will
burn with a flame like gunpowder. I make this experiment
because you can make it well at home. Now I want you
to see what will be the result of the combustion of this zinc.
Here it is burning — burning beautifully like a candle, I may
say. But what is all that smoke, and what are those little
clouds of wool which will come to you if you can not come to
them, and make themselves sensible to you in the form of
the old philosophic wool, as it was called. We shall have
left in that crucible, also, a quantity of this woolly matter.
But I will take a piece of this same zinc, and make an ex-
periment a little more closely at home, as it were. You will
have here the same thing happening. Here is the piece of
zinc; there [pointing to a jet of hydrogen] is the furnace,
and we will set to work and try and burn the metal. It
glows, you see; there is the combustion; and there is the
white substance into which it burns. And so, if I take that
flame of hydrogen as the representative of a candle, and
show you a substance like zinc burning in the flame, you
will see that it was merely during the action of combustion
that this substance glowed — while it was kept hot; and if I
take a flame of hydrogen and put this white substance from
the zinc into it, look how beautifully it glows, and just
because it is a solid substance.
I will now take such a flame as I had a moment since, and
set free from it the particles of carbon. Here is some cam-
phene, which will burn with a smoke; but if I send these
particles of smoke through this pipe into the hydrogen
flame you will see they will burn and become luminous,
because we heat them a second time. There they afe. Those
are the particles of carbon reignited a second time. They
and perchlorate of potassa. The oxide of chlorine inflames the sulphuret of
antimony, which is a combustible body, and the whole mass instantly bursts
into flame.
114 FARADAY
are those particles which you can easily see by holding a
piece of paper behind them, and which, while they are in the
flame, are ignited by the heat produced, and, when so
ignited, produce this brightness. When the particles are not
separated you get no brightness. The flame of coal-gas
owes its brightness to the separation, during combustion,
of these particles of carbon, which are equally in that as in
a candle. I can very quickly alter that arrangement. Here,
for instance, is a bright flame of gas. Supposing I add so
much air to the flarme as to cause it all to burn before those
particles are set free, I shall not have this brightness; and
I can do that in this way: If I place over the jet this wire-
gauze cap, as you see, and then light the gas over it, it burns
with a non-luminous flame, owing to its having plenty of
air mixed with it before it burns; and if I raise the gauze,
you see it does not burn below. ( 10 ) There is plenty of
carbon in the gas ; but, because the atmosphere can get to it,
and mix with it before it burns, you see how pale and blue
the flame is. And if I blow upon a bright gas-flame, so as to
consume all this carbon before it gets heated to the glowing
point, it will also burn blue. [The lecturer illustrated his
remarks by blowing on the gas-light.] The only reason why
J have not the same bright light when I thus blow upon the
flame is that the carbon meets with sufficient air to burn it
before it gets separated in the flame in a free state. The
difference is solely due to the solid particles not being sepa-
rated before the gas is burnt.
You observe that there are certain products as the result
of the combustion of a candle, and that of these products
one portion may be considered as charcoal, or soot; that
charcoal, when afterward burnt, produces some other
product; and it concerns us very much now to ascertain
what that other product is. We showed that something was
going away; and I want you now to understand how much
is going up into the air ; and for that purpose we will have
10 The " air-burner," which is of such value in the laboratory, owes its
advantage to this principle. It consists of a cylindrical metal chimney, cov-
ered at the top with a piece of rather coarse iron wire gauze. This is sup-
ported over an Argand burner in such a manner that the gas may mix in
the chimney with an amount of air sufficient to burn the carbon and hydrogen
simultaneously, so that there may be no separation of carbon in the flame
with consequent deposition of soot. The flame, being unable to pass through
the wire gauze, burns in a steady, nearly invisible manner above.
CHEMICAL HISTORY OF A CANDLE
115
combustion on a little larger scale. From that candle
ascends heated air, and two or three experiments will show
you the ascending current ; but, in order to give you a notion
of the quantity of matter which ascends in this way, I will
make an experiment by which I shall try to imprison some
of the products of this combustion. For this purpose I have
here what boys call a fire-balloon; I use this fire-balloon
merely as a sort of measure of the re-
sult of the combustion we are con-
sidering; and I am about to make a
flame in such an easy and simple
manner as shall best serve my present
purpose. This plate shall be the
"cup," we will so say, of the candle;
this spirit shall be our fuel ; and I am
about to place this chimney over it,
because it is better for me to do so
than to let things proceed at random.
Mr. Anderson will now light the
fuel, and here at the top we shall get
the results of the combustion. What
we get at the top of that tube is ex-
actly the same, generally speaking, as
you get from the combustion of a
candle; but we do not get a luminous
flame here, because we use a sub-
stance which is feeble in carbon. I
am about to put this balloon — not
into action, because that is not my
object — but to show you the effect which results from the
action of those products which arise from the candle, as they
arise here from the furnace. [The balloon was held over
the chimney, when it immediately commenced to fill.] You
see how it is disposed to ascend ; but we must not let it up ;
because it might come in contact with those upper gaslights,
and that would be very inconvenient. [The upper gas-
lights were turned out at the request of the lecturer, and
the balloon was allowed to ascend.] Does not that show
you what a large bulk of matter is being evolved? Now
there is going through this tube [placing a large glass tube
Fig. 64
116 FARADAY
over a candle] all the products of that candle, and you will
presently see that the tube will become quite opaque.
Suppose I take another candle, and place it under a jar,
and then put a light on the other side, just to show you
what is going on. You see that the sides of the jar become
cloudy, and the light begins to burn feebly. It is the
products, you see, which make the light so dim, and
this is the same thing which makes the sides of the jar so
opaque. If you go home, and take a spoon that has been in
the cold air, and hold it over a candle — not so as to soot it —
you will find that it becomes dim just as that jar is dim.
If you can get a silver dish, or something of that kind, you
will make the experiment still better; and now, just to carry
your thoughts forward to the time we shall next meet, let
me tell you that it is water which causes the dimness, and
when we next meet I will show you that we can make it,
without difficulty, assume the form of a liquid.
LECTURE III
PRODUCTS: WATER FROM THE COMBUSTION-
NATURE OF WATER— A COMPOUND-
HYDROGEN
I DARE say you well remember that when we parted we
had just mentioned the word " products" from the
candle ; for when a candle burns we found we were able,
by nice adjustment, to get various products from it. There
was one substance which was not obtained when the candle
was burning properly, which was charcoal or smoke, and there
was some other substance that went upward from the flame
which did not appear as smoke, but took some other form,
and made part of that general current which, ascending
from the candle upward, becomes invisible, and escapes.
There were also other products to mention. You remember
that in that rising current having its origin at the candle
we found that one part was condensable against a cold spoon,
or against a clean plate, or any other cold thing, and an-
other part was incondensable.
We will first take the condensable part, and examine it,
and, strange to say, we find that that part of the product is
just water — nothing but water. On the last occasion I spoke
of it incidentally, merely saying that water was produced
among the condensable products of the candle; but to-day I
wish to draw your attention to water, that we may examine it
carefully, especially in relation to this subject, and also with
respect to its general existence on the surface of the globe.
Now, having previously arranged an experiment for the
purpose of condensing water from the products of the
candle, my next point will be to show you this water; and
perhaps one of the best means that I can adopt for showing
its presence to so many at once is to exhibit a very visible
117
118
FARADAY
action of water, and then to apply that test to what is col-
lected as a drop at the bottom of that vessel. I have here a
chemical substance, discovered by Sir Humphrey Davy,
which has a very energetic action upon water, which I shall
use as a test of the presence of water. If I take a little
piece of it — it is called potassium, as coming from potash-—
if I take a little piece of it, and throw it into that basin, you
see how it shows the presence of water by lighting up and
floating about, burning with a violet flame. I am now
going to take away the candle which has been burning
beneath the vessel containing ice and salt, and you see a
drop of water — a condensed product of the candle — hanging
from the under surface of the dish. I will show you that
potassium has the same action upon it as upon the water
in that basin in the experiment we have just tried. See !
it takes fire, and burns in just the same manner. I will take
another drop upon this glass slab, and when
I put the potassium on to it, you see at once,
from its taking fire, that there is water
present. Now tihat water was produced by
the candle. In the same manner, if I put this
spirit-lamp under that jar, you will soon see
the latter become damp from the dew which
is deposited upon it — that dew being the
result of combustion; and I have no doubt
you will shortly see, by the drops of water
which fall upon the paper below, that there
is a good deal of water produced from the
combustion of the lamp. I will let it remain,
and you can afterward see how much water
has been collected. So, if I take a gas-lamp,
and put any cooling arrangement over it, I shall
get water — water being likewise produced from the com-
bustion of gas. Here, in this bottle, is a quantity of water —
perfectly pure, distilled water, produced from the combus-
tion of a gas-lamp — in no point different from the water
that you distill from the river, or ocean, or spring, but
exactly the same thing. Water is one individual thing; it
never changes. We can add to it by careful adjustment
for a little while, or we can take it apart and get other
Fig. 65
CHEMICAL HISTORY OF A CANDLE 119
things from it; but water, as water, remains always the
same, either in a solid, liquid, or fluid state. Here again
[holding another bottle] is some water produced by the
combustion of an oil-lamp. A pint of oil, when burnt fairly
and properly, produces rather more than a pint of water.
Here, again, is some water, produced by a rather long
experiment, from a wax candle. And so we can go on
with almost all combustible substances, and find that if they
burn with a flame, as a candle, they produce water. You
may make these experiments yourselves: the head of a poker
is a very good thing to try with, and if it remains cold long
enough over the candle, you may get water condensed in
drops on it; or a spoon, or ladle, or any thing else may be
used, provided it be clean, and can carry off the heat, and
so condense the water.
And now — to go into the history of this wonderful pro-
duction of water from combustibles, and by combustion — I
must first of all tell you that this water may exist in different
conditions; and although you may now be acquainted with
all its forms, they still require us to give a little attention
to them for the present; so that we may perceive how the
water, while it goes through its Protean changes, is entirely
and absolutely the same thing, whether it is produced from
a candle, by combustion, or from the rivers or ocean.
First of all, water, when at the coldest, is ice. Now we
philosophers — I hope that I may class you and myself to-
gether in this case — speak of water as water, whether it be
in its solid, or liquid, or gaseous state — we speak of it chemi-
cally as water. Water is a thing compounded of two sub-
stances, one of which we have derived from the candle, and
the other we shall find elsewhere. Water may occur as ice ;
and you have had most excellent opportunities lately of see-
ing this. Ice changes back into water — for we had on our
last Sabbath a strong instance of this change by the sad
catastrophe which occurred in our own house, as well as in
the houses of many of our friends — ice changes back into
water when the temperature is raised; water also changes
into steam when it is warmed enough. The water which we
have here before us is in its densest state ;( n ) and, although
11 Water is in its densest state at a temperature of 39. i° Fahrenheit.
120 FARADAY
it changes in weight, in condition, in form, and in many
other qualities, it still is water ; and whether we alter it into
ice by cooling, or whether we change it into steam by heat,
it increases in volume — in the one case very strangely and
powerfully, and in the other case very largely and wonder-
fully. For instance, I will now take this thin cylinder, and
pour a little water into it, and, seeing how much water I
pour in, you may easily estimate for yourselves how high it
will rise in the vessel: it will cover the bottom about two
inches. I am now about to convert the water into steam
for the purpose of showing to you the different volumes
which water occupies in its different states of water and
steam.
Let us now take the case of water changing into ice : we
can effect that by cooling it in a mixture of salt and
pounded ice( 12 ) — and I shall do so to show you the expan-
sion of water into a thing of larger bulk when it is so
changed. These bottles [holding one] are made of strong
cast iron, very strong and very thick — I suppose they are
the third of an inch in thickness; they are very carefully
filled with water, so as to exclude all air, and then they are
screwed down tight. We shall see that when we freeze the
water in these iron vessels, they will not be able to hold the
ice, and the expansion within them will break them in pieces
as these [pointing to some fragments] are broken, which
have been bottles of exactly the same kind. I am about to
put these two bottles into that mixture of ice and salt for
the purpose of showing that when water becomes ice it
changes in volume in this extraordinary way.
In the mean, time, look at the change which has taken
place in the water to which we have applied heat ; it is losing
its fluid state. You may tell this by two or three circum-
stances. I have covered the mouth of this glass flask, in
which water is boiling, with a watch-glass. Do you see
what happens? It rattles away like a valve chattering, be-
cause the steam rising from the boiling water sends the
valve up and down, and forces itself out, and so makes it
clatter. You can very easily perceive that the flask is quite
"A mixture of salt and pounded ice reduces the temperature from 32 ° F.
to zero, the ice at the same time becoming fluid.
CHEMICAL HISTORY OF A CANDLE 121
full of steam, or else it would not force its way out. You
see also that the flask contains a substance very much larger
than the water, for it fills the whole of the flask over and
over again, and there it is blowing away into the air; and
yet you can not observe any great diminution in the bulk
of the water, which shows you that its change of bulk is
very great when it becomes steam.
I have put our iron bottles containing water into this
freezing mixture, that you may see what happens. No
communication will take place, you observe, between the
water in the bottles and the ice in the outer vessel. But
there will be a conveyance of heat from the one to the other,
and if we are successful — we are making our experiment
in very great haste — I expect you will by-and-by, so soon
as the cold has taken possession of the bottles and their
contents, hear a pop on the occasion of the bursting of the
one bottle or the other, and, when we come to examine the
bottles, we shall find their contents masses of ice, partly
inclosed by the covering of iron which is too small for them,
because the ice is larger in bulk than the water. You know
very well that ice floats upon water; if a boy falls through
a hole into the water, he tries to get on the ice again to
float him up. Why does the ice float? Think of that, and
philosophize. Because the ice is larger than the quantity
of water which can produce it, and therefore the ice weighs
the lighter and the water is the heavier.
To return now to the action of heat on water. See what a
stream of vapor is issuing from this tin vessel. You observe,
we must have made it quite full of steam to have it sent out
in that great quantity. And now, as we can convert the
water into steam by heat, we convert it back into liquid
water by the application of cold. And if we take a glass,
or any other cold thing, and hold it over this steam, see
how soon it gets damp with water : it will condense it until
the glass is warm — it condenses the water which is now
running down the sides of it. I have here another ex-
periment to show the condensation of water from a va-
porous state back into a liquid state, in the same way
as the vapor, one of the products of the candle, was
condensed against the bottom of the dish and obtained
122
FARADAY
in the form of water; and to show you how truly and
thoroughly these changes take place, I will take this tin
flask, which is now full of steam, and close the top. We
shall see what takes place when we cause this water or
steam to return back to the fluid state by pouring some cold
water on the outside. [The lecturer poured the cold water
Fig. 66
over the vessel, when it immediately collapsed.] You see
what has happened. If I had closed the stopper, and still
kept the heat applied to it, it would have burst the vessel;
yet, when the steam returns to the state of water, the vessel
collapses, there being a vacuum produced inside by the con-
densation of the steam. I show you these experiments for
the purpose of pointing out that in all these occurrences
there is nothing that changes the water into any other
thing; it still remains water; and so the vessel is obliged
to give way, and is crushed inward, as in the other case, by
the farther application of heat, it would have been blown
outward.
And what do you think the bulk of that water is when it
assumes the vaporous condition ? You see that cube [point-
CHEMICAL HISTORY OF A CANDLE 123
ing to a cubic foot]. There, by its side, is a cubic inch,
exactly the same shape as the cubic foot, and that bulk of
water [the cubic inch] is sufficient to expand into that bulk
[the cubic foot] of steam; and,
on the contrary, the application
of cold will contract that large
quantity of steam into this small
quantity of water. [One of the
iron bottles burst at that mo-
ment] Ah ! There is one of our
bottles burst, and here, you see,
is a crack down one side an FlG * 6?
eighth of an inch in width. [The other now exploded, send-
ing the freezing mixture in all directions.] This other bottle
is also broken; although the iron was nearly half an inch
thick, the ice has burst it asunder. These changes always
take place in water ; they do not require to be always produced
by artificial means ; we only use them here because we want
to produce a small winter round that little bottle instead
of a long and severe one. But if you go to Canada, or to
the North, you will find the temperature there out of doors
will do the same thing as has been done here by the freezing
mixture.
To return to our quiet philosophy. We shall not in future
be deceived, therefore, by any changes that are produced in
water. Water is the same every where, whether produced
from the ocean or from the flame of the candle. Where,
then, is this water which we get from a candle? I must
anticipate a little, and tell you. It evidently comes, as to
part of it, from the candle, but is it within the candle before-
hand? No, it is not in the candle; and it is not in the air
around about the candle which is necessary for its com-
bustion. It is neither in one nor the other, but it comes
from their conjoint action, a part from the candle, a part
from the air; and this we have now to trace, so that we
may understand thoroughly what is the chemical history of
a candle when we have it burning on our table. How shall
we get at this? I myself know plenty of ways, but I want
you to get at it from the association in your own minds of
what I have already told you.
124 FARADAY
I think you can see a little in this way. We had Just now
the case of a substance which acted upon the water in the
way that Sir Humphrey Davy showed us,( M ) and which I
am now going to recall to your minds by making again an
experiment upon that dish. It is a thing which we have to
handle very carefully ; for you see, if I allow a little splash
of water to come upon this mass, it sets fire to part of it;
and if there were free access of air, it would quickly set fire
to the whole. Now this is a metal — a beautiful and bright
metal — which rapidly changes in the air, and, as you know,
rapidly changes in water. I will put a piece on the water,
and you see it burns beautifully, making a floating lamp,
using the water in the place of air. Again, if we take a
few iron filings or turnings and put them in water, we find
that they likewise undergo an alteration. They do not
change so much as this potassium does, but they change
somewhat in the same way; they become rusty, and show
an action upon the water, though in a different degree of
intensity to what this beautiful metal does; but they act
upon the water in the same manner generally as this potas-
sium. I want you to put these different facts together in
your minds. I have another metal here [zinc], and when
we examined it with regard to the solid substance produced
by its combustion, we had an opportunity of seeing that it
burned; and I suppose, if I take a little strip of this zinc
and put it over the candle, you will see something half way,
as it were, between the combustion of potassium on the
water and the action of iron — you see there is a sort of com-
bustion. It has burned, leaving a white ash or residuum;
and here also we find that the metal has a certain amount
of action upon water.
By degrees we have learned how to modify the action of
these different substances, and to make them tell us what
we want to know. And now, first of all, I take iron. It is
a common thing in all chemical reactions, where we get
any result of this kind, to find that it is increased by the
action of heat; and if we want to examine minutely and
13 Potassium, the metallic basis of potash, was discovered by Sir Humphrey
Davy in 1807, who succeeded in separating it from potash by means of a
powerful voltaic battery. Its great affinity for oxygen causes it to decompose
water with evolution of hydrogen, which takes fire with the heat produced.
CHEMICAL HISTORY,, OF A CANDLE
125
carefully the action of bodies one upon another, we often
have to refer to the action of heat. You are aware, I be-
lieve, that iron filings burn beautifully in the air; but I am
about to show you an experiment of this kind, because it
will impress upon you what I am going to say about iron
in its action on water. If I take a flame and make it hol-
low — you know why, because I want to get air to it and
into it, and therefore I make it hollow — and then take a few
iron filings and drop them into the flame, you see how well
they burn. That combustion results from the chemical
Fig. 68
action which is going on when we ignite those particles.
And so we proceed to consider these different effects, and
ascertain what iron will do when it meets with water. It
will tell us the story so beautifully, so gradually and regu-
larly, that I think it will please you very much.
I have here a furnace with a pipe going through it like
an iron gun-barrel, and I have stuffed that barrel full of
bright iron turnings, and placed it across the fire to be
made red-hot. We can either send air through the barrel to
come in contact with the iron, or we can send steam from,
this little boiler at the end of the barrel. Here is a stop-
cock which shuts off the steam from the barrel until we
wish to admit it. There is some water in these glass jars,
which I have colored blue, so that you may see what hap-
pens. Now you know very well that any steam I might send
126 FARADAY
through that barrel, if it went through into the water, would
be condensed; for you have seen that steam can not retain
its gaseous form if it be cooled down; you saw it here
[pointing to the tin flask] crushing itself into a small bulk,
and causing the flask holding it to collapse; so that if I
were to send steam through that barrel it would be con-
densed, supposing the barrel were cold; it is therefore
heated to perform the experiment I am now about to show
you. I am going to send the steam through the barrel in
small quantities, and you shall judge for yourselves, when
you see it issue from the other end, whether it still remains
steam. Steam is condensible into water, and when you
lower the temperature of steam you convert it back into
fluid water; but I have lowered the temperature of the gas
which I have collected in this jar by passing it through
water after it has traversed the iron barrel, and still it does
not change back into water. I will take another test and
apply to this gas. (I hold the jar in an inverted position,
or my substance would escape.) If I now apply a light to
the mouth of the jar, it ignites with a slight noise. That
tells you that it is not steam ; steam puts out a fire : it does
not burn; but you saw that what I had in that jar burnt.
We may obtain this substance equally from water produced
from the candle flame as from any other source. When it
is obtained by the action of the iron upon the aqueous vapor,
it leaves the iron in a state very similar to that in which
these filings were after they were burnt. It makes the iron
heavier than it was before. So long as the iron remains
in the tube and is heated, and is cooled again without the
access of air or water, it does not change in its weight; but
after having had this current of steam passed over it, it
then comes out heavier than it was before, having taken
something out of the steam, and having allowed something
else to pass forth, which we see here. And now, as we have
another jar full, I will show you something most interest-
ing. It is a combustible gas; and I might at once take this
jar and set fire to the contents, and show you that it is com-
bustible; but I intend to show you more, if I can. It is
also a very light substance. Steam will condense ; this body
will rise in the air, and not condense. Suppose I take
CHEMICAL HISTORY OF A CANDLE
127
Fig. 69
another glass jar, empty of all but air: if I examine it with
a taper I shall find that it contains nothing but air. I will
now take this jar full of the gas that I am speaking of, and
deal with it as though it were
a light body; I will hold both
upside down, and turn the one
up under the other; and that
which did contain the gas pro-
cured from the steam, what does
it contain now? You will find
it now only contains air. But
look! Here is the combustible
substance [taking the other jar]
which I have poured out of the one jar into the other. It
still preserves its quality, and condition, and independence,
and therefore is the more worthy of our consideration, as
belonging to the products of a candle.
Now this substance which we have just prepared by the
action of iron on the steam or water, we can also get by
means of those other things which you have already seen act
so well upon the water. If I take a piece of potassium, and
make the necessary arrangements, it will produce this gas;
and if instead a piece of zinc, I find, when I come to exam-
ine it very carefully, that the main reason why this zinc can
not act upon the water continuously as the other metal does
is because the result of the action of the water envelops the
zinc in a kind of protecting coat. We have learned in con-
sequence, that if we put into our vessel only the zinc and
water, they, by themselves, do not give rise to much action,
and we get no result. But suppose I proceed to dissolve off
this varnish — this encumbering substance — which I can do
by a little acid; the moment I do this I find the zinc acting
upon the water exactly as the iron did, but at the common
temperature. The acid in no way is altered except in its
combination with the oxide of zinc which is produced. I
have now poured the acid into the glass, and the effect is as
though I were applying heat to cause this boiling up. There
is something coming off from the zinc very abundantly,
which is not steam. There is a jar full of it; and you will
find that I have exactly the same combustible substance re-
128
FARADAY
maining in the vessel, when I hold it upside down, that I
produced during the experiment with the iron barrel. This
is what we get from water, the same substance which is
contained in the candle.
Let us now trace distinctly the connection between these
two points. This is hydrogen — a body classed among those
Fig. 70
things which in chemistry we call elements, because we can
get nothing else out of them. A candle is not an elementary
body, because we can get carbon out of it; we can get this
hydrogen out of it, or at least out of the water which it sup-
plies. And this gas has been so named hydrogen, because it
is that element which, in association with another, generates
water.* Mr. Anderson having now been able to get two or
three jars of gas, we shall have a few experiments to make,
and I want to show you the best way of making these
experiments. I am not afraid to show you, for I wish you to
make experiments, if you will only make them with care and
attention, and the assent of those around you. As we advance
in chemistry we are obliged to deal with substances which
are rather injurious if in their wrong places; the acids, and
heat, and combustible things we use might do harm if care-
lessly employed. If you want to make hydrogen, you can
make it easily from bits of zinc, and sulphuric or muriatic
acid. Here is what in former times was called the " philoso-
pher's candle." It is a little phial with a cork and a tube or
pipe passing through it. And I am now putting a few little
pieces of zinc into it. This little instrument I am going to
apply to a useful purpose in our demonstrations, for I want
to show you that you can prepare hydrogen, and make some
* 'YSwp, M water," and yewaut, " I generate."
CHEMICAL HISTORY OF A CANDLE 129
experiments with it as you please, at your own homes. Let
me here tell you why I am so careful to fill this phial nearly,
and yet not quite full. I do it because the evolved gas, which,
as you have seen, is very combustible, is explosive to a con-
siderable extent when mixed with air, and might ^
lead to harm if you were to apply a light to the end
of that pipe before all the air had been swept out
of the space above the water. I am now about to
pour in the sulphuric acid. I have used very little
zinc and more sulphuric acid and water, because I
want to keep it at work for some time. I therefore
take care in this way to modify the proportions of the
ingredients so that I may have a regular supply — not
too quick and not too slow. Supposing I now take a
glass and put it upside down over the end of the tube,
because the hydrogen is light I expect that it will re-
main in that vessel a little while. We will now test the IG * 7I
contents of our glass to see if there be hydrogen in it; I think
I am safe in saying we have caught some [applying a light].
There it is, you see. I will now apply a light to the top of the
tube. There is the hydrogen burning. There is our philo-
sophical candle. It is a foolish, feeble sort of a flame, you
may say, but it is so hot that scarcely any common flame gives
out so much heat. It goes on burning regularly, and I am now
about to put that flame to burn under a certain arrangement,
in order that we may examine its results and make use of the
information which we may thereby acquire. Inasmuch as the
candle produces water, and this gas comes out of the water,
let us see what this gives us by the same process of com-
bustion that the candle went through when it burnt in the
atmosphere, and for that purpose I am going to put the lamp
under this apparatus, in order to condense whatever may
arise from the combustion within it. In the course of a short
time you will see moisture appearing in the cylinder, and you
will get the water running down the side, and the water from
this hydrogen flame will have absolutely the same effect upon
all our tests, being obtained by the same general process as
in the former case. This hydrogen is a very beautiful sub-
stance. It is so light that it carries things up; it is far
lighter than the atmosphere, and I dare say I can show you
H C VOL. XXX — E
130 FARADAY
this by an experiment which, if you are very clever, some
of you may even have skill enough to repeat. Here is our
generator of hy-
" s **^-^ drogen, and here
are some soap-
suds. I have an
India-rubber tube connected with the hydrogen gen-
erator, and at the end of the tube is a tobacco-pipe. I
can thus put the pipe into the suds and blow bubbles
by means of the hydrogen. You observe how the
bubbles fall downward when I blow them with my
warm breath; but notice the difference when I blow
them with hydrogen. [The lecturer here blew bubbles
Fig. 72 with hydrogen, which rose to the roof of the theatre.]
It shows you how light this gas must be in order to carry
with it not merely the ordinary soap-bubble, but the larger
portion of a drop hanging to the bottom of it. I can show its
lightness in a better way than this ; larger bubbles than these
may be so lifted up; indeed, in former times balloons used to
be filled with this gas. Mr. Anderson will fasten this tube on
to our generator, and we shall have a stream of hydrogen
here with which we can charge this balloon made of col-
lodion. I need not even be very careful to get all the air out,
for I know the power of this gas to carry it up. [Two collo-
dion balloons were inflated and sent up, one being held by a
string.] Here is another larger one, made of thin mem-
brane, which we will fill and allow to ascend; you will see
they will all remain floating about until the gas escapes.
What, then, are the comparative weights of these sub-
stances ? I have a table here which will show you the propor-
tion which their weights bear to each other. I have taken
a pint and a cubic foot as the measures, and have placed
opposite to them the respective figures. A pint measure
of this hydrogen weighs three quarters of our smallest
weight, a grain, and a cubic foot weighs one twelfth of
an ounce; whereas a pint of water weighs 8750 grains,
and a cubic foot of water weighs almost 1000 ounces.
You see, therefore, what a vast difference there is be-
tween the weight of a cubic foot of water and a cubic
foot of hydrogen.
CHEMICAL HISTORY OF A CANDLE 131
Hydrogen gives rise to no substance that can become solid,
either during combustion or afterward as a product of its
combustion; but when it burns it produces water only; and if
we take a cold glass and put it over the flame, it becomes
damp, and you have water produced immediately in apprecia-
ble quantity; and nothing is produced by its combustion but
the same water which you have seen the flame of the candle
produce. It is important to remember that this hydrogen is
the only thing in nature which furnishes water as the sole
product of combustion.
And now we must endeavor to find some additional proof
of the general character and composition of water, and for
this purpose I will keep you a little longer, so that at our
next meeting we may be better prepared for the subject.
We have the power of arranging the zinc which you
have seen acting upon the water by the assistance of an
acid, in such a manner as to cause all the power to be
evolved in the place where we require it. I have behind
me a voltaic pile, and I am just about to show you, at the
end of this lecture, its character and power, that you may
see what we shall have to deal with when next we meet.
I hold here the extremities of the wires which transport
the power from behind me, and which I shall cause to act
on the water.
We have previously seen what a power of combustion is
possessed by the potassium, or the zinc, or the iron filings;
but none of them show such energy as this. [The lecturer
here made contact between the two terminal wires of the
battery, when a brilliant flash of light was produced.] This
light is, in fact, produced by a forty-zinc power of burning;
it is a power that I can carry about in my hands through
these wires at pleasure, although if I applied it wrongly to
myself it would destroy me in an instant, for it is a most
intense thing, and the power you see here put forth while
you count five [bringing the poles in contact and exhibit-
ing the electric light] is equivalent to the power of sev-
eral thunder-storms, so great is its force ("). And that
you may see what intense energy it has, I will take the
14 Professor Faraday has calculated that there is as much electricity re-
quired to decompose one grain of water as there is in a very powerful
flash of lightning.
132 FARADAY
ends of the wires which convey the power from the bat-
tery, and with it I dare say I can burn this iron file. Now
this is a chemical power, and one which, when we next
meet, I shall apply to water, and show you what results we
are able to produce.
LECTURE IV
HYDROGEN IN THE CANDLE— BURNS INTO
WATER— THE OTHER PART OF
WATER— OXYGEN
I SEE you are not tired of the candle yet, or I am sure
you would not be interested in the subject in the way you
are. When our candle was burning we found it pro-
duced water exactly like the water we have around us ; and by
farther examination of this water we found in it that curious
body, hydrogen — that light substance of which there is some
in this jar. We afterward saw the burning powers of that
hydrogen, and that it produced water. And I think I intro-
duced to your notice an apparatus which I very briefly said
was an arrangement of chemical force, or power, or energy,
so adjusted as to convey its power to us in these wires; and
I said I should use that force to pull the water to pieces, to
see what else there was in the water besides hydrogen;
because, you remember, when we passed the water through
the iron tube, we by no means got the weight of water back
which we put in in the form of steam, though we had a very
large quantity of gas evolved. We have now to see what is
the other substance present. That you may understand the
character and use of this instrument, let us make an experi-
ment or two. Let us put together, first of all, some sub-
stances, knowing what they are, and then see what that
instrument does to them. There is some copper (observe the
various changes which it can undergo), and here is some
nitric acid, and you will find that this, being a strong chemi-
cal agent, will act very powerfully when I add it to the
copper. It is now sending forth a beautiful red vapor;
but as we do not want that vapor, Mr. Anderson will hold it
near the chimney for a short time, that we may have the use
133
134 FARADAY
and beauty of the experiment without the annoyance. The
copper which I have put into the flask will dissolve it : it will
change the acid and the water into a blue fluid containing
copper and other things, and I purpose then showing you how
this voltaic battery deals with it; and in the mean time we
will arrange another kind of experiment for you to see what
power it has. This is a substance which is to us like water —
that is to say, it contains bodies which we do not know of
as yet, as water contains a body which we do not know as
yet. Now this solution of a salt( 15 ) I will put upon paper and
spread about, and apply the power of the battery to it, and
observe what will happen. Three or four important things
will happen which we shall take advantage of. I place this
wetted paper upon a sheet of tin-foil, which is convenient
for keeping all clean, and also for the advantageous applica-
tion of the power; and this solution, you see, is not at all
affected by being put upon the paper or tin-foil, nor by any
thing else I have brought in contact with it yet, and which,
therefore, is free to us to use as regards that instrument.
But first let us see that our instrument is in order. Here are
our wires. Let us see whether it is in the state in which
it was last time. We can soon tell. As yet, when I bring
them together, we have no power, because the conveyers —
what we call the electrodes — the passages or ways for the
electricity — are stopped; but now Mr. Anderson by that
[referring to a sudden flash at the ends of the wires] has
given me a telegram to say that it is ready. Before I begin
our experiment I will get Mr. Anderson to break contact
again at the battery behind me, and we will put a platinum
wire across to connect the poles, and then if I find I can
ignite a pretty good length of this wire we shall be safe in
our experiment. Now you will see the power. [The con-
nection was established, and the intermediate wire became
red-hot.] There is the power running beautifully through the
wire, which I have made thin on purpose to show you that we
have those powerful forces; and now, having that power,
we will proceed with it to the examination of water.
15 A solution af acetate of lead submitted to the action of the voltaic cur-
rent yields lead at the negative pole, and brown peroxide of lead at the posi-
tive pole. A solution of nitrate of silver, under the same circumstances,
yields silver at the negative pole, and peroxide of silver at the positive pole.
CHEMICAL HISTORY OF A CANDLE 135
I have here two pieces of platinum, and if I lay them
down upon this piece of paper [the moistened paper on the
tin-foil] you will see no action ; and if I take them up there
is no change that you can see, but the arrangement remains
just as it was before. But now see what happens: if I
take these two poles and put either one or the other of them
down separately on the platinum plates, they do nothing for
me; both are perfectly without action; but if I let them
both be in contact at the same moment, see what happens
[a brown spot appeared under each pole of the battery].
Look here at the effect that takes place, and see how I have
pulled something apart from the white — something brown;
and I have no doubt, if I were to arrange it thus, and
were to put one of the poles to the tin-foil on the other
side of the paper — why, I get such a beautiful action upon
the paper that I am going to see whether I can not write
with it — a telegram, if you please. [The lecturer here
traced the word " juvenile " on the paper with one of
the terminal wires.] See there how beautifully we can
get our results.
You see we have here drawn something which we have not
known about before out of this solution. Let us now take
that flask from Mr. Anderson's hands, and see what we
can draw out of that. This, you know, is a liquid which
we have just made up from copper and nitric acid while our
other experiments were in hand; and though I am making
this experiment very hastily, and may bungle a little, yet I
prefer to let you see what I do rather than prepare it
beforehand.
Now see what happens. These two platinum plates are the
two ends (or I will make them so immediately) of this
apparatus; and I am about to put them in contact with that
solution, just as we did a moment ago on the paper. It does
not matter to us whether the solution be on the paper or
whether it be in the jar, so long as we bring the ends of
the apparatus to it. If I put the two platinums in by them-
selves they come out as clean and as white as they go in
[inserting them into the fluid without connecting them with
the battery] ; but when we take the power and lay that on
[the platinums were connected with the battery and again
136 FARADAY
dipped into the solution], this, you see [exhibiting one of the
platinums], is at once turned into copper, as it were; it
has become like a plate of copper; and that [exhibiting the
other piece of platinum] has come out quite clean. If I
take this coppered piece and change sides, the copper
will leave the right-hand side and come over to the left
side; what was before the coppered plate comes out clean,
and the plate which was clean comes out coated with cop-
per; and thus you see that the same copper we put into
this solution we can also take out of it by means of this
instrument.
Putting that solution aside, let us now see what effect this
instrument will have upon water. Here are two little plati-
num plates which I intend to make the ends of the bat-
tery, and this (C) is a little vessel so shaped as to enable
me to take it to pieces, and show you its construction. In
these two cups (A and B) I pour mercury, which touches the
ends of the wires connected with the platinum plates. In
the vessel (C) I pour some water containing a little acid (but
which is put in only for the purpose of facilitating the
action; it undergoes no change in the process), and con-
nected with the top of the vessel is a bent glass tube (D),
which may remind you of the pipe which was connected with
the gun-barrel in our furnace experiment, and which now
passes under the jar (F). I have now adjusted this appara-
tus, and we will proceed to affect the water in some way or
other. In the other case I sent the water through a tube
which was made red-hot; I am now going to pass the elec-
tricity through the contents of this vessel. Perhaps I may
boil the water; if I do boil the water I shall get steam; and
you know that steam condenses when it gets cold, and you
will therefore see by that whether I do boil the water or not.
Perhaps, however, I shall not boil the water, but produce
some other effect. You shall have the experiment and see.
There is one wire which I will put to this side (A), and here
is the other wire which I will put to the other side (B), and
you will soon see whether any disturbance takes place. Here
it is seeming to boil up famously; but does it boil? Let us
see whether that which goes out is steam or not. I think you
will soon see the jar (F) will be filled with vapor, if that
CHEMICAL HISTORY OF A CANDLE
137
which rises from the water is steam. But can it be steam?
Why, certainly not; because there it remains, you see,
unchanged. There it is standing over the water, and it can
not therefore be steam, but must be a permanent gas of some
sort. What is it? Is it hydrogen? Is it anything else?
Well, we will examine it. If it is hydrogen it will burn.
Fig. 73
[The lecturer then ignited a portion of the gas collected,
which burnt with an explosion.] It is certainly something
combustible, but not combustible in the way that hydrogen is.
Hydrogen would not have given you that noise; but the
color of that light, when the thing did burn, was like that
of hydrogen; it will, however, burn without contact with
the air. That is why I have chosen this other form of appa-
ratus, for the purpose of pointing out to you what are the
particular circumstances of this experiment. In place of an
open vessel, I have taken one that is closed (our battery is
so beautifully active that we are even boiling the mercury,
and getting all things right — not wrong, but vigorously
right) ; and I am going to show you that that gas, what-
ever it may be, can burn without air, and in that respect
differs from a candle, which can not burn without the air.
And our manner of doing this is as follows: I have here a
glass vessel (G) which is fitted with two platinum wires
(IK) through which I can apply electricity; and we can
put the vessel on the air-pump and exhaust the air; and
when we have taken the air out we can bring it here and
138 FARADAY
fasten it on to this jar (F), and let into the vessel that gas
which was formed by the action of the voltaic battery upon
the water, and which we have produced by changing the
water into it; for I may go as far as this, and say we have
really, by that experiment, changed the water into that gas.
We have not only altered its condition, but we have
changed it really and truly into that gaseous substance, and
all the water is there which was decomposed by the experi-
ment. As I screw this vessel (G H) on here (H), and
make the tubes well connected, and when I open the stop-
cocks (H H H), if you watch the level of the water
(in F), you will see that the gas will rise. I will now
close the stop-cocks, as I have drawn up as much as the
vessel can hold, and being safely conveyed into that cham-
ber, I will pass into it an electric spark from this Ley-
den jar (L), when the vessel, which is now quite clear
and bright, will become dim. There will be no sound, for
the vessel is strong enough to confine the explosion. [A
spark was then passed through the jar, when the explo-
sive mixture was ignited.] Did you see that brilliant
light? If I again screw the vessel on to the jar, and
open these stop-cocks, you will see that the gas will rise a
second time. [The stop-cocks were then opened.] Those
gases [referring to the gases first collected in the jar, and
which had just been ignited by the electric spark] have dis-
appeared, as you see; their place is vacant, and fresh gas
has gone in. Water has been formed from them; and if
we repeat our operation [repeating the last experiment], I
shall have another vacancy, as you will see by the water
rising. I always have an empty vessel after the explosion,
because the vapor or gas into which that water has been
resolved by the battery explodes under the influence of the
spark, and changes into water; and by-and-by you will see
in this upper vessel some drops of water trickling down the
sides and collecting at the bottom.
We are here dealing with water entirely, without refer-
ence to the atmosphere. The water of the candle had the
atmosphere helping to produce it; but in this way it can be
produced independently of the air. Water, therefore, ought
to contain that other substance which the candle takes from
CHEMICAL HISTORY OF A CANDLE
139
the air, and which, combining with the hydrogen, produces
water.
Just now you saw that one end of this battery took hold
of the copper, extracting it from the vessel which contained
the blue solution. It was effected by this wire; and surely
we may say, if the battery has such power with a metallic
solution which we made and unmade, may we not find that
it is possible to split asunder the component parts of the
water, and put them into this place and that place? Sup-
pose I take the poles — the metallic ends of this battery —
and see what will happen with o H
the water in this apparatus (Fig.
74), where we have separated
the two ends far apart. I place
one here (at A), and the other
there (at B) ; and I have little
shelves with holes which I can
put upon each pole, and so ar-
range them that whatever es-
capes from the two ends of the
battery will appear as separate
gases; for you saw that the
water did not become vaporous, but gaseous. The wires
are now in perfect and proper connection with the vessel
containing the water, and you see the bubbles rising; let
us collect these bubbles and see what they are. Here is
a glass cylinder (O) ; I fill it with water and put it over
one end (A) of the pile, and I will take another (H), and
put it over the other end (B) of the pile. And so now we
have a double apparatus, with both places delivering gas.
Both these jars will fill with gas. There they go, that to the
right (H) filling very rapidly; the one to the left (O) filling
not so rapidly; and, though I have allowed some bubbles to
escape, yet still the action is going on pretty regularly; and
were it not that one is rather smaller than the other, you
would see that I should have twice as much in this (H) as
I have in that (O). Both these gases are colorless; they
stand over the water without condensing ; they are alike in all
things — I mean in all apparent things ; and we have here an
opportunity of examining these bodies and ascertaining what
Fig. 74
140 FARADAY
they are. Their bulk is large, and we can easily apply experi-
ments to them. I will take this jar (H) first, and will ask
you to be prepared to recognize hydrogen.
Think of all its qualities — the light gas which stood well
in inverted vessels, burning with a pale flame at the mouth
of the jar, and see whether this gas does not satisfy all these
conditions. If it be hydrogen it will remain here while I hold
this jar inverted. [A light was then applied, when the
hydrogen burnt.] What is there now in the other jar? You
know that the two together made an explosive mixture.
But what can this be which we find as the other constituent
in water, and which must therefore be that substance which
made the hydrogen burn? We know that the water we put
into the vessel consisted of the two things together. We find
one of these is hydrogen : what must that other be which was
in the water before the experiment, and which we now have
by itself? I am about to put this lighted splinter of wood
into the gas. The gas itself will not burn, but it will make the
splinter of wood burn. [The lecturer ignited the end of the
wood and introduced it into the jar of gas.] See how it
invigorates the combustion of the wood, and how it makes it
burn far better than the air would make it burn; and now
you see by itself that every other substance which is con-
tained in the water, and which, when the water was formed
by the burning of the candle, must have been taken from the
atmosphere. What shall we call it, A, B, or C? Let us call
it O — call it " Oxygen ; " it is a very good, distinct-sounding
name. This, then, is the oxygen which was present in the
water, forming so large a part of it.
We shall now begin to understand more clearly our
experiments and researches, because when we have examined
these things once or twice we shall soon see why a candle
burns in the air. When we have in this way analyzed the
water — that is to say, separated or electrolyzed its parts
out of it, we get two volumes of hydrogen and one of
the body that burns it. And these two are represented
to us on the following diagram, with their weights also
stated; and we shall find that the oxygen is a very heavy
body by comparison with the hydrogen. It is the other
element in water.
CHEMICAL HISTORY OF A CANDLE
141
I
Hydrogen
8
Oxygen
9
Oxygen 88.9
Hydrogen . . . . n.i
Water
100. o
I had better, perhaps, tell you now how we get this oxygen
abundantly, having shown you how we can separate it from
the water. Oxygen, as you will immediately imagine, exists
in the atmosphere; for how should the candle burn to pro-
duce water without it? Such a thing would be absolutely
impossible, and chemically impossible without oxygen. Can
we get it from the air? Well, there are some very compli-
cated and difficult processes by which we can get it from
> the air ; but we have
~~~ --—*--"- better processes. There
is a substance called the black oxide of man-
ganese; it is a very black-looking mineral,
but very useful, and when made red-hot it
gives out oxygen. Here is an iron bottle
which has had some of this substance put
into it, and there is a tube fixed to it, and
a fire ready made, and Mr. Anderson will
put that retort into the fire, for it is made
of iron, and can stand the heat. Here is
a salt called chlorate of potassa, which is
FlG * 75 now made in large quantities for bleach-
ing, and chemical and medical uses, and for pyrotechnic
and other purposes. I will take some and mix it with
some of the oxide of manganese (oxide of copper, or
oxide of iron would do as well), and if I put these to-
gether in a retort, far less than a red heat is sufficient
to evolve this oxygen from the mixture. I am not pre-
paring to make much, because we only want sufficient for
our experiments; only, as you will see immediately, if I
142 FARADAY
use too small a charge the first portion of the gas will be
mixed with the air already in the retort, and I should be
obliged to sacrifice the first portion of the gas because it
would be so much diluted with air; the first portion must
therefore be thrown away. You will find in this case that
a common spirit-lamp is quite sufficient for me to get the
oxygen, and so we shall have two processes going on for
its preparation. See how freely the gas is coming over
from that small portion of the mixture. We will examine
it and see what are its properties. Now in this way we are
Q producing, as you will observe, a gas just like the one
we had in the experiment with the battery, transparent,
undissolved by water, and presenting the ordinary vis-
ible properties of the atmosphere. (As this first jar
contains the air, together with the first portions of the
oxygen set free during the preparation, we will carry it
out of the way, and be prepared to make our experi-
ments in a regular, dignified manner.) And inasmuch
as that power of making wood, wax, or other things
burn, was so marked in the oxygen we obtained by
means of the voltaic battery from water, we may expect
to find the same property here. We will try it. You
76°' see there is the combustion of a lighted taper in air, and
here is its combustion in this gas [lowering the taper into
the jar.] See how brightly and how beautifully it burns!
You can also see more than this: you will perceive it is
a heavy gas, while the hydrogen would go up like a balloon,
or even faster than a balloon, when not encumbered with
the weight of the envelope. You may easily see that although
we obtained from water twice as much in volume of the
hydrogen as of oxygen, it does not follow that we have twice
as much in weight, because one is heavy and the other a
very light gas. We have means of weighing gases or air;
but, without stopping to explain that, let me just tell you
what their respective weights are. The weight of a pint of
hydrogen is three quarters of a grain; the weight of the
same quantity of oxygen is nearly twelve grains. This is a
very great difference. The weight of a cubic foot of hydro-
gen is one twelfth of an ounce; and the weight of a cubic
foot of oxygen is one ounce and a third. And so on we
CHEMICAL HISTORY OF A CANDLE 143
might come to masses of matter which may be weighed in the
balance, and which we can take account of as to hundred-
weights and as to tons, as you will see almost immediately.
Now, as regards this very property of oxygen supporting
combustion, which we may compare to air, I will take a piece
of candle to show it you in a rough way — and the result will
be rough. There is our candle burning in the air : how will it
burn in oxygen? I have here a jar of this gas, and I am
about to put it over the candle for you to compare the
action of this gas with that of the air. Why, look at it; it
looks something like the light you saw at the poles of the
voltaic battery. Think how vigorous that action must be.
And yet, during all that action, nothing more is produced
than what is produced by the burning of the candle in air.
We have the same production of water, and the same phe-
nomena exactly, when we use this gas instead of air, as we
have when the candle is burnt in air.
But now we have got a knowledge of this new substance,
we can look at it a little more distinctly, in order to satisfy
ourselves that we have got a good general understanding of
this part of the product of a candle. It is wonderful how
great the supporting powers of this substance are as regards
combustion. For instance, here is a lamp which, simple
though it be, is the original, I may say, of a great variety
of lamps which are constructed for divers purposes — for
light-houses, microscopic illuminations, and other uses; and
if it was proposed to make it burn very brightly, you would
say, "If a candle burnt better in oxygen, will not a lamp
do the same ? " Why, it will do so. Mr. Anderson will give
me a tube coming from our oxygen-reservoir, and I am
about to apply it to this flame, which I will previously make
burn badly on purpose. There comes the oxygen: what a
combustion that makes ! But if I shut it off, what becomes
of the lamp? [The flow of oxygen was stopped, and the lamp
relapsed to its former dimness.] It is wonderful how, by
means of oxygen, we get combustion accelerated. But it
does not affect merely the combustion of hydrogen, or car-
bon, or the candle, but it exalts all combustions of the com-
mon kind. We will take one which relates to iron, for
instance, as you have already seen iron burn a little in the
144
FARADAY
atmosphere. Here is a jar of oxygen, and this is a piece of
iron wire ; but if it were a bar as thick as my wrist, it would
burn the same. I first attach a little piece of
wood to the iron; I then set the wood on fire,
and let them both down together into the jar.
The wood is now alight, and there it burns as
wood should burn in oxygen; but it will soon
communicate its combustion to the iron. The
iron is now burning brilliantly, and will con-
tinue so for a long time. As long as we supply
IG# 77 oxygen, so long can we carry on the combustion
of the iron, until the latter is consumed.
We will now put that on one side, and take some other sub-
stance; but we must limit our experiments, for we have not
time to spare for all the illustrations you would have a right
to if we had more time. We will take a piece of sulphur:
you know how sulphur burns in the air; well, we put it into
the oxygen, and you will see that whatever can burn in air
can burn with a far greater intensity in oxygen, leading
you to think that perhaps the atmosphere itself owes all its
power of combustion to this gas. The sulphur is
now burning very quietly in the oxygen; but you
can not for a moment mistake the very high
and increased action which takes place when it
is so burnt, instead of being burnt merely in
common air.
I am now about to show you the combustion of
another substance — phosphorus. I can do it better
for you here than you can do it at home. This
is a very combustible substance; and if it be so
combustible in air, what might you expect it would be in
oxygen? I am about to show it to you not in its fullest
intensity, for if I did so we should almost blow the appa-
ratus up; I may even now crack the jar, though I do not
want to break things carelessly. You see how it burns in
the air. But what a glorious light it gives out when I intro-
duce it into oxygen ! [Introducing the lighted phosphorus
into the jar of oxygen.] There you see the solid particles
going off which cause that combustion to be so brilliantly
luminous.
Fig. 78
CHEMICAL HISTORY OF A CANDLE 145
Thus far we have tested this power of oxygen, and the
high combustion it produces by means of other substances.
We must now, for a little while longer, look at it as respects
the hydrogen. You know, when we allowed the oxygen
and the hydrogen derived from the water to mix and burn
together, we had a little explosion. You remember also that
when I burnt the oxygen and the hydrogen in a jet together,
we got very little light, but great heat; I am now about to
set fire to oxygen and hydrogen mixed in the proportion in
which they occur in water. Here is a vessel containing one
volume of oxygen and two volumes of hydrogen. This
mixture is exactly of the same nature as the gas we just
now obtained from the voltaic battery; it would be far too
much to burn at once; I have therefore arranged to blow
soap-bubbles with it and burn those bubbles, that we may
see by a general experiment or two how this oxygen sup-
ports the combustion of the hydrogen. First of all we will
see whether we can blow a bubble. Well, there goes the
gas [causing it to issue through a tobacco-pipe into some
soap-suds]. Here I have a bubble. I am receiving them
on my hand, and you will perhaps think I am acting oddly
in this experiment, but it is to show you that we must not
always trust to noise and sounds, but rather to real facts.
[Exploding a bubble on the palm of his hand.] I am afraid
to fire a bubble from the end of the pipe, because the explo-
sion would pass up into the jar and blow it to pieces. This
oxygen, then, will unite with the hydrogen, as you see by
the phenomena, and hear by the sound, with the utmost
readiness of action, and all its powers are then taken up in
its neutralization of the qualities of the hydrogen.
So now I think you will perceive the whole history of
water with reference to oxygen and the air from what we
have before said. Why does a piece of potassium decom-
pose water? Because it finds oxygen in the water. What
is set free when I put it in the water, as I am about to do
again? It sets free hydrogen, and the hydrogen burns; but
the potassium itself combines with oxygen ; and this piece of
potassium, in taking the water apart — the water, you may
say, derived from the combustion of the candle — takes
away the oxygen which the candle took from the air,
146 FARADAY
and so sets the hydrogen free; and even if I take
a piece of ice, and put a piece of potassium upon it, the
beautiful affinities by which the oxygen and the hydrogen
are related are such that the ice will absolutely set fire to
the potassium. I show this to you to-day in order to enlarge
your ideas of these things, and that you may see how
greatly results are modified by circumstances. There is the
potassium on the ice, producing a sort of volcanic action.
It will be my place when next we meet, having pointed
out these anomalous actions, to show you that none of these
extra and strange effects are met with by us — that none of
these strange and injurious actions take place when we are
burning, not merely a candle, but gas in our streets, or fuel
in our fireplaces, so long as we confine ourselves within the
laws that Nature has made for our guidance.
LECTURE V
OXYGEN PRESENT IN THE AIR— NATURE OF THE
ATMOSPHERE — ITS PROPERTIES — OTHER
PRODUCTS FROM THE CANDLE — CARBONIC
ACID — ITS PROPERTIES
WE have now seen that we can produce hydrogen
and oxygen from the water that we obtained from
the candle. Hydrogen, you know, comes from the
candle, and oxygen, you believe, comes from the air. But
then you have a right to ask me, " How is it that the air
and the oxygen do not equally well burn the candle ? " If
you remember what happened when I put a jar of oxygen
over a piece of candle, you recollect there was a very dif-
ferent kind of combustion to that which took place in the
air. Now why is this? It is a very important question, and
one I shall endeavor to make you understand ; it relates most
intimately to the nature of the atmosphere, and is most
important to us.
We have several tests for oxygen besides the mere burn-
ing of bodies; you have seen a candle burnt in oxygen or
in the air; you have seen phosphorus burnt in the air or in
oxygen, and you have seen iron filings burnt in oxygen.
But we have other tests besides these, and I am about to
refer to one or two of them for the purpose of carrying
your conviction and your experience farther. Here we have
a vessel of oxygen. I will show its presence to you: if I
take a little spark and put it into that oxygen, you know by
the experience you gained the last time we met what will
happen — if I put that spark into the jar, it will tell you
whether we have oxygen here or not. Yes ! We have
proved it by combustion ; and now here is another test for
oxygen, which is a very curious and useful one. I have
here two jars full of gas, with a plate between them to
147
148 FARADAY
prevent their mixing; I take the plate away, and the gases
are creeping one into the other. "What happens?" say
you; "they together produce no such combustion as was
seen in the case of the candle." But see how the presence
of oxygen is told by its association with this other sub-
stance.^ 6 ) What a beautifully colored gas I have obtained
in this way, showing me the presence of the oxygen! In
the same way we can try this experiment by mixing common
air with this test-gas. Here is a jar containing air — such
air as the candle would burn in, and here is a jar or bottle
containing the test-gas. I let them come together over
water, and you see the result: the contents of the test-bottle
are flowing into the jar of air, and you see I obtain exactly
the same kind of action as before, and that shows me that
there is oxygen in the air — the very same substance that has
been already obtained by us from the water produced by
the candle. But then, beyond that, how is it that the candle
does not burn in air as well as in oxygen? We will come
to that point at once. I have here two jars ; they are filled
to the same height with gas, and the appearance to the eye
is alike in both, and I really do not know at present which
of these jars contains oxygen and which contains air,
although I know they have previously been filled with these
gases. But here is our test-gas, and I am going to work
with the two jars, in order to examine whether there is any
difference between them in the quality of reddening this
gas. I am now going to turn this test-gas into one of the
jars, and observe what happens: There is reddening, you
see; there is, then, oxygen present. We will now test the
other jar; but you see this is not so distinctly red as the
first, and, farther, this curious thing happens: if I take
these two gases and shake them well together with water,
we shall absorb the red gas ; and then, if I put in more of
this test-gas and shake again, we shall absorb more; and I
can go on as long as there be any oxygen present to pro-
duce that effect. If I let in air, it will not matter; but the
moment I introduce water, the red gas disappears; and I
16 The gas which is thus employed as a test for the presence of oxygen is
the binoxide of nitrogen, or nitrous oxide. It is a colorless gas, which, when
brought in contact with oxygen, unites with it, forming hyponitric acid, the
red gas referred to.
CHEMICAL HISTORY OF A CANDLE 149
may go on in this way, putting in more and more of the
test-gas, until I come to something left behind which will
not redden any longer by the use of that particular body that
rendered the air and the oxygen red. Why is that? You
see in a moment it is because there is, besides oxygen,
something else present which is left behind. I will let a
little more air into the jar, and if it turns red you will
know that some of that reddening gas is still present, and
that, consequently, it was not for the want of this pro-
ducing body that that air was left behind.
Now you will begin to understand what I am about to
say. You saw that when I burnt phosphorus in a jar,
as the smoke produced by the phosphorus and the oxy-
gen of the air condensed, it left a good deal of gas unburnt,
just as this red gas left something untouched; there was,
in fact, this gas left behind, which the phosphorus can not
touch, which the reddening gas can not touch, and this
something is not oxygen, and yet is part of the atmosphere.
So that is one way of opening out air into the two things
of which it is composed — oxygen, which burns our candles,
our phosphorus, or any thing else, and this other substance
— nitrogen — which will not burn them. This other part of
the air is by far the larger proportion, and it is a very
curious body when we come to examine it; it is remarkably
curious, and yet you say, perhaps, that it is very uninterest-
ing. It is uninteresting in some respects because of this,
that it shows no brilliant effects of combustion. If I test it
with a taper as I do oxygen and hydrogen, it does not burn
like hydrogen, nor does it make the taper burn like oxygen.
Try it in any way I will, it does neither the one thing nor
the other; it will not take fire; it will not let the taper
burn; it puts out the combustion of every thing. There is
nothing that will burn in it in common circumstances. It
has no smell; it is not sour; it does not dissolve in water;
it is neither an acid nor an alkali; it is as indifferent to all
our organs as it is possible for a thing to be. And you
might say, "It is nothing; it is not worth chemical atten-
tion; what does it do in the air?" Ah! then come our
beautiful and fine results shown us by an observant phi-
losophy. Suppose, in place of having nitrogen, or nitrogen
150 FARADAY
and oxygen, we had pure oxygen as our atmosphere; what
would become of us? You know very well that a piece of
iron lit in a jar of oxygen goes on burning to the end.
When you see a fire in an iron grate, imagine where the
grate would go to if the whole of the atmosphere were
oxygen. The grate would burn up more powerfully than
the coals; for the iron of the grate itself is even more com-
bustible than the coals which we burn in it. A fire put into
the middle of a locomotive would be a fire in a magazine
of fuel, if the atmosphere were oxygen. The nitrogen
lowers it down and makes it moderate and useful for us,
and then, with all that, it takes away with it the fumes that
you have seen produced from the candle, disperses them
throughout the whole of the atmosphere, and carries them
away to places where they are wanted to perform a great
and glorious purpose of good to man, for the sustenance of
vegetation, and thus does a most wonderful work, although
you say, on examining it, " Why, it is a perfectly indifferent
thing." This nitrogen in its ordinary state is an inactive
element; no action short of the most intense electric force,
and then in the most infinitely small degree, can cause the
nitrogen to combine directly with the other element of the
atmosphere, or with other things round about it; it is a
perfectly indifferent, and therefore to say, a safe substance.
But, before I take you to that result, I must tell you
about the atmosphere itself. I have written on this diagram
the composition of one hundred parts of atmospheric air:
Bulk. Weight.
Oxygen 20 23.3
Nitrogen 80 77.7
100 100.0
It is a true analysis of the atmosphere so far as regards the
quantity of oxygen and the quantity of nitrogen present.
By our analysis, we find that 5 pints of the atmosphere con-
tain only 1 pint of oxygen, and 4 pints, or 4 parts, of nitro-
gen by bulk. That is our analysis of the atmosphere. It
requires all that quantity of nitrogen to reduce the oxygen
down, so as to be able to supply the candle properly with
fuel, so as to supply us with an atmosphere which our lungs
CHEMICAL HISTORY OF A CANDLE
151
can healthily and safely breathe; for it is just as important
to make the oxygen right for us to breathe, as it is to make
the atmosphere right for the burning of the fire and the
candle.
But now for this atmosphere. First of all, let me tell
you the weight of these gases. A pint of nitrogen weighs
10 4-10 grains, or a cubic foot weighs 1 1-6 ounces. That is
the weight of the nitrogen. The oxygen is
heavier: a pint of it weighs 11 9-10 grs., and a
cubic foot weighs 1 3-4 oz. A pint of air weighs
about 10 7-10 grs., and a cubic foot 1 1-5 oz.
You have asked me several times, and I am
very glad you have, "How do you weigh gases ?"
I will show you: it is very simple, and easily
done. Here is a balance, and here a copper bottle
made as light as we can consistent with due
strength, turned very nicely in the lathe, and
made perfectly air-tight, with a stop-cock, which
we can open and shut, which at present is open,
and, therefore, allows the bottle to be full of
air. I have here a nicely-adjusted balance in
which I think the bottle, in its present condition,
will be balanced by the weight on the other side.
And here is a pump by which we can force the
air into this bottle, and with it we will force in
a certain number of volumes of air as measured
by the pump. [Twenty measures were pumped
in.] We will shut that in and put it in the balance.
See how it sinks ; it is much heavier than it was.
By the air that we have forced into it by the pump. There
is not a greater bulk of air, but there is the same bulk of
heavier air, because we have forced in air upon it. And
that you may have a fair notion in your mind as to how
much this air measures, here is a jar full of water. We
will open that copper vessel into this jar, and let the air
return to its former state. All I have to do now is to
screw them tightly together, and to turn the taps, when
there, you see, is the bulk of the twenty pumps of air which
I forced into the bottle ; and to make sure that we have been
quite correct in what we have been doing, we will take the
Fig. 79
By what?
152
FARADAY
bottle again to the balance, and if it is now counterpoised
by the original weight, we shall be quite sure we have made
our experiment correctly. It is bal-
anced; so, you see, we can find out
the weight of the extra volumes of
air forced in in that way, and by
that means we are able to ascertain
that a cubic foot of air weighs i 1-5
oz. But that small experiment will
by no means convey to your mind
the whole literal truth of this mat-
ter. It is wonderful how it accumu-
lates when you come to larger vol-
umes. This bulk of air [a cubic foot]
weighs 1 1-5 oz. What do you think
of the contents of that box above
there which I have had made for the
purpose? The air which is within
that box weighs one pound — a full
pound; and I have calculated the
weight of the air in this room: you
would hardly imagine it, but it is above a ton. So rapidly
do the weights rise up, and so important is the presence of
the atmosphere, and of the oxygen and the nitrogen in
it, and the use it performs in conveying things to and fro
from place to place, and carrying bad vapors to places where
they will do good instead of harm.
Having given you that little illustration with respect to
the weight of the air, let me show you certain consequences
of it. You have a right to them, because you would not
understand so much without it. Do you remember this kind
of experiment? have you ever seen it? Suppose I take a
pump somewhat similar to the one I had a little while ago
to force air into the bottle, and suppose I place it in such
a manner that by certain arrangements I can apply my
hand to it. My hand moves about in the air so easily that
it seems to feel nothing, and I can hardly get velocity
enough by any motion of my own in the atmosphere to
make sure that there is much resistance to it. But when I
put my hand here [on the air-pump receiver, which was
Fig. 80
CHEMICAL HISTORY OF A CANDLE
153
afterward exhausted] you see what happens. Why is my
hand fastened to this place, and why am I able to pull this
Fig. 8i
pump about ? And see ! how is it that I can hardly get my
hand away? Why is this? It is the weight of the air —
the weight of the air that is above. I have another experi-
ment here, which I think will explain to you more about it.
When the air is pumped from underneath the bladder which
is stretched over this glass, you will see the effect in another
shape : the top is quite flat at present, but I will make a very
little motion with the pump, and now look at it; see how it
has gone down; see how it is bent in; you will see the
bladder go in more and more, until, at last, I expect it will
be driven in and broken by the force
of the atmosphere pressing upon it
[the bladder, at last, broke with a
loud report]. Now that was done
entirely by the weight of the air
pressing on it, and you can easily
understand how that is. The par-
ticles that are piled up in the at-
mosphere stand upon each other, as
these five cubes do; you can easily
conceive that four of these five cubes
are resting upon the bottom one, and
if I take that away the others will
all sink down. So it is with the
atmosphere; the air that is above is
sustained by the air that is beneath, and when the air is
pumped away from beneath them, the change occurs which
you saw when I placed my hand on the air-pump, and which
Fig. 82
154 FARADAY
you saw in the case of the bladder, and which you shall see
better here. I have tied over this jar a piece of sheet India-
rubber, and I am now about to take away the air from the
inside of the jar; and if you will watch the India-rubber —
which acts as a partition between the air below and the air
above — you will see, when I pump, how the pressure shows
itself. See where it is going to: I can actually put my hand
into the jar; and yet this result is only caused by the great
and powerful action of the air above. How beautifully it
shows this curious circumstance !
Here is something that you can have a pull at when I
have finished to-day. It is a little apparatus of two hollow
brass hemispheres, closely fitted together, and having con-
nected with it a pipe and a cock, through which we can
exhaust the air from the inside; and although the two
halves are so easily taken apart while the air is left within,
yet you will see, when we exhaust it by-and-by, no power
of any two of you will be able to pull them apart. Every
square inch of surface that is contained in the area of that
vessel sustains fifteen pounds by weight, or nearly so, when
the air is taken out, and you may try your strength presently
in seeing whether you can overcome that pressure of the
atmosphere.
Here is another very pretty thing — the boys' sucker, only
refined by the philosopher. We young ones have a perfect
right to take toys, and make them into philosophy, inasmuch
as nowadays we are turning philosophy into toys. Here is
a sucker, only it is made of India-rubber. If I clap it upon
the table, you see at once it holds. Why does it hold? I
can slip it about, and yet if I try to pull it up, it seems as
if it would pull the table with it. I can easily make it slip
about from place to place, but only when I bring it to the
edge of the table can I get it off. It is only kept down by
the pressure of the atmosphere above; we have a couple of
them, and if you take these two and press them together,
you will see how firmly they stick. And, indeed, we may
use them as they are proposed to be used, to stick against
windows or against walls, where they will adhere for an
evening, and serve to hang any thing on that you want. I
think, however, that you boys ought to be shown experi-
CHEMICAL HISTORY OF A CANDLE 155
merits that you can make at home; and so here is a very
pretty experiment in illustration of the pressure of the
atmosphere. Here is a tumbler of water. Suppose I were
to ask you to turn that tumbler upside down so that the
water should not fall out, and yet not be kept in by your
hand, but merely by using the pressure of the atmosphere;
could you do that? Take a wine-glass, either quite full or
half full of water, and put a flat card on the top; turn it
upside down, and then see what becomes of the card and
of the water. The air can not get in because the water, by
its capillary attraction round the edge, keeps it out.
I think this will give you a correct notion of what you
may call the materiality of the air ; and when I tell you that
that box holds a pound of it, and this room more than a ton,
you will begin to think that air is something very serious.
I will make another experiment to convince you of this
positive resistance. There is that beautiful experiment of
the popgun, made so well and so easily, you know, out of a
quill, or a fcube, or any thing of that kind; where we take
a slice of potato, for instance, or an apple, and take
the tube and cut out a pellet, as I have now done, and push
it to one end. I have made that end tight ; and now I take
another piece and put it in: it will confine the air that is
within the tube perfectly and completely for our- purpose;
and I shall now find it absolutely impossible by any force
of mine to drive that little pellet close up to the other. It
can not be done; I may press the air to a certain extent,
but if I go on pressing, long before it comes to the second
the confined air will drive the front one out with a force
something like that of gunpowder ; for gunpowder is in part
dependent upon the same action that you see here exem-
plified.
I saw the other day an experiment which pleased me
much, as I thought it would serve our purpose here. (I
ought to have held my tongue for four or five minutes
before beginning this experiment, because it depends upon
my lungs for success.) By the proper application of air, I
expect to be able to drive this egg out of one cup into the
other by the force of my breath ; but if I fail it is in a good
cause, and I do not promise success, because I have been
156 FARADAY
talking- more than I ought to do to make the experiment
succeed.
[The lecturer here tried the experiment, and succeeded in
blowing the egg from one egg-cup to the other.]
You see that the air which I blow goes downward between
the egg and the cup, and makes a blast under the egg, and
is thus able to lift a heavy thing; for a full egg is a very
heavy thing for air to lift. If you want to make the experi-
ment, you had better boil the egg quite hard first, and then
you may very safely try to blow it from one cup to the
other with a little care.
I have now kept you long enough upon this property of
the weight of the air, but there is another thing I should like
to mention. You saw the way in which, in this popgun, I
was able to drive the second piece of potato half or two
thirds of an inch before the first piece started, by virtue of
the elasticity of the air, just as I pressed into the copper
bottle the particles of air by means of the pump. Now this
depends upon a wonderful property in the air, namely, its
elasticity, and I should like to give you a good illustration
of this. If I take any thing that confines the air properly,
as this membrane, which also is able to contract and expand
so as to give us a measure of the elasticity of the air, and
confine in this bladder a certain portion of air; and then, if
we take the atmosphere off from the outside of it, just as in
these cases we put the pressure on — if we take the pressure
off, you will see how it will then go on expanding and ex-
panding, larger and larger, until it will fill the whole of this
bell-jar, showing you that w T onderful property of the air, its
elasticity, its compressibility, and expansibility, to an exceed-
ingly large extent, and which is very essential for the pur-
poses and services it performs in the economy of creation.
We will now turn to another very important part of our
subject, remembering that we have examined the candle in
its burning, and have found that it gives rise to various
products. We have the products, you know, of soot, of
water, and of something else, which you have not yet exam-
ined. We have collected the water, but have allowed the
other things to go into the air. Let us now examine some
of these other products.
CHEMICAL HISTORY OF A CANDLE
157
Here is an experiment which I think will help you in part
in this way. We will put our candle there, and place over it
a chimney, thus. I think my candle will go on burning,
because the air-passage is open at the bottom and the top.
In the first place, you see the moisture appearing — that you
know about. It is water produced from the candle by the
action of the air upon its hydrogen. But, besides that, some-
thing is going out at the top: it is not moisture — it is not
water — it is not condensible; and yet, after all, it has very
singular properties. You will find that the air coming out
Fig. 83
of the top of our chimney is nearly sufficient to blow the
light out I am holding to it ; and if I put the light fairly
opposed to the current, it will blow it quite out. You will
say, that is as it should be, and I am supposing that you
think it ought to do so, because the nitrogen does not sup-
port combustion, and ought to put the candle out, since the
candle will not burn in nitrogen. But is there nothing else
there than nitrogen ? I must now anticipate — that is to say, I
must use my own knowledge to supply you with the means
that we adopt for the purpose of ascertaining these things,
and examining such gases as these. I will take an empty
bottle — here is one — and if I hold it over this chimney, I
158 FARADAY
shall get the combustion of the candle below sending its
results into the bottle above ; and we shall soon find that this
bottle contains not merely an air that is bad as regards the
combustion of a taper put into it, but having other
properties.
Let me take a little quick-lime and pour some common
water on to it — the commonest water will do. I will stir it a
moment, then pour it upon a piece of filtering paper in a
funnel, and we shall very quickly have a clear water pro-
ceeding to the bottle below, as I have here. I have plenty of
this water in another bottle, but nevertheless I should like to
use the lime-water that was prepared before you, so that you
may see what its uses are. If I take some of this beautiful
clear lime-water, and pour it into this jar which has collected
the air from the candle, you will see a change coming about.
Do you see that the water has become quite milky? Ob-
serve, that will not happen with air merely. Here is a bot-
tle filled with air; and if I put a little lime-water into it,
neither the oxygen nor the nitrogen, nor any thing else that
is in that quantity of air, will make any change in the lime-
water; it remains perfectly clear, and no shaking of that
quantity of lime-water with that quantity of air in its com-
mon state will cause any change; but if I take this bottle
with the lime-water and hold it so as to get the general
products of the candle in contact with it, in a very short time
we shall have it milky; there is the chalk, consisting of the
lime which we used in making the lime-water, combined
with something that came from the candle — that other
product which we are in search of, and which I want to
tell you about today. This is a substance made visible to us
by its action, which is not the action of the lime-water either
upon the oxygen or upon the nitrogen, nor upon the water
itself, but it is something new to us from the candle. And
then we find this white powder, produced by the lime-water
and the vapor from the candle, appears to us very much like
whitening or chalk, and when examined it does prove to be
exactly the same substance as whitening or chalk. So we
are led, or have been led, to observe upon the various cir-
cumstances of this experiment, and to trace this production
of chalk to its various causes, to give us the true knowledge
CHEMICAL HISTORY OF A CANDLE 159
of the nature of this combustion of the candle — to find that
this substance issuing from the candle is exactly the same
as that substance which would issue from a retort if I were
to put some chalk into it with a little moisture and make it
red-hot ; you would then find that exactly the same substance
would issue from it as from the candle.
But we have a better means of getting this substance, and
in greater quantity, so as to ascertain what its general char-
acters are. We find this substance in very great abundance
in a multitude of cases where you would least expect it.
All limestones contain a great deal of this gas which issues
from the candle, and which we call carbonic acid. All
chalks, all shells, all corals, contain a great quantity of this
curious air. We find it fixed in these stones, for which
reason Dr. Black called it " fixed air " — finding it in these
fixed things like marble and chalk — he called it fixed air
because it lost its quality of air, and assumed the condition
of a solid body. We can easily get this air from marble.
Here is a jar containing a little muriatic acid, and here is
a taper which, if I put it into that jar, will show only the
presence of common air. There is, you see, pure air down
to the bottom; the jar is full of it. Here is a substance —
marble ( 17 ) — a very beautiful and superior marble; and if I
put these pieces of marble into the jar, a great boiling
apparently goes on. That, however, is not steam ; it is a
gas that is rising up; and if I now search the jar by a
candle, I shall have exactly the same effect produced upon
the taper as I had from the air which issued from the end
of the chimney over the burning candle. It is exactly the
same action, and caused by the very same substance that
issued from the candle ; and in this way we can get carbonic
acid in great abundance — we have already nearly filled the
jar. We also find that this gas is not merely contained in
marble. Here is a vessel in which I have put some common
whitening — chalk which has been washed in water and de-
prived of its coarser particles, and so supplied to the plas-
terer as whitening — here is a large jar containing this
whitening and water ; and I have here some strong sulphuric
* 7 Marble is a compound of carbonic acid and lime. The muriatic acid,
being the stronger of the two, takes the place of the carbonic acid, which
escapes as a gas, the residue forming muriate of lime or chloride of calcium.
160 FARADAY
acid, which is the acid you might have to use if you were
to make these experiments (only, in using this acid with
limestone, the body that is produced is an insoluble sub-
stance, whereas the muriatic acid produces a soluble sub-
stance that does not so much thicken the water). And you
will seek out a reason why I take this kind of apparatus for
the purpose of showing this experiment. I do it because
you may repeat in a small way what I am about to do in a
large one. You will have here just the same kind of action;
and I am evolving in this large jar carbonic acid exactly
the same in its nature and properties as the gas which we
obtained from the combustion of the candle in the atmos-
phere. And, no matter how different the two methods by
which we prepare this carbonic acid, you will see, when we
get to the end of our subject, that it is all exactly the same,
whether prepared in the one way or in the other.
We will now proceed to the next experiment with regard
to this gas. What is its nature? Here is one of the vessels
full, and we will try it as we have done so many other gases
— by combustion. You see it is not combustible, nor does it
support combustion. Neither, as we know, does it dissolve
much in water, because we collect it over water very easily.
Then you know that it has an effect, and becomes white in
contact with lime-water; and when it does become white
in that way, it becomes one of the constituents to make car-
bonate of lime or limestone.
The next thing I must show you is that it really does dis-
solve a little in water, and therefore that it is unlike oxygen
and hydrogen in that respect. I have here an apparatus by
which we can produce this solution. In the lower part of
this apparatus is marble and acid, and in the upper part cold
water. The valves are so arranged that the gas can get
from one to the other. I will set it in action now, and you
can see the gas bubbling up through the water, as it has been
doing all night long, and by this time we shall find that we
have this substance dissolved in the water. II I take a glass
and draw off some of the water, I find that it tastes a little
acid to the mouth ; it is impregnated with carbonic acid ; and
if I now apply a little lime-water to it, that will give us a
test of its presence. This water will make the lime-water
CHEMICAL HISTORY OF A CANDLE
161
turbid and white, which is proof of the presence of carbonic
acid.
Then it is a very weighty gas; it is heavier than the
atmosphere. I have put their respective weights at the
lower part of this table, along with, for comparison, the
weights of the other gases we have been examining:
Pint. Cubic Foot.
Hydrogen f grs. T V oz.
Oxygen 11^ ii-
Nitrogen io^y I J
Air 10^ l£
Carbonic acid l6i
TV
A pint of it weighs 16 1-3 grs., and a cubic foot weighs 1 9-10
oz., almost two ounces. You can see by many experiments
that this is a heavy gas. Suppose I take a glass contain-
ing nothing else but air, and from this vessel containing
the carbonic acid I attempt to pour a little of this gas into
that glass — I wonder whether any has gone in or not. I
can not tell by the appearance, but I can in this way [intro-
ducing the taper]. Yes, there it is, you see; and if I were to
examine it by lime-water, I should find it by that test also.
I will take this little bucket, and put it
down into the well of carbonic acid —
indeed, we too often have real wells of
carbonic acid — and now, if there is
any carbonic acid, I must have got to
it by this time, and it will be in this
bucket, which we will examine with a
taper. There it is, you see: it is full
of carbonic acid.
There is another experiment by
which I will show you its weight.
I have here a jar suspended at one
end of a balance — it is now equipoised ;
but when I put this carbonic acid into
the jar on the one side which now con-
tains air, you will see it sink down
the carbonic acid that I pour into
Fig. 84
at once because of
it. And now, if I
examine this jar with the lighted taper, I shall find that
the carbonic acid has fallen into it, and it no longer
has any power of supporting the combustion. If I blow
hc vol. xxx — F
162
FARADAY
a soap-bubble, which of course will be filled with air,
and let it fall into this jar of carbonic acid it will
float. But I shall first of all take one of these little
balloons filled with air. I am not quite sure where the
carbonic acid is; we will just try the depth, and see where-
abouts is its level. There, you see, we have this bladder
Fig. 85
floating on the carbonic acid; and if I evolve some more of
the carbonic acid, the bladder will be lifted up higher.
There it goes; the jar is nearly full, and now I will see
whether I can blow a soap-bubble on that and float it in the
same way. [The lecturer here blew a soap-bubble and
allowed it to fall into the jar of carbonic acid, when it
floated in it midway.] It is floating as the balloon floated
by virtue of the greater weight of the carbonic acid than of
the air. And now, having so far given you the history of
the carbonic acid, as to its sources in the candle, as to its
physical properties and weight, when we next meet I shall
show you of what it is composed, and where it gets its ele-
ments from.
LECTURE VI
CARBON OR CHARCOAL— COAL-GAS— RESPIRA-
TION AND ITS ANALOGY TO THE BURN-
ING OF A CANDLE— CONCLUSION
A LADY who honors me by her presence at these lectures
has conferred a still farther obligation by sending me
these two candles, which are from Japan, and, I pre-
sume, are made of that substance to which I referred in a
former lecture. You see that they are even far more highly
ornamented than the French candles, and, I suppose, are
candles of luxury, judging from their appearance. They have
a remarkable peculiarity about them, namely, a hollow wick —
that beautiful peculiarity which Argand introduced into the
lamp and made so valuable. To those who receive such
presents from the East, I may just say that this and such
like materials gradually undergo a change which gives them
on the surface a dull and dead appearance; but they may
easily be restored to their original beauty if the surface be
rubbed with a clean cloth or silk handkerchief, so as to
polish the little rugosity or roughness: this will restore the
beauty of the colors. I have so rubbed one of these can-
dles, and you see the difference between it and the other
which has not been polished, but which may be restored by
the same process. Observe, also, that these moulded candles
from Japan are made more conical than the moulded candles
in this part of the world.
I told you, when we last met, a good deal about carbonic
acid. We found by the lime-water test that when the vapor
from the top of the candle or lamp was received into bottles
and tested by this solution of lime-water (the composition of
which I explained to you, and which you can make for your-
selves), we had that white opacity which was in fact cal-
163
164 FARADAY
careous matter, like shells and corals, and many of the rocks
and minerals in the earth. But I have not yet told you fully
and clearly the chemical history of this substance, carbonic
acid, as we have it from the candle, and I must now resume
that subject. We have seen the products, and the nature of
them, as they issue from the candle. We have traced the
water to its elements, and now we have to see where are the
elements of the carbonic acid supplied by the candle : a few
experiments will show this. You remember that when a
candle burns badly it produces smoke; but if it is burning
well, there is no smoke. And you know that the brightness
of the candle is due to this smoke, which becomes ignited.
Here is an experiment to prove this: so long as the smoke
remains in the flame of the candle and becomes ignited, it
gives a beautiful light, and never appears to us in the form
of black particles. I will light some fuel which is extrava-
gant in its burning: this will serve our purpose — a little
turpentine on a sponge. You see the smoke rising from it,
and floating in L o the air in large quantities; and remember
now, the carbonic acid that we have from the candle is from
such smoke as that. To make that evident to you, I will
introduce this turpentine burning on the sponge into a flask
where I have plenty of oxygen, the rich part of the atmos-
phere, and you now see that the smoke is all consumed. This
is the first part of our experiment; and now what follows?
The carbon which you saw flying off from the turpentine
flame in the air is now entirely burned in this oxygen, and
we shall find that it will, by this rough and temporary experi-
ment, give us exactly the same conclusion and result as we
had from the combustion of the candle. The reason why
I make the experiment in this manner is solely that I may
cause the steps of our demonstration to be so simple that you
can never for a moment lose the train of reasoning, if you
only pay attention. All the carbon which is burned in
oxygen, or air, comes out as carbonic acid, while those par-
ticles which are not so burned show you the second substance
in the carbonic acid, namely, the carbon — that body which
made the flame so bright while there was plenty of air, but
which was thrown off in excess when there was not oxygen
enough to burn it.
CHEMICAL HISTORY OF A CANDLE 165
I have also to show you a little more distinctly the his-
tory of carbon and oxygen in their union to make carbonic
acid. You are now better able to understand this than
before, and I have prepared three or four experiments by
way of illustration. This jar is filled with oxygen, and here
is some carbon which has been placed in a crucible for the
purpose of being made red-hot. I keep my jar dry, and ven-
ture to give you a result imperfect in some degree, in order
that I may make the experiment brighter. I am about to
put the oxygen and the carbon together. That this is carbon
(common charcoal pulverized) you will see by the way in
which it burns in the air [letting some of the red-hot char-
coal fall out of the crucible]. I am now about to burn it in
oxygen gas, and look at the difference. It may appear to you
at a distance as if it were burning with a flame : but it is not
so. Every little piece of charcoal is burning as a spark, and
while it so burns it is producing carbonic acid. I specially
want these two or three experiments to point out what I
shall dwell upon more distinctly by-and-by — that carbon burns
in this way, and not as a flame.
Instead of taking many particles of carbon to burn I will
take a rather large piece, which will enable you to see the
form and size, and to trace the effects very decidedly. Here
is the jar of oxygen, and here is the piece of charcoal, to
which I have fastened a little piece of wood, which I can set
fire to, and so commence the combustion, which I could not
conveniently do without. You now see the charcoal burn-
ing, but not as a flame (or if there be a flame it is the
smallest possible one, which I know the cause of, namely,
the formation of a little carbonic oxide close upon the sur-
face of the carbon). It goes on burning, you see, slowly
producing carbonic acid by the union of this carbon or
charcoal (they are equivalent terms) with the oxygen. I
have here another piece of charcoal, a piece of bark, which
has the quality of being blown to pieces — exploding — as it
burns. By the effect of the heat we shall reduce the lump
of carbon into particles that will fly off; still every particle,
equally with the whole mass, burns in this peculiar way —
it burns as a coal and not like a flame. You observe a mul-
titude of little combustions going on, but no flame. I do
166 FARADAY
not know a finer experiment than this to show that carbon
burns with a spark.
Here, then, is carbonic acid formed from its elements. It
is produced at once; and if we examined it by lime-water
you will see that we have the same substance which I have
previously described to you. By putting together 6 parts
of carbon by weight (whether it comes from the flame of a
candle or from powdered charcoal) and 16 parts of oxygen by
weight, we have 22 parts of carbonic acid; and, as we saw
last time, the 22 parts of carbonic acid combined with 28 parts
of lime, produced common carbonate of lime. If you were to
examine an oyster-shell and weigh the component parts, you
would find that every 50 parts would give 6 of carbon and 16
of oxygen combined with 28 of lime. However, I do not want
to trouble you with these minutiae ; it is only the general phi-
losophy of the matter that we can now go into. See how finely
the carbon is dissolving away [pointing to the lump of char-
coal burning quietly in the jar of oxygen]. You may say
that the charcoal is actually dissolving in the air round
about; and if that were perfectly pure charcoal, which we
can easily prepare, there would be no residue whatever.
When we have a perfectly cleansed and purified piece of car-
bon, there is no ash left. The carbon burns as a solid dense
body, that heat alone can not change as to its solidity, and
yet it passes away into vapor that never condenses into solid
or liquid under ordinary circumstances; and what is more
curious still is the fact that the oxygen does not change
in its bulk by the solution of the carbon in it. Just as the
bulk is at first, so it is at last, only it has become carbonic
acid.
There is another experiment which I must give you before
you are fully acquainted with the general nature of carbonic
acid. Being a compound body, consisting of carbon and
oxygen, carbonic acid is a body that we ought to be able to
take asunder. And so we can. As we did with water, so
we can with carbonic acid — take the two parts asunder.
The simplest and quickest way is to act upon the carbonic
acid by a substance that can attract the oxygen from it,
and leave the carbon behind. You recollect that I took
potassium and put it upon water or ice, and you saw that it
CHEMICAL HISTORY OF A CANDLE 167
could take the oxygen from the hydrogen. Now, suppose we
do something of the same kind here with this carbonic acid.
You know carbonic acid to be a heavy gas: I will not test
it with lime-water, as that will interfere with our subsequent
experiments, but I think the heaviness of the gas and the
power of extinguishing flame will be sufficient far our pur-
pose. I introduce a flame into the gas, and you will see
whether it will be put out. You see the light is extinguished.
Indeed, the gas may, perhaps, put out phosphorus, which
you know has a pretty strong combustion. Here is a piece
of phosphorus heated to a high degree. I introduce it into
gas, and you observe the light is put out, but it will take
fire again in the air, because there it re-enters into combus-
tion. Now let me take a piece of potassium, a substance
which even at common temperatures can act upon carbonic
acid, though not sufficiently for our present purpose, be-
cause it soon gets covered with a protecting coat; but if we
warm it up to the burning point in air, as we have a fair
right to do, and as we have done with phosphorus, you will
see that it can burn in carbonic acid; and if it burns, it
will burn by taking oxygen, so that you will see what is left
behind. I am going, then, to burn this potassium in the
carbonic acid, as a proof of the existence of oxygen in the
carbonic acid. [In the preliminary process of heating the
potassium exploded.] Sometimes we get an awkward piece
of potassium that explodes, or something like it, when it
burns. I will take another piece, and now that it is heated
I introduce it into the jar, and you perceive that it burns in
the carbonic acid — not so well as in the air, because the
carbonic acid contains the oxygen combined; but it does
burn, and takes away the oxygen. If I now put this potas-
sium into water, I find that besides the potash formed
(which you need not trouble about) there is a quantity of
carbon produced. I have here made the experiment in a
very rough way, but I assure you that if I were to make it
carefully, devoting a day to it instead of five minutes, we
should get all the proper amount of charcoal left in the
spoon, or in the place where the potassium was burned, so
that there could be no doubt as to the result. Here, then,
is the carbon obtained from the carbonic acid, as a common
168 FARADAY
black substance; so that you have the entire proof of the
nature of carbonic acid as consisting of carbon and oxygen.
And now I may tell you, that whenever carbon burns under
common circumstances it produces carbonic acid.
Suppose I take this piece of wood, and put it into a
bottle with lime-water. I might shake that lime-water up
with wood and the atmosphere as long as I pleased, it would
still remain clear as you see it; but suppose I burn the
piece of wood in the air of that bottle. You, of course,
know I get water. Do I get carbonic acid? [The experi-
ment was performed.] There it is, you see — that is to say,
the carbonate of lime, which results from carbonic acid, and
that carbonic acid must be formed from the carbon which
comes from the wood, from the candle, or any other thing.
Indeed, you have yourselves frequently tried a very pretty
experiment, by which you may see the carbon in wood.
If you take a piece of wood, and partly burn it, and then
blow it out, you have carbon left. There are things that
do not show carbon in this way. A candle does not so show
it, but it contains carbon. Here also is a jar of coal-gas,
which produces carbonic acid abundantly; you do not see
the carbon, but we can soon show it to you. I will light it,
and as long as there is any gas in the cylinder it will go on
burning. You see no carbon, but you see a flame, and be-
cause that is bright it will lead you to guess that there is
carbon in the flame. But I will show it to you by another
process. I have some of the same gas in another vessel,
mixed with a body that will burn the hydrogen of the gas,
but will not burn the carbon. I will light them with a burn-
ing taper, and you perceive the hydrogen is consumed, but
not the carbon, which is left behind as a dense black smoke.
I hope that by these three or four experiments you will
learn to see when carbon is present, and understand what
are the products of combustion when gas or other bodies
are thoroughly burned in the air.
Before we leave the subject of carbon, let us make a few
experiments and remarks upon its wonderful condition as
respects ordinary combustion. I have shown you that the
carbon, in burning, burns only as a solid body, and yet you
perceive that, after it is burned, it ceases to be a solid. There
CHEMICAL HISTORY OF A CANDLE 169
are very few fuels that act like this. It is, in fact, only that
great source of fuel, the carbonaceous series, the coals,
charcoals, and woods, that can do it. I do not know that
there is any other elementary substance besides carbon that
burns with these conditions; and if it had not been so, what
would happen to us? Suppose all fuel had been like iron,
which, when it burns, burns into a solid substance. We
could not then have such a combustion as you have in this
fireplace. Here also is another kind of fuel which burns
very well — as well as, if not better, than carbon — so well,
indeed, as to take fire of itself when it is in the air, as you
see. [Breaking a tube full of lead pyrophorus.] This sub-
stance is lead, and you see how wonderfully combustible it
is. It is very much divided, and is like a heap of coals in
the fireplace : the air can get to its surface and inside, and so
it burns. But why does it not burn in that way now when it
is lying in a mass? [Emptying the contents of the tube in a
heap on to a plate of iron.] Simply because the air can not
get to it. Though it can produce a great heat, the great heat
which we want in our furnaces and under our boilers, still
that which is produced can not get away from the portion
which remains unburned underneath, and that portion,
therefore, is prevented from coming in contact with the
atmosphere, and can not be consumed. How different is
that from carbon! Carbon burns just in the same way as
this lead does, and so gives an intense fire in the furnace,
or wherever you choose to burn it; but then the body pro-
duced by its combustion passes away, and the remaining
carbon is left clear. I showed you how carbon went on
dissolving in the oxygen, leaving no ash, whereas here
[pointing to the heap of pyrophorus] we have actually more
ash than fuel, for it is heavier by the amount of the oxygen
which has united with it. Thus you see the difference
between carbon and lead or iron — if we choose iron, which
gives so wonderful a result in our applications of this fuel,
either as light or heat. If, when the carbon burnt, the prod-
uct went off as a solid body, you would have had the room
filled with an opaque substance, as in the case of the phos-
phorus; but when carbon burns, every thing passes up into
the atmosphere. It is in a fixed, almost unchangeable con-
170
FARADAY
dition before the combustion; but afterward it is in the form
of gas, which it is very difficult (though we have succeeded)
to produce in a solid or liquid state.
Now I must take you to a very interesting part of our sub-
ject — to the relation between the combustion of a candle
and that living kind of combustion which goes on within us.
In every one of us there is a living process of combustion
going on very similar to that of a candle, and I must try to
Fig. 86
make that plain to you. For it is not merely true in a
poetical sense — the relation of the life of man to a taper;
and if you will follow, I think I can make this clear. In
order to make the relation very plain, I have devised a little
apparatus which we can soon build up before you. Here is
a board, and a groove cut in it, and I can close the groove at
the top part by a little cover ; I can then continue the groove
as a channel by a glass tube at each end, there being a
free passage through the whole. Suppose I take a taper or
candle (we can now be liberal in our use of the word
" candle," since we understand what it means), and place it
in one of the tubes ; it will go on, you see, burning very well.
You observe that the air which feeds the flame passes down
the tube at one end, then goes along the horizontal tube,
and ascends the tube at the other end in which the taper
is placed. If I stop the aperture through which the air
enters, I stop combustion, as you perceive. I stop the supply
of air, and consequently the candle goes out. But now what
will you think of this fact? In a former experiment I
showed you the air going from one burning candle to a
CHEMICAL HISTORY OF A CANDLE
171
second candle. If I took the air proceeding from another
candle, and sent it down by a complicated arrangement into
this tube, I should put this burning candle out. But what
will you say when I tell you that my breath will put out
that candle? I do not mean by blowing at all, but simply
that the nature of my breath is such that a candle can not
burn in it. I will now hold my mouth over the aperture,
and without blowing the flame in any way, let no air enter
the tube but what comes from my mouth. You see the
result. I did not blow the candle out.
I merely let the air which I expired
pass into the aperture, and the result
was that the light went out for want of
oxygen, and for no other reason. Some-
thing or other — namely, my lungs — had
taken away the oxygen from the air and
there was no more to supply the com-
bustion of the candle. It is, I think, very
pretty to see the time it takes before
the bad air which I throw into this part
of the apparatus has reached the can-
dle. The candle at first goes on burn-
ing, but so soon as the air has had time
to reach it it goes out. And now I
will show you another experiment, be-
cause this is an important part of our
philosophy. Here is a jar which contains fresh air, as
you can see by the circumstance of a candle or gas-
light burning it. I make it close for a little time, and
by means of a pipe I get my mouth over it so that I
can inhale the air. By putting it over water, in the
way that you see, I am able to draw up this air (supposing
the cork to be quite tight), take it into my lungs, and throw
it back into the jar: we can then examine it, and see the
result. You observe, I first take up the air, and then throw
it back, as is evident from the ascent and descent of the
water; and now, by putting a taper into the air, you will
see the state in which it is by the light being extinguished.
Even one inspiration, you see, has completely spoiled this air,
so that it is no use my trying to breathe it a second time.
Fig. 87
172
FARADAY
Now you understand the ground of the impropriety of many
of the arrangements among the houses of the poorer classes,
by which the air is breathed over and over again for the
want of a supply, by means of proper ventilation, sufficient
to produce a good result. You see how bad the air becomes
by a single breathing, so that you can easily understand how
essential fresh air is to us.
To pursue this a little farther, let us see what will happen
with lime-water. Here is a globe which contains a little
lime-water, and it is so arranged as re-
gards the pipes as to give access to the
air within, so that we can ascertain the
effect of respired or unrespired air upon
it. Of course I can either draw in air
(through A), and so make the air that
feeds my lungs go through the lime-water,
or I can force the air out of my lungs
through the tube (B), which goes to the
bottom, and so show its effect upon the
lime-water. You will observe that how-
ever long I draw the external air into the
lime-water and then through it to my
I shall produce no effect upon the water — it will
not make the lime-water turbid; but if I throw the air
from my lungs through the lime-water several times in
succession, you see how white and milky the water is get-
ting, showing the effect which expired air has had upon
it; and now you begin to know that the atmosphere which
we have spoiled by respiration is spoiled by carbonic acid,
for you see it here in contact with the lime-water.
I have here two bottles, one containing lime-water and the
other common water, and tubes which pass into the bottles
and connect them. The apparatus is very rough, but it is
useful notwithstanding. If I take these two bottles, in-
haling here and exhaling there, the arrangement of the tubes
will prevent the air going backward. The air coming in
will go to my mouth and lungs, and in going out will pass
through the lime-water, so that I can go on breathing, and
making an experiment very refined in its nature and very
good in its results. You will observe that the good air has
Fig. 88
lungs,
CHEMICAL HISTORY OF A CANDLE
173
done nothing to the lime-water; in the other case, nothing
has come to the lime-water but my respiration, and you see
the difference in the two cases.
Let us now go a little farther. What is all this process
going on within us which we can not do without, either day
or night, which is so provided
for by the Author of all things
that He has arranged that it
shall be independent of all will?
If we restrain our respiration,
as we can to a certain extent,
we should destroy ourselves.
When we are asleep the organs
of respiration and the parts
that are associated with them
still go on with their action,
so necessary is this process of
respiration to us, this contact
of the air with the lungs. I
must tell you, in the briefest
possible manner, what this
process is. We consume food; the food goes through that
strange set of vessels and organs within us, and is brought
into various parts of the system, into the digestive parts
especially; and alternately the portion which is so changed
is carried through our lungs by one set of vessels,
while the air that we inhale and exhale is drawn into
and thrown out of the lungs by another set of vessels, so
that the air and the food come close together, separated
only by an exceedingly thin surface : the air can thus act
upon the blood by this process, producing precisely the same
results in kind as we have seen in the case of the candle.
The candle combines with parts of the air forming carbonic
acid, and evolves heat; so in the lungs there is this curious,
wonderful change taking place. The air entering, combines
with the carbon (not carbon in a free state, but, as in this
case, placed ready for action at the moment), and makes
carbonic acid and is so thrown out into the atmosphere, and
thus this singular result takes place; we may thus look
upon the food as fuel. Let me take that piece of sugar, which
Fig. 89
17 4 FARADAY
will serve my purpose. It is a compound of carbon, hydrogen,
and oxygen, similar to a candle, as containing the same
elements, though not in the same proportion, the proportions
being as shown in this table :
Sugar.
Carbon 72
Hy dro § en "U
Oxygen oo)
This is, indeed, a very curious thing, which you can well
remember, for the oxygen and hydrogen are in exactly the
proportions which form water, so that sugar may be said to
be compounded of 72 parts of carbon and 99 parts of water;
and it is the carbon in the sugar that combines with the
oxygen carried in by the air in the process of respiration,
so making us like candles ; producing these actions, warmth,
and far more wonderful results besides, for the sustenance
of the system, by a most beautiful and simple process. To
make this still more striking, I will take a little sugar; or,
to hasten the experiment, I will use some sirup, which con-
tains about three fourths of sugar and a little water. If
I put a little oil of vitriol on it, it takes away the water,
and leaves the carbon in a black mass. [The lecturer mixed
the two together.] You see how the carbon is coming out,
and before long we shall have a solid mass of charcoal, all
of which has come out of sugar. Sugar, as you know, is
food, and here we have absolutely a solid lump of carbon
where you would not have expected it. And if I make
arrangements so as to oxidize the carbon of sugar we shall
have a much more striking result. Here is sugar, and I
have here an oxidizer — a quicker one than the atmosphere;
and so we shall oxidize this fuel by a process different from
respiration in its form, though not different in its kind. It is
the combustion of the carbon by the contact of oxygen
which the body has supplied to it. If I set this into action
at once, you will see combustion produced. Just what oc-
curs in my lungs — taking in oxygen from another source,
namely, the atmosphere, takes place here by a more rapid
process.
You will be astonished when I tell you what this curious
play of carbon amounts to. A candle will burn some four,
CHEMICAL HISTORY OF A CANDLE 175
five, six, or seven hours. What, then, must be the daily
amount of carbon going up into the air in the way of
carbonic acid! What a quantity of carbon must go from
each of us in respiration! What a wonderful change of
carbon must take place under these circumstances of com-
bustion or respiration! A man in twenty- four hours con-
verts as much as seven ounces of carbon into carbonic acid;
a milch cow will convert seventy ounces, and a horse seventy-
nine ounces, solely by the act of respiration. That is, the
horse in twenty- four hours burns seventy-nine ounces of
charcoal, or carbon, in his organs of respiration to supply
his natural warmth in that time. All the warm-blooded
animals get their warmth in this way, by the conversion of
carbon, not in a free state, but in a state of combination.
And what an extraordinary notion this gives us of the
alterations going on in our atmosphere. As much as 5,000,-
000 pounds, or 548 tons, of carbonic acid is formed by res-
piration in London alone in twenty-four hours. And where
does all this go? Up into the air. If the carbon had been
like the lead which I showed you, or the iron which, in
burning, produces a solid substance, what would happen?
Combustion could not go on. As charcoal burns it becomes
a vapor and passes off into the atmosphere, which is the
great vehicle, the great carrier for conveying it away to
other places. Then what becomes of it? Wonderful is it
to find that the change produced by respiration, which
seems so injurious to us (for we can not breathe air twice
over), is the very life and support of plants and vegetables
that grow upon the surface of the earth. It is the same
also under the surface, in the great bodies of water; for
fishes and other animals respire upon the same principle,
though not exactly by contact with the open air.
Such fish as I have here [pointing to a globe of gold-fish]
respire by the oxygen which is dissolved from the air by the
water, and form carbonic acid, and they all move about
to produce the one great work of making the animal and
vegetable kingdoms subservient to each other. And all the
plants growing upon the surface of the earth, like that
which I have brought here to serve as an illustration, absorb
carbon; these leaves are taking up their carbon from the
17 6 FARADAY
atmosphere to which we have given it in the form of car-
bonic acid, and they are growing and prospering. Give
them a pure air like ours, and they could not live in it ; give
them carbon with other matters, and they live and rejoice.
This piece of wood gets all its carbon, as the trees and plants
get theirs, from the atmosphere, which, as we have seen,
carries away what is bad for us and at the same time good
for them — what is disease to the one being health to the
other. So are we made dependent not merely upon our fel-
low-creatures, but upon our fellow-existers, all Nature be-
ing tied together by the laws that make one part conduce
to the good of another.
There is another little point which I must mention before
we draw to a close — a point which concerns the whole of
these operations, and most curious and beautiful it is to see
it clustering upon and associated with the bodies that con-
cern us — oxygen, hydrogen, and carbon, in different states
of their existence. I showed you just now some powdered
lead, which I set burning; ( 18 ) and you saw that the mo-
ment the fuel was brought to the air it acted, even before
it got out of the bottle — the moment the air crept in it
acted. Now there is a case of chemical affinity by which
all our operations proceed. When we breathe, the same
operation is going on within us. When we burn a candle,
the attraction of the different parts one to the other is go-
ing on. Here it is going on in this case of the lead, and
it is a beautiful instance of chemical affinity. If the prod-
ucts of combustion rose off from the surface, the lead would
take fire, and go on burning to the end; but you remember
that we have this difference between charcoal and lead —
that, while the lead can start into action at once if there be
access of air to it, the carbon will remain days, weeks,
months, or years. The manuscripts of Herculaneum were
written with carbonaceous ink, and there they have been for
1800 years or more, not having been at all changed by the
atmosphere, though coming in contact with it under various
18 Lead pyrophorus is made by heating dry tartrate of lead in a glass tube
(closed at one end, and drawn out to a fine point at the other) until no more
vapors are evolved. The open end of the tube is then to be sealed before
the blowpipe. When the tube is broken and the contents shaken out into the
air, they burn with a red flash.
CHEMICAL HISTORY OF A CANDLE 177
circumstances. Now, what is the circumstance which makes
the lead and carbon differ in this respect? It is a striking
thing to see that the matter which is appointed to serve the
purpose of fuel waits in its action; it does not start off
burning, like the lead and many other things that I could
show you, but which I have not encumbered the table with;
but it waits for action. This waiting is a curious and
wonderful thing. Candles — those Japanese candles, for in-
stance — do not start into action at once like the lead or
iron (for iron finely divided does the same thing as lead),
but there they wait for years, perhaps for ages, without
undergoing any alteration. I have here a supply of coal-gas.
The jet is giving forth the gas, but you see it does not
take fire — it comes out into the air, but it waits till it is
hot enough before it burns. If I make it hot enough, it
takes fire. If I blow it out, the gas that is issuing forth
waits till the light is applied to' it again. It is curious to see
how different substances wait — how some will wait till the
temperature is raised a little, and others till it is raised a
good deal. I have here a little gunpowder and some gun-
cotton ; even these things differ in the conditions under which
they will burn. The gunpowder is composed of carbon and
other substances, making it highly combustible ; and the gun-
cotton is another combustible preparation. They are both
waiting, but they will start into activity at different de-
grees of heat, or under different conditions. By applying
a heated wire to them, we shall see which will start first
[touching the gun-cotton with the hot iron]. You see the
gun-cotton has gone off, but not even the hottest part of the
wire is now hot enough to fire the gunpowder. How
beautifully that shows you the difference in the degree in
which bodies act in this way ! In the one case the substance
will wait any time until the associated bodies are made active
by heat; but in the other, as in the process of respiration, it
waits no time. In the lungs, as soon as the air enters, it
unites with the carbon; even in the lowest temperature
which the body can bear short of being frozen, the action
begins at once, producing the carbonic acid of respiration;
and so all things go on fitly and properly. Thus you see the
analogy between respiration and combustion is rendered still
178 FARADAY
more beautiful and striking. Indeed, all I can say to you at
the end of these lectures (for we must come to an end at one
time or other) is to express a wish that you may, in your
generation, be fit to compare to a candle; that you may,
like it, shine as lights to those about you; that, in all your
actions, you may justify the beauty of the taper by making
your deeds honorable and effectual in the discharge of
your duty to your fellow-men.
ON THE CONSERVATION OF FORCE
ICE AND GLACIERS
BY
HERMANN VON HELMHOLTZ
INTRODUCTORY NOTE
Hermann Ludwig Ferdinand von Helmholtz was born at
Potsdam, near Berlin, on August 31, 1821. His father was a
man of high culture, a teacher in the gymnasium, whose in-
fluence ensured to his son the foundations of a broad general
education. His mother was a descendant from William Penn,
the English Quaker.
Helmholtz early showed mathematical ability, and wished
to devote his life to the study of physics; but practical
considerations led him to take up medicine, and he became
a surgeon in the Prussian army. He began the publication of
original contributions to science in 1842, and for fifty-two
years, till his death in 1894, he continued to produce in an
unbroken stream. He held a succession of academic positions,
teaching physiology at Konigsberg, Bonn, and Heidelberg, and
for the last twenty-three years of his life filling the chair of
physics at Berlin.
The titles of his professorships, however, give a very inad-
equate idea of his range. His contributions to science cover
medicine, physiology, optics, acoustics, mathematics, mechanics,
and electricity. His interests in science and art came together
in his work on esthetics, and he had a lively appreciation of
painting, poetry, and music.
The practice of popular lecturing on scientific subjects was
almost unknown in Germany when Helmholtz began, and he did
much to give it dignity and to set a standard. His own lectures,
as the reader of the following papers will perceive, are master-
pieces of their kind. "The matter', 3 says a biographer, "is dis-
cussed by a master, who brings to bear upon it all his wealth
of learning and research, while there is the ever-enduring in-
terest that attaches to an exposition by one who is giving forth
from his own treasury." It is fortunate for the layman when
a scientist and thinker of the first order has the skill and the
inclination to share with the outside world the rich harvest of
his brilliant and laborious research.
180
ON THE CONSERVATION
OF FORCE
INTRODUCTION TO A SERIES OF LECTURES
DELIVERED AT CARLSRUHE IN THE
WINTER OF 1862-1863
AS I have undertaken to deliver here a series of lec-
Z\ tures, I think the best way in which I can discharge
-* — ^- that duty will be to bring before you, by means of a
suitable example, some view of the special character of those
sciences to the study of which I have devoted myself. The
natural sciences, partly in consequence of their practical
applications, and partly from their intellectual influence on
the last four centuries, have so profoundly, and with such
increasing rapidity, transformed all the relations of the
life of civilised nations; they have given these nations such
increase of riches, of enjoyment of life, of the preservation
of health, of means of industrial and of social intercourse,
and even such increase of political power, that every edu-
cated man who tries to understand the forces at work in
the world in which he is living, even if he does not wish
to enter upon the study of a special science, must have some
interest in that peculiar kind of mental labour, which works
and acts in the sciences in question.
On a former occasion I have already discussed the char-
acteristic differences which exist between the natural and
the mental sciences as regards the kind of scientific work.
I then endeavoured to show that it is more especially in the
thorough conformity with law which natural phenomena and
natural products exhibit, and in. the comparative ease with
which laws can be stated, that this difference exists. Not
that I wish by any means to deny, that the mental life of
181
182 HELMHOLTZ
individuals and peoples is also in conformity with law,
as is the object of philosophical, philological, historical, mor-
al, and social sciences to establish. But in mental life, the
influences are so interwoven, that any definite sequence can
but seldom be demonstrated. In Nature the converse is the
case. It has been possible to discover the law of the origin
and progress of many enormously extended series of natural
phenomena with such accuracy and completeness that we
can predict their future occurrence with the greatest cer-
tainty; or in cases in which we have power over the con-
ditions under which they occur, we can direct them just
according to our will. The greatest of all instances of
what the human mind can effect by means of a well-recog-
nised law of natural phenomena is that afforded by modern
astronomy. The one simple law of gravitation regulates the
motions of the heavenly bodies not only of our own planetary
system, but also of the far more distant double stars; from
which, even the ray of light, the quickest of all messengers,
needs years to reach our eye; and, just on account of this
simple conformity with law, the motions of the bodies in
question can be accurately predicted and determined both
for the past and for future years and centuries to a fraction
of a minute.
On this exact conformity with law depends also the
certainty with which we know how to tame the impetuous
force of steam, and to make it the obedient servant of our
wants. On this conformity depends, moreover, the intellectual
fascination which chains the physicist to his subjects. It is
an interest of quite a different kind to that which mental
and moral sciences afford. In the latter it is man in the
various phases of his intellectual activity who chains us.
Every great deed of which history tells us, every mighty
passion which art can represent, every picture of manners,
of civic arrangements, of the culture of peoples of distant
lands or of remote times, seizes and interests us, even if
there is no exact scientific connection among them. We
continually find points of contact and comparison in our
conceptions and feelings; we get to know the hidden capac-
ities and desires of the mind, which in the ordinary peace-
ful course of civilised life remain unawakened.
ON THE CONSERVATION OF FORCE 183
It is not to be denied that, in the natural sciences, this kind
of interest is wanting. Each individual fact, taken by itself,
can indeed arouse our curiosity or our astonishment, or be
useful to us in its practical applications. But intellectual
satisfaction we obtain only from a connection of the whole,
just from its conformity with law. Reason we call that faculty
innate in us of discovering laws and applying them with
thought. For the unfolding of the peculiar forces of pure
reason in their entire certainty and in their entire bearing,
there is no more suitable arena than inquiry into Nature in
the wider sense, the mathematics included. And it is not
only the pleasure at the successful activity of one of our
most essential mental powers; and the victorious subjections
to the power of our thought and will of an external world,
partly unfamiliar, and partly hostile, which is the reward of
this labour; but there is a kind, I might almost say, of
artistic satisfaction, when we are able to survey the enor-
mous wealth of Nature as a regularly-ordered whole — a kos-
mos, an image of the logical thought of our own mind.
The last decades of scientific development have led us
to the recognition of a new universal law of all natural
phenomena, which, from its extraordinarily extended range,
and from the connection which it constitutes between natural
phenomena of all kinds, even of the remotest times and the
most distant places, is especially fitted to give us an idea of
what I have described as the character of the natural
sciences, which I have chosen as the subject of this lecture.
This law is the Law of the Conservation of Force, a term
the meaning of which I must first explain. It is not abso-
lutely new; for individual domains of natural phenomena
it was enunciated by Newton and Daniel Bernoulli; and
Rum ford and Humphry Davy have recognised distinct
features of its presence in the laws of heat.
The possibility that it was of universal application was
first stated by Dr. Julius Robert Mayer, a Schwabian physi-
cian (now living in Heilbronn), in the year 1842, while
almost simultaneously with, and independently of him, James
Prescot Joule, an English manufacturer, made a series of
important and difficult experiments on the relation of heat
to mechanical force, which supplied the chief points in
184 HELMHOLTZ
which the comparison of the new theory with experience
was still wanting.
The law in question asserts, that the quantity of force
which can be brought into action in the whole of Nature
is unchangeable, and can neither be increased nor dimin-
ished. My first object will be to explain to you what is
understood by quantity of force; or, as the same idea is
more popularly expressed with reference to its technical
application, what we call amount of work in the mechanical
sense of the word.
The idea of work for machines, or natural processes, is
taken from comparison with the working power of man; and
we can therefore best illustrate from human labour the
most important features of the question with which we are
concerned. In speaking of the work of machines and of
natural forces we must, of course, in this comparison elimi-
nate anything in which activity of intelligence comes into
play. The latter is also capable of the hard and intense
work of thinking, which tries a man just as mus-
cular exertion does. But whatever of the actions of intelli-
gence is met with in the work of machines, of course is due
to the mind of the constructor and cannot be assigned to
the instrument at work.
Now, the external work of man is of the most varied
kind as regards the force or ease, the form and rapidity,
of the motions used on it, and the kind of work produced.
But both the arm of the blacksmith who delivers his power-
ful blows with the heavy hammer, and that of the violinist
who produces the most delicate variations in sound, and
the hand of the lace-maker who works with threads so
fine that they are on the verge of the invisible, all these
acquire the force which moves them in the same manner
and by the same organs, namely the muscles of the arms.
An arm the muscles of which are lamed is incapable of
doing any work; the moving force of the muscle must be
at work in it, and these must obey the nerves, which bring
to them orders from the brain. That member is then capa-
ble of the greatest variety of motions; it can compel the
most varied instruments to execute the most diverse tasks.
Just so it is with machines: they are used for the most
ON THE CONSERVATION OF FORCE 185
diversified arrangements. We produce by their agency
an infinite variety of movements, with the most various
degrees of force and rapidity, from powerful steam-ham-
mers and rolling-mills, where gigantic masses of iron are
cut and shaped like butter, to spinning and weaving-frames,
the work of which rivals that of the spider. Modem
mechanism has the richest choice of means of transferring
the motion of one set of rolling wheels to another with
greater or less velocity; of changing the rotating motion
of wheels into the up-and-down motion of the piston-rod, of
the shuttle, of falling hammers and stamps; or, conversely,
of changing the latter into the former; or it can, on the
other hand, change movements of uniform into those of
varying velocity, and so forth. Hence this extraordinarily
rich utility of machines for so extremely varied branches
of industry. But one thing is common to all these differ-
ences; they all need a moving force, which sets and keeps
them in motion, just as the works of the human hand all
need the moving force of the muscles.
Now, the work of the smith requires a far greater and
more intense exertion of the muscles than that of the violin-
player; and there are in machines corresponding differences
in the power and duration of the moving force required.
These differences, which correspond to the different degree
of exertion of the muscles in human labour, are alone what
we have to think of when we speak of the amount of work
of a machine. We have nothing to do here with the mani-
fold character of the actions and arrangements which the
machines produce; we are only concerned with an expendi-
ture of force.
This very expression which we use so fluently, ' expendi-
ture of force/ which indicates that the force applied has
been expended and lost, leads us to a further characteristic
analogy between the effects of the human arm and those
of machines. .The greater the exertion, and the longer it
lasts, the more is the arm tired, and the more is the store of
its moving force for the time exhausted. We shall see
that this peculiarity of becoming exhausted by work is also
met with in the moving forces of inorganic nature; indeed,
that this capacity of the human arm of being tired is only
186 HELMHOLTZ
one of the consequences of the law with which we are now
concerned. When fatigue sets in, recovery is needed, and
this can only be effected by rest and nourishment. We shall
find that also in the inorganic moving forces, when their
capacity for work is spent, there is a possibility of repro-
duction, although in general other means must be used to
this end than in the case of the human arm.
From the feeling of exertion and fatigue in our muscles,
we can form a general idea of what we understand by
amount of work; but we must endeavour, instead of the
indefinite estimate afforded by this comparison, to form a
clear and precise idea of the standard by which we have
to measure the amount of work. This we can do better by
the simplest inorganic moving forces than by the actions
of our muscles, which are a very complicated apparatus,
acting in an extremely intricate manner.
Let us now consider that moving force which we know
best, and which is simplest — gravity. It acts, for example,
as such in those clocks which are driven by a weight. This
weight, fastened to a string, which is wound round a pulley
connected with the first toothed wheel of the clock, cannot
obey the pull of gravity without setting the whole clockwork
in motion. Now I must beg you to pay special attention
to the following points: the weight cannot put the clock in
motion without itself sinking; did the weight not move, it
could not move the clock, and its motion can only be such
a one as obeys the action of gravity. Hence, if the clock
is to go, the weight must continually sink lower and lower,
and must at length sink so far that the string which sup-
ports it is run out. The clock then stops. The usual effect
of its weight is for the present exhausted. Its gravity is
not lost or diminished ; it is attracted by the earth as before,
but the capacity of this gravity to produce the motion of the
clockwork is lost. It can only keep the weight at rest in
the lowest point of its path, it cannot farther put it in
motion.
But we can wind up the clock by the power of the arm,
by which the weight is again raised. When this has been
done, it has regained its former capacity, and can again
set the clock in motion.
ON THE CONSERVATION OF FORCE 187
We learn from this that a raised weight possesses a
moving force, but that it must necessarily sink if this force
is to act; that by sinking, this moving force is exhausted,
but by using another extraneous moving force — that of the
arm — its activity can be restored.
The work which the weight has to perform in driving
the clock is not indeed great. It has continually to overcome
the small resistances which the friction of the axles and
teeth, as well as the resistance of the air, oppose to the
motion of the wheels, and it has to furnish the force for
the small impulses and sounds which the pendulum pro-
duces at each oscillation. If the weight is detached from
the clock, the pendulum swings for a while before coming
to a rest, but its motion becomes each moment feebler, and
ultimately ceases entirely, being gradually used up by the
small hindrances I have mentioned. Hence, to keep the
clock going, there must be a moving force, which, though
small, must be continually at work. Such a one is the
weight.
We gtt, moreover, from this example, a measure for the
amount of work. Let us assume that a clock is driven by a
weight of a pound, which falls five feet in twenty-four
hours. If we fix ten such clocks, each with a weight of
one pound, then ten clocks will be driven twenty-four
hours; hence, as each has to overcome the same resistances
in the same time as the others, ten times as much work is
performed for ten pounds fall through five feet. Hence,
we conclude that the height of the fall being the same, the
work increases directly as the weight.
Now, if we increase the length of the string so that the
weight runs down ten feet, the clock will go two days instead
of one; and, with double the height of fall, the weight will
overcome on the second day the same resistances as on the
first, and will therefore do twice as much work as when it
can only run down five feet. The weight being the same,
the work increases as the height of fall. Hence, we may take
the product of the weight into the height of fall as a meas-
ure of work, at any rate, in the present case. The applica-
tion of this measure is, in fact, not limited to the individual
case, but the universal standard adopted in manufactures
188 HELMHOLTZ
for measuring magnitude of work is a foot pound — that
is, the amount of work which a pound raised through a
foot can produce. 1
We may apply this measure of work to all kinds of
machines, for we should be able to set them all in motion
by means of a weight sufficient to turn a pulley. We could
thus always express the magnitude of any driving force, for
any given machine, by the magnitude and height of fall of
such a weight as would be necessary to keep the machine
going with its arrangements until it had performed a cer-
tain work. Hence it is that the measurement of work by
foot pounds is universally applicable. The use of such a
weight as a driving force would not indeed be practically
advantageous in those cases in which we were compelled to
raise it by the power of our own arm; it would in that case
be simpler to work the machine by the direct action of the
arm. In the clock we use a weight so that we need not
stand the whole day at the clockwork, as we should have
to do to move it directly. By winding up the clock we accu-
mulate a store of working capacity in it, which is sufficient
for the expenditure of the next twenty-four hours.
The case is somewhat different when Nature herself
raises the weight, which then works for us. She does not
do this with solid bodies, at least not with such regularity
as to be utilised; but she does it abundantly with water,
which, being raised to the tops of mountains by meteorolog-
ical processes, returns in streams from them. The gravity
of water we use as moving force, the most direct applica-
tion being in what are called overshot wheels, one of which
is represented in Fig. 90. Along the circumference of such
a wheel are a series of buckets, which act as receptacles
for the water, and, on the side turned to the observer, have
the tops uppermost; on the opposite side the tops of the
buckets are upside-down. The water flows at M into the
buckets of the front of the wheel, and at F, where the mouth
begins to incline downwards, it flows out. The buckets on
the circumference are filled on the side turned to the ob-
server, and empty on the other side. Thus the former are
1 This is the technical measure of work; to convert it into scientific
measure it must be multiplied by the intensity of gravity.
ON THE CONSERVATION OF FORCE 189
weighted by the water contained in them, the latter not;
the weight of the water acts continuously on only one side
of the wheel, draws this down, and thereby turns the wheel;
Fig. 90
the other side of the wheel offers no resistance, for it con-
tains no water. It is thus the weight of the falling water
which turns the wheel, and furnishes the motive power. But
you will at once see that the mass of water which turns the
wheel must necessarily fall in order to do so, and that
though, when it has reached the bottom, it has lost none of its
gravity, it is no longer in a position to drive the wheel, if it
is not restored to its original position, either by the^ power
of the human arm or by means of some other natural force.
If it can flow from the mill-stream to still lower levels, it
190 HELMHOLTZ
may be used to work other wheels. But when it has reached
its lowest level, the sea, the last remainder of the moving
force is used up, which is due to gravity — that is, to the
attraction of the earth, and it cannot act by its weight
until it has been again raised to a high level. As this is
actually effected by meteorological processes, you will at
once observe that these are to be considered as sources of
moving force.
Water-power was the first inorganic force which man
learnt to use instead of his own labour or of that of domestic
animals. According to Strabo, it was known to iGng
Mithridates of Pontus, who was also otherwise celebrated
for his knowledge of Nature; near his palace there was a
water-wheel. Its use was first introduced among the
Romans in the time of the first Emperors. Even now we
find water-mills in all mountains, valleys, or wherever there
are rapidly-flowing regularly-filled brooks and streams. We
find water-power used for all purposes which can possibly
be effected by machines. It drives mills which grind corn,
saw-mills, hammers and oil-presses, spinning-frames and
looms, and so forth. It is the cheapest of all motive powers,
it flows spontaneously from the inexhaustible stores of
Nature; but it is restricted to a particular place, and only
in mountainous countries is it present in any quantity; in
level countries extensive reservoirs are necessary for dam-
ming the rivers to produce any amount of water-power.
Before passing to the discussion of other motive forces I
must answer an objection which may readily suggest itself.
We all know that there are numerous machines, systems of
pulleys, levers and cranes, by the aid of which heavy bur-
dens may be lifted by a comparatively small expenditure
of force. We have all of us often seen one or two workmen
hoist heavy masses of stones to great heights, which they
would be quite unable to do directly; in like manner, one
or two men, by means of a crane, can transfer the largest
and heaviest chests from a ship to the quay. Now, it may
be asked, If a large, heavy weight had been used for driving
a machine, would it not be very easy, by means of a crane
or a system of pulleys, to raise it anew, so that it could
again be used as a motor, and thus acquire motive power,
ON THE CONSERVATION OF FORCE
191
without being compelled to use a corresponding exertion in
raising the weight?
The answer to this is, that all these machines, in that
degree in which for the moment they facilitate the exertion,
also prolong it, so that by their help
no motive power is ultimately gained.
Let us assume that four labourers
have to raise a load of four hun-
dredweight by means of a rope pass-
ing over a single pulley. Every time
the rope is pulled down through four
feet, the load is also raised through
four feet. But now, for the sake of
comparison, let us suppose the same
load hung to a block of four pulleys,
as represented in Fig. 91. A single
labourer would now be able to raise
the load by the same exertion of
force as each one of the four put
forth. But when he pulls the rope
through four feet, the load only rises
one foot, for the length through
which he pulls the rope, at a, is uni-
formly distributed in the block over
four ropes, so that each of these
is only shortened by a foot. To raise
the load, therefore, to the same
height, the one man must necessarily
work four times as long as the four
together did. But the total expendi-
ture of work is the same, whether
four labourers work for a quarter of
an hour or one works for an hour.
If, instead of human labour, we
introduce the work of a weight, and
hang to the block a load of 400, and
at a, where otherwise the labourer
works, a weight of 100 pounds, the
block is then in equilibrium, and, without any appreciable
exertion of the arm, may be set in motion. The weight of
Fig. 9
192 HELMHOLTZ
ioo pounds sinks, that of 400 rises. Without any meas-
urable expenditure of force, the heavy weight has been
raised by the sinking of the smaller one. But observe that
the smaller weight will have sunk through four times the
distance that the greater one has risen. But a fall of 100
Fig. 92
pounds through four feet is just as much 400 foot-pounds as
a fall of 400 pounds through one foot.
The action of levers in all their various modifications is
precisely similar. Let a b, Fig. 92, be a simple lever, sup-
ported at c, the arm c b being four times as long as the other
arm a c. Let a weight of one pound be hung at b, and a
weight of four pounds at a, the lever is then in equilibrium,
and the least pressure of the finger is sufficient, without any
appreciable exertion of force, to place it in the position
a 1 If, in which the heavy weight of four pounds has been
raised, while the one-pound weight has sunk. But here,
also, you will observe no work has been gained, for while the
heavy weight has been raised through one inch, the lighter
one has fallen through four inches ; and four pounds through
one inch is, as work, equivalent to the product of one pound
through four inches.
Most other fixed parts of machines may be regarded as
modified and compound levers; a toothed-wheel, for instance
as a series of levers, the ends of which are represented by
ON THE CONSERVATION OF FORCE
193
the individual teeth, and one after the other of which is
put in activity in the degree in which the tooth in question
seizes or is seized by the adjacent pinion. Take, for in-
stance, the crabwinch, represented in Fig. 93. Suppose the
pinion on the axis of the barrel of the winch has twelve
Fig. 93
teeth, and the toothed-wheel, H H, seventy-two teeth, that
is six times as many as the former. The winch must now
be turned round six times before the toothed-wheel, H, and
the barrel, D, have made one turn, and before the rope which
raises the load has been lifted by a length equal to the cir-
cumference of the barrel. The workman thus requires six
times the time, though to be sure only one-sixth of the exer-
tion, which he would have to use if the handle were directly
applied to the barrel, D. In all these machines, and parts of
machines, we find it confirmed that in proportion as the
velocity of the motion increases its power diminishes, and
that when the power increases the velocity diminishes, but
that the amount of work is never thereby increased.
HC VOL. XXX — G
194
HELMHOLTZ
In the overshot mill-wheel, described above, water acts
by its weight. But there is another form of mill-wheels,
Fig. 94
what is called the undershot wheel, in which it only acts
by its impact, as represented in Fig. 94. These are used
where the height from which the water comes is not great
enough to flow on the upper part of the wheel. The lower
part of undershot wheels dips in the flowing water which
strikes against their float-boards and carries them along.
Such wheels are used in swift-flowing streams which have
a scarcely perceptible fall, as, for instance, on the Rhine.
In the immediate neighborhood of such a wheel, the water
need not necessarily have a great fall if it only strikes with
considerable velocity. It is the velocity of the water, exert-
ing an impact against the float-boards, which acts in this
case, and which produces the motive power.
Windmills, which are used in the great plains of Holland
and North Germany to supply the want of falling water,
afford another instance of the action of velocity. The sails
are driven by air in motion — by wind. Air at rest could
just as little drive a windmill as water at rest a water-wheel.
ON THE CONSERVATION OF FORCE 195
The driving force depends here on the velocity of moving
masses.
A bullet resting in the hand is the most harmless thing in
the world; by its gravity it can exert no great effect; but
when fired and endowed with great velocity it drives through
all obstacles with the most tremendous force.
If I lay the head of a hammer gently on a nail, neither
its small weight nor the pressure of my arm is quite sufficient
to drive the nail into the wood; but if I swing the hammer
and allow it to fall with great velocity, it acquires a new
force, which can overcome far greater hindrances.
These examples teach us that the velocity of a moving
mass can act as motive force. In mechanics, velocity in so
far as it is motive force, and can produce work, is called
vis viva. The name is not well chosen ; it is too apt to
suggest to us the force of living beings. Also in this case
you will see, from the instances of the hammer and of the
bullet, that velocity is lost, as such, when it produces work-
ing power. In the case of the water-mill, or of the wind-
mill, a more careful investigation of the moving masses of
water and air is necessary to prove that part of their
velocity has been lost by the work which they have per-
formed.
The relation of velocity to working power is most simply
and clearly seen in a simple pendulum, such as can be con-
structed by any weight which we suspend to a cord. Let M,
Fig. 95, be such a weight, of a spherical form; A B, a hori-
zontal line drawn through the centre of the sphere; P the
point at which the cord is fastened. If now I draw the
weight M on one side towards A, it moves in the arc M a,
the end of which, a, is somewhat higher than the point A
in the horizontal line. The weight is thereby raised to the
height A a. Hence my arm must exert a certain force to
bring the weight to a. Gravity resists this motion, and
endeavours to bring back the weight to M, the lowest point
which it can reach.
Now, if after I have brought the weight to a I let it go, it
obeys this force of gravity and returns to M, arrives there
with a certain velocity, and no longer remains quietly hang-
ing at M as it did before, but swings beyond M towards b 9
196
HELMHOLTZ
where its motion stops as soon as it has traversed on the
side of B an arc equal in length to that on the side of A,
and after it has risen to a distance B b above the horizontal
line, which is equal to the height A a, to which my arm had
previously raised it. In b the pendulum returns, swings the
same way back through M towards a, and so on, until its
oscillations are gradually diminished, and ultimately annulled
by the resistance of the air and by friction.
You see here that the reason why the weight, when it
comes from a to M, and does not stop there, but ascends to
by in opposition to the action of gravity, is only to be sought
in its velocity. The velocity which it has acquired in mov-
ing from the height A a is capable of again raising it to an
equal height, B b. The velocity of the moving mass, M, is
thus capable of raising this mass; that is to say, in the
language of mechanics, of performing work. This would
also be the case if we had imparted such a velocity to the
suspended weight by a blow.
From this we learn further how to measure the working
ON THE CONSERVATION OF FORCE 197
power of velocity — or, what is the same thing, the vis viva
of the moving mass. It is equal to the work, expressed in
foot pounds, which the same mass can exert after its velocity
has been used to raise it, under the most favourable circum-
stances, to as great a height as possible. 2 This does not
depend on the direction of the velocity; for if we swing a
weight attached to a thread in a circle, we can even change
a downward motion into an upward one.
The motion of the pendulum shows us very distinctly how
the forms of working power hitherto considered — that of a
raised weight and that of a moving mass — may merge into
one another. In the points a and b, Fig. 95, the mass has
no velocity; at the point M it has fallen as far as possible,
but possesses velocity. As the weight goes from a to m
the work of the raised weight is changed into vis viva; as
the weight goes further from m to b the vis viva is changed
into the work of a raised weight. Thus the work which the
arm originally imparted to the pendulum is not lost in these
oscillations provided we may leave out of consideration the
influence of the resistance of the air and of friction. Neither
does it increase, but it continually changes the form of its
manifestation.
Let us now pass to other mechanical forces, those of
elastic bodies. Instead of the weights which drive our
clocks, we find in time-pieces and in watches, steel springs
which are coiled in winding up the clock, and are uncoiled
by the working of the clock. To coil up the spring we con-
sume the force of the arm ; this has to overcome the resisting
elastic force of the spring as we wind it up, just as in the
clock we have to overcome the force of gravity which the
weight exerts. The coiled spring can, however, perform
work; it gradually expends this acquired capability in driv-
ing the clockwork.
If I stretch a crossbow and afterwards let it go, the
stretched string moves the arrow; it imparts to it force in
the form of velocity. To stretch the cord my arm must
work for a few seconds ; this work is imparted to the arrow
2 The measure of vis viva in theoretical mechanics is half the product of
the weight into the square of the velocity. To reduce it to the technical
measure of the work we must divide it by the intensity of gravity; that is,
by the velocity at the end of the first second of a freely falling body.
198 HELMHOLTZ
at the moment it is shot off. Thus the crossbow concen-
trates into an extremely short time the entire work which
the arm had communicated in the operation of stretching;
the clock, on the contrary, spreads it over one or several
days. In both cases no work is produced which my arm did
not originally impart to the instrument, it is only expended
more conveniently.
The case is somewhat different if by any other natural
process I can place an elastic body in a state of tension with-
out having to exert my arm. This is possible and is most
easily observed in the case of gases.
If, for instance, I discharge a firearm loaded with gun-
powder, the greater part of the mass of the powder is con-
verted into gases at a very high temperature, which have a
powerful tendency to expand, and can only be retained in the
narrow space in which they are formed, by the exercise of
the most powerful pressure. In expanding with enormous
force they propel the bullet, and impart to it a great velocity,
which we have already seen is a form of work.
In this case, then, I have gained work which my arm has
not performed. Something, however, has been lost — the
gunpowder, that is to say, whose constituents have changed
into other chemical compounds, from which they cannot,
without further ado, be restored to their original condition.
Here, then, a chemical change has taken place, under the
influence of which work has been gained.
Elastic forces are produced in gases by the aid of heat, on
a far greater scale.
Let us take, as the most simple instance, atmospheric air.
In Fig. 96 an apparatus is represented such as Regnault
used for measuring the expansive force of heated gases. If
no great accuracy is required in the measurement, the ap-
paratus may be arranged more simply. At C is a glass globe
filled with dry air, which is placed in a metal vessel, in
which it can be heated by steam. It is connected with the
U-shaped tube, S s, which contains a liquid, and the limbs
of which communicate with each other when the stop-cock
R is closed. If the liquid is in equilibrium in the tube S ^
when the globe is cold, it rises in the leg s, and ultimately
overflows when the globe is heated. If, on the contrary,
ON THE CONSERVATION OF FORCE
199
when the globe is heated, equilibrium be restored by allowing
some of the liquid to flow out at R, as the globe cools it
will be drawn up towards n. In both cases liquid is raised,
and work thereby produced.
The same experiment is continuously repeated on the
largest scale in steam-engines, though, in order to keep
Fig. 96
up a continual disengagement of compressed gases from the
boiler, the air in the globe in Fig. 96, which would soon
reach the maximum of its expansion, is replaced by water,
which is gradually changed into steam by the application of
heat. But steam, so long as it remains as such, is an elastic
gas which endeavours to expand exactly like atmospheric
air. And instead of the column of liquid which was raised
in our last experiment, the machine is caused to drive a
solid piston which imparts its motion to other parts of the
machine. Fig. 97 represents a front view of the working
200 HELMHOLTZ
parts of a high-pressure engine, and Fig. 98 a section. The
boiler in which steam is generated is not represented; the
steam passes through the tube z z, Fig. 98, to the cylinder
A A, in which moves a tightly fitting piston C. The parts
between the tube z z and the cylinder A A, that is the slide
valve in the valve-chest K K, and the two tubes d and e
allow the steam to pass first below and then above the piston,
while at the same time the steam has free exit from the
other half of the cylinder. When the steam passes under the
piston, it forces it upward; when the piston has reached the
top of its course the position of the valve in K K changes,
and the steam passes above the piston and forces it down
again. The piston-rod acts by means of the connecting-rod
P, on the crank Q of the fly-wheel X and sets this in motion.
By means of the rod s, the motion of the rod regulates the
opening and closing of the valve. But we need not here
enter into those mechanical arrangements, however ingeni-
ously they have been devised. We are only interested in the
manner in which heat produces elastic vapour, and how this
vapour, in its endeavour to expand, is compelled to move
the solid parts of the machine, and furnish work.
You all know how powerful and varied are the effects of
which steam-engines are capable ; with them has really begun
the great development of industry which has characterised
our century before all others. Its most essential superiority
over motive powers formerly known is that it is not re-
stricted to a particular place. The store of coal and the
small quantity of water which are the sources of its power
can be brought everywhere, and steam-engines can even be
made movable, as is the case with steam-ships and loco-
motives. By means of these machines we can develop mo-
tive power to almost an indefinite extent at any place on
the earth's surface, in deep mines and even on the middle
of the ocean ; while water and wind mills are bound to special
parts of the surface of the land. The locomotive transports
travellers and goods over the land in numbers and with a
speed which must have seemed an incredible fable to our
forefathers, who looked upon the mail-coach with its six
passengers in the inside, and its ten miles an hour, as an
enormous progress. Steam-engines traverse the ocean in-
ON THE CONSERVATION OF FORCE 201
Fig. 97
202 HELMHOLTZ
dependency of the direction of the wind, and, successfully
resisting storms which would drive sailing-vessels far away,
reach their goal at the appointed time. The advantages
which the concourse of numerous and variously skilled
workmen in all branches offers in large towns where wind
and water power are wanting, can be utilised, for steam-
engines find place everywhere, and supply the necessary
crude force; thus the more intelligent human force may be
spared for better purposes ; and, indeed, wherever the nature
of the ground or the neighbourhood of suitable lines of
communication present a favourable opportunity for the de-
velopment of industry, the motive power is also present in
the form of steam-engines.
We see, then, that heat can produce mechanical power;
but in the cases which we have discussed we have seen
that the quantity of force which can be produced by a given
measure of a physical process is always accurately defined,
and that the further capacity for work of the natural forces
is either diminished or exhausted by the work which has
been performed. How is it no^v with Heat in this respect?
This question was of decisive importance in the endeavour
to extend the law of the Conservation of Force to all natural
processes. In the answer lay the chief difference between
the older and newer views in these respects. Hence it is that
many physicists designate that view of Nature corresponding
to the law of the conservation of force with the name of
Mechanical Theory of Heat.
The older view of the nature of heat was that it is a sub-
stance, very fine and imponderable indeed, but indestructible,
and unchangeable in quantity, which is an essential funda-
mental property of all matter. And, in fact, in a large
number of natural processes, the quantity of heat which can
be demonstrated by the thermometer is unchangeable.
By conduction and radiation, it can indeed pass from
hotter to colder bodies; but the quantity of heat which the
former lose can be shown by the thermometer to have re-
appeared in the latter. Many processes, too, were known,
especially in the passage of bodies from the solid to the
liquid and gaseous states, in which heat disappeared — at any
rate, as regards the thermometer. But when the gaseous
ON THE CONSERVATION OF FORCE 203
Fig. 98
204 HELMHOLTZ
body was restored to the liquid, and the liquid to the solid
state, exactly the same quantity of heat reappeared which
formerly seemed to have been lost. Heat was said to have
become latent. On this view, liquid water differed from
solid ice in containing a certain quantity of heat bound,
which, just because it was bound, could not pass to the
thermometer, and therefore was not indicated by it. Aque-
ous vapour contains a far greater quantity of heat thus
bound. But if the vapour be precipitated, and the liquid
water restored to the state of ice, exactly the same amount
of heat is liberated as had become latent in the melting of
the ice and in the vaporisation of the water.
Finally, heat is sometimes produced and sometimes dis-
appears in chemical processes. But even here it might be
assumed that the various chemical elements and chemical
compounds contain certain constant quantities of latent heat,
which, when they change their composition, are sometimes
liberated and sometimes must be supplied from external
sources. Accurate experiments have shown that the quan-
tity of heat which is developed by a chemical process — for
instance, in burning a pound of pure carbon into carbonic
acid — is perfectly constant, whether the combustion is slow
or rapid, whether it takes place all at once or by inter-
mediate stages. This also agreed very well with the assump-
tion, which was the basis of the theory of heat, that heat
is a substance entirely unchangeable in quantity. The natural
processes which have here been briefly mentioned, were the
subject of extensive experimental and mathematical investi-
gations, especially of the great French physicists in the last
decade of the former, and the first decade of the present,
century; and a rich and accurately-worked chapter of
physics had been developed, in which everything agreed
excellently with the hypothesis — that heat is a substance.
On the other hand, the invariability in the quantity of heat
in all these processes could at that time be explained in no
other manner than that heat is a substance.
But one relation of heat — namely, that to mechanical work
— had not been accurately investigated. A French engineer,
Sadi Carnot, son of the celebrated War Minister of the
Revolution, had indeed endeavoured to deduce the work
ON THE CONSERVATION OF FORCE 205
which heat performs, by assuming that the hypothetical
caloric endeavoured to expand like a gas; and from this
assumption he deduced in fact a remarkable law as
to the capacity of heat for work, which even now, though
with an essential alteration introduced by 'Clausius, is
among the bases of the modern mechanical theory of
heat, and the practical conclusions from which, so far
as they could at that time be compared with experiments,
have held good.
But it was already known that whenever two bodies in
motion rubbed against each other, heat was developed anew,
and it could not be said whence it came.
The fact is universally recognised ; the axle of a carriage
which is badly greased and where the friction is great,
becomes hot — so hot, indeed, that it may take fire; machine-
wheels with iron axles going at a great rate may become so
hot that they weld to their sockets. A powerful degree of
friction is not, indeed, necessary to disengage an appreci-
able degree of heat; thus, a lucifer-match, which by rubbing
is so heated that the phosphoric mass ignites, teaches this
fact. Nay, it is enough to rub the dry hands together to
feel the heat produced by friction, and which is far greater
than the heating which takes place when the hands lie gently
on each other. Uncivilised people use the friction of two
pieces of wood to kindle a fire. With this view, a sharp
spindle of hard wood is made to revolve rapidly on a base
of soft wood in the manner represented in Fig. 99.
So long as it was only a question of the friction of solids,
in which particles from the surface become detached and
compressed, it might be supposed that some changes in
structure of the bodies rubbed might here liberate latent
heat, which would thus appear as heat of friction.
But heat can also be produced by the friction of liquids,
in which there could be no question of changes in structure,
or of the liberation of latent heat. The first decisive ex-
periment of this kind was made by Sir Humphry Davy in
the commencement of the present century. In a cooled
space he made two pieces of ice rub against each other, and
thereby caused them to melt. The latent heat which the
newly formed water must have here assimilated could not
206
HELMHOLTZ
have been conducted to it by the cold ice, or have been pro-
duced by a change of structure; it could have come from no
other cause than from friction, and must have been created
by friction.
Heat can also be produced by the impact of imperfectly
elastic bodies as well as by friction. This is the case, for
Fig. 99
instance, when we produce fire by striking flint against steel,
or when an iron bar is worked for some time by powerful
blows of the hammer.
If we inquire into the mechanical effects of friction and
of inelastic impact, we find at once that these are the proc-
esses by which all terrestrial movements are brought to
rest. A moving body whose motion was not retarded by
any resisting force would continue to move to all eternity.
The motions of the planets are an instance of this. This is
apparently never the case with the motion of the terrestrial
bodies, for they are always in contact with other bodies
which are at rest, and rub against them. We can, indeed,
ON THE CONSERVATION OF FORCE 207
very much diminish their friction, but never completely
annul it. A wheel which turns about a well-worked axle,
once set in motion, continues it for a long time; and the
longer, the more truly and smoother the axle is made to
turn, the better it is greased, and the less the pressure it has
to support. Yet the vis viva of the motion which we have
imparted to such a wheel when we started it, is gradually
lost in consequence of friction. It disappears, and if we
do not carefully consider the matter, it seems as if the vis
viva which the wheel had possessed had been simply de-
stroyed without any substitute.
A bullet which is rolled on a smooth horizontal surface
continues to roll until its velocity is destroyed by friction on
the path, caused by the very minute impacts on its little
roughnesses.
A pendulum which has been put in vibration can con-
tinue to oscillate for hours if the suspension is good, with-
out being driven by a weight ; but by the friction against the
surrounding air, and by that at its place of suspension, it
ultimately comes to rest.
A stone which has fallen from a height has acquired a
certain velocity on reaching the earth; this we know is the
equivalent of a mechanical work; so long as this velocity
continues as such, we can direct it upwards by means of
suitable arrangements, and thus utilise it to raise the stone
again. Ultimately the stone strikes against the earth and
comes to rest; the impact has destroyed its velocity, and
therewith apparently also the mechanical work which this
velocity could have affected.
If we review the results of all these instances, which
each of you could easily add to from your own daily experi-
ence, we shall see that friction and inelastic impact are
processes in which mechanical work is destroyed, and heat
produced in its place.
The experiments of Joule, which have been already men-
tioned, lead us a step further. He has measured in foot
pounds the amount of work which is destroyed by the fric-
tion of solids and by the friction of liquids; and, on the
other hand, he has determined the quantity of heat which
is thereby produced, and has established a definite relation
208 HELMHOLTZ
between the two. His experiments show that when heat is
produced by the consumption of work, a definite quantity
of work is required to produce that amount of heat which is
known to physicists as the unit of heat; the heat, that is to
say, which is necessary to raise one gramme of water
through one degree centigrade. The quantity of work
necessary for this is, according to Joule's best experiments,
equal to the work which a gramme would perform in fall-
ing through a height of 425 metres.
In order to show how closely concordant are his numbers,
I will adduce the results of a few series of experiments
which he obtained after introducing the latest improve-
ments in his methods.
1. A series of experiments in which water was heated
by friction in a brass vessel. In the interior of this vessel
a vertical axle provided with sixteen paddles was rotated,
the eddies thus produced being broken by a series of pro-
jecting barriers, in which parts were cut out large enough
for the paddles to pass through. The value of the equiva-
lent was 424.9 metres.
2. Two similar experiments, in which mercury in an iron
vessel was substituted for water in a brass one, gave 425
and 426.3 metres.
3. Two series of experiments, in which a conical ring
rubbed against another, both surrounded by mercury, gave
426.7 and 425.6 metres.
Exactly the same relations between heat and work were
also found in the reverse process — that is, when work was
produced by heat. In order to execute this process under
physical conditions that could be controlled as perfectly as
possible, permanent gases and not vapours were used,
although the latter are, in practice, more convenient for
producing large quantities of work, as in the case of the
steam-engine. A gas which is allowed to expand with mod-
erate velocity becomes cooled. Joule was the first to show
the reason of this cooling. For the gas has, in expanding,
to overcome the resistance which the pressure of the atmos-
phere and the slowly yielding side of the vessel oppose to
it; or, if it cannot of itself overcome this resistance, it sup-
ports the arm of the observer which does it. Gas thus per-
ON THE CONSERVATION OF FORCE 209
forms work, and this work is produced at the cost of its
heat. Hence the cooling. If, on the contrary, the gas is
suddenly allowed to issue into a perfectly exhausted space
where it finds no resistance, it does not become cool, as
Joule has shown; or if individual parts of it become cool,
others become warm; and, after the temperature has become
equalised, this is exactly as much as before the sudden ex-
pansion of the gaseous mass.
How much heat the various gases disengage when they
are compressed, and how much work is necessary for their
compression; or, conversely, how much heat disappears
when they expand under a pressure equal to their own
counterpressure, was partly known from the older physical
experiments, and has partly been determined by the recent
experiments of Regnault by extremely perfect methods. Cal-
culations with the best data of this kind give us the value
of the thermal equivalent from experiments : —
With atmospheric air 426.0 metres
" oxygen 425.7 "
" nitrogen 431.3 "
" hydrogen 425.3 "
Comparing these numbers with those which determine the
equivalence of heat and mechanical work in friction, as
close an agreement is seen as can at all be expected from
numbers which have been obtained by such varied investi-
gations of different observers.
Thus then : a certain quantity of heat may be changed into
a definite quantity of work; this quantity of work can also
be retransformed into heat, and, indeed, into exactly the
same quantity of heat as that from which it originated; in
a mechanical point of view, they are exactly equivalent.
Heat is a new form in v/hich a quantity of work may appear.
These facts no longer permit us to regard heat as a sub-
stance, for its quantity is not unchangeable. It can be pro-
duced anew from the vis viva of motion destroyed; it can
be destroyed, and then produces motion. We must rather
conclude from this that heat itself is a motion, an internal
invisible motion of the smallest elementary particles of
bodies. If, therefore, motion seems lost in friction and
impact, it is not actually lost, but only passes from the great
210 HELMHOLTZ
visible masses to their smallest particles; while in steam-
engines the internal motion of the heated gaseous particles
is transferred to the piston of the machine, accumulated in
it, and combined in a resultant whole.
But what is the nature of this internal motion can only
be asserted with any degree of probability in the case of
gases. Their particles probably cross one another in rec-
tilinear paths in all directions, until, striking another par-
ticle, or against the side of the vessel, they are reflected in
another direction. A gas would thus be analogous to a
swarm of gnats, consisting, however, of particles infinitely
small and infinitely more closely packed. This hypothesis,
which has been developed by Kronig, Clausius, and Max-
well, very well accounts for all the phenomena of gases.
What appeared to the earlier physicists to be the constant
quantity of heat is nothing more than the whole motive
power of the motion of heat, which remains constant so
long as it is not transformed into other forms of work, or
results afresh from them.
We turn now to another kind of natural forces which
can produce work — I mean the chemical. We have to-day
already come across them. They are the ultimate cause of
the work which gunpowder and the steam-engine produce;
for the heat which is consumed in the latter, for example,
originates in the combustion of carbon — that is to say, in a
chemical process. The burning of coal is the chemical union
of carbon with the oxygen of the air, taking place under
the influence of the chemical affinity of the two substances.
We may regard this force as an attractive force between
the two, which, however, only acts through them with extra-
ordinary power, if the smallest particles of the two sub-
stances are in closest proximity to each other. In combus-
tion this force acts; the carbon and oxygen atoms strike
against each other and adhere firmly, inasmuch as they form
a new compound — carbonic acid — a gas known to all of you
as that which ascends from all fermenting and fermented
liquids — from beer and champagne. Now this attraction
between the atoms of carbon and of oxygen performs work
just as much as that which the earth in the form of gravity
exerts upon a raised weight. When the weight falls to the
ON THE CONSERVATION OF FORCE 211
ground, it produces an agitation, which is partly trans-
mitted to the vicinity as sound waves, and partly remains as
the motion of heat. The same result we must expect from
chemical action. When carbon and oxygen atoms have
rushed against each other, the newly- formed particles of
carbonic acid must be in the most violent molecular motion
— that is, in the motion of heat. And this is so. A pound
of carbon burned with oxygen to form carbonic acid, gives
as much heat as is necessary to raise 80.9 pounds of water
from the freezing to the boiling point; and just as the same
amount of work is produced when a weight falls, whether
it falls slowly or fast, so also the same quantity of heat is
produced by the combustion of carbon, whether this is slow
or rapid, whether it takes place all at once, or by successive
stages.
When the carbon is burned, we obtain in its stead, and in
that of the oxygen, the gaseous product of combustion — car-
bonic acid. Immediately after combustion it is incandescent.
When it has afterwards imparted heat to the vicinity, we
have in the carbonic acid the entire quantity of carbon and
the entire quantity of oxygen, and also the force of affinity
quite as strong as before. But the action of the latter is
now limited to holding the atoms of carbon and oxygen
firmly united; they can no longer produce either heat or
work any more than a fallen weight can do work if it has
not been again raised by some extraneous force. When the
carbon has been burnt we take no further trouble to retain
the carbonic acid; it can do no more service, we endeavour
to get it out of the chimneys of our houses as fast as we
can.
Is it possible, then, to tear asunder the particles of car-
bonic acid, and give to them once more the capacity of
work which they had before they were combined, just as
we can restore the potentiality of a weight by raising it from
the ground? It is indeed possible. We shall afterwards
see how it occurs in the life of plants; it can also be effected
by inorganic processes, though in roundabout ways, the
explanation of which would lead us too far from our present
course.
This can, however, be easily and directly shown for an-
212
HELMHOLTZ
other element, hydrogen, which can be burnt just like carbon.
Hydrogen with carbon is a constituent of all combustible
vegetable substances, among others, it is also an essential
constituent of the gas which is used for lighting our streets
and rooms; in the free state it is also a gas, the lightest of
all, and burns when ignited with a feebly luminous blue
flame. In this combustion — that is, in the chemical com-
bination of hydrogen with oxygen, a very considerable quan-
tity of heat is produced; for a given weight of hydrogen,
four times as much heat as in the combustion of the same
weight of carbon. The product of combustion is water,
which, therefore, is not of itself further combustible, for
the hydrogen in it is completely saturated with oxygen.
The force of affinity, therefore, of hydrogen for oxygen,
Fig. ioo
like that of carbon for oxygen, performs work in combus-
tion, which appears in the form of heat. In the water which
has been formed during combustion, the force of affinity
is exerted between the elements as before, but its capacity
for work is lost. Hence the two elements must be again
separated, their atoms torn apart, if new effects are to be
produced from them.
This we can do by the aid of currents of electricity. In
the apparatus depicted in Fig. ioo, we have two glass vessels
filled with acidulated water a and ai, which are separated in
the middle by a porous plate moistened with water. In both
ON THE CONSERVATION OF FORCE
213
sides are fitted platinum wires, k, which are attached to
platinum plates, i and iu As soon as a galvanic current is
transmitted through the water by the platinum wires, fc, you
see bubbles of gas ascend from the plates i and iu These
bubbles are the two elements of water, hydrogen on the one
hand, and oxygen on the other. The gases emerge through
the tubes g and gu If we wait until the upper part of the
vessels and the tubes have been filled with it, we can inflame
hydrogen at one side; it burns with a blue flame. If I
bring a glimmering spill near the mouth of the other tube,
it bursts into flame, just as happens with oxygen gas, in
Fig. io i
which the processes of combustion are far more intense
than in atmospheric air, where the oxygen mixed with nitro-
gen is only one-fifth of the whole volume.
If I hold a glass flask filled with water over the hydrogen
flame, the water, newly formed in combustion, condenses
upon it.
If a platinum wire be held in the almost non-luminous
flame, you see how intensely it is ignited; in a plentiful
current of a mixture of the gases, hydrogen and oxygen,
which have been liberated in the above experiment, the
almost infusible platinum might even be melted. The
214 HELMHOLTZ
hydrogen which has here been liberated from the water by
the electrical current has regained the capacity of producing
large quantities of heat by a fresh combination with oxygen ;
its affinity for oxygen has regained for it its capacity for
work.
We here become acquainted with a new source of work,
the electric current which decomposes water. This current
is itself produced by a galvanic battery, Fig. ioi. Each of
the four vessels contains nitric acid, in which there is a
hollow cylinder of very compact carbon. In the middle of
the carbon cylinder is a cylindrical porous vessel of white
clay, which contains dilute sulphuric acid; in this dips a zinc
cylinder. Each zinc cylinder is connected by a metal ring
with the carbon cylinder of the next vessel, the last zinc
cylinder, n, is connected with one platinum plate, and the
first carbon cylinder, p, with the other platinum plate of the
apparatus for the decomposition of water.
If now the conducting circuit of this galvanic apparatus
is completed, and the decomposition of water begins, a
chemical process takes place simultaneously in the cells of
the voltaic battery. Zinc takes oxygen from the surround-
ing water and undergoes a slow combustion. The product of
combustion thereby produced, oxide of zinc, unites further
with sulphuric acid, for which it has a powerful affinity, and
sulphate of zinc, a saline kind of substance, dissolves in the
liquid. The oxygen, moreover, which is withdrawn from it
is taken by the water from the nitric acid surrounding the
cylinder of carbon, which is very rich in it, and readily gives
it up. Thus, in the galvanic battery zinc burns to sulphate
of zinc at the cost of the oxygen of nitric acid.
Thus, while one product of combustion, water, is again
separated, a new combustion is taking place — that of zinc.
While we there reproduce chemical affinity which is capa-
ble of work, it is here lost. The electrical current is, as it
were, only the carrier which transfers the chemical force of
the zinc uniting with oxygen and acid to water in the decom-
posing cell, and uses it for overcoming the chemical force
of hydrogen and oxygen.
In this case, we can restore work which has been lost, but
only by using another force, that of oxidising zinc.
ON THE CONSERVATION OF FORCE 215
Here we have overcome chemical forces by chemical
forces, through the instrumentality of the electrical current.
But we can attain the same object by mechanical forces, if
we produce the electrical current by a magneto-electrical
machine, Fig. 102. If we turn the handle, the anker R R 1 , on
which is coiled copper-wire, rotates in front of the poles of
the horse-shoe magnet, and in these coils electrical cur-
rents are produced, which can be led from the points a and b.
If the ends of these wires are connected with the apparatus
for decomposing water, we obtain hydrogen and oxygen,
though in far smaller quantity than by the aid of the bat-
tery which we used before. But this process is interesting,
for the mechanical force of the arm which turns the wheel
produces the work which is required for separating the com-
bined chemical elements. Just as the steam-engine changes
chemical into mechanical force the magneto-electrical ma-
chine transforms mechanical force into chemical.
The application of electrical currents opens out a large
number of relations between the various natural forces. We
have decomposed water into its elements by such currents,
and should be able to decompose a large number of other
chemical compounds. On the other hand, in ordinary gal-
vanic batteries electrical currents are produced by chemical
forces.
In all conductors through which electrical currents pass
they produce heat; I stretch a thin platinum wire between
the ends n and p of the galvanic battery, Fig. ioi ; it becomes
ignited and melts. On the other hand, electrical currents
are produced by heat in what are called thermo-electric
elements.
Iron which is brought near a spiral of copper wire, tra-
versed by an electrical current, becomes magnetic, and then
attracts other pieces of iron, or a suitably placed steel mag-
net. We thus obtain mechanical actions which meet with
extended applications in the electrical telegraph, for instance.
Fig. 103, represents a Morse's telegraph in one-third of the
natural size. The essential part is a horse-shoe shaped iron
core, which stands in the copper spirals b b. Just over the
top of this is a small steel magnet c c, which is attracted the
moment an electrical current, arriving by the telegraph
216 HELMHOLTZ
wire, traverses the spirals b b. The magnet cc is rigidly
fixed in the lever d d, at the other end of which is a style;
Fig.
this makes a mark on a paper band, drawn by a clock-work,
as often and as long as c c is attracted by the magnetic action
of the electrical current. Conversely, by reversing the mag-
netism in the iron core of the spirals b b, we should obtain
ON THE CONSERVATION OF FORCE
217
in them an electrical current just as we have obtained such
currents in the magneto-electrical machine, Fig. 102; in the
lllllliliHIllllllMllUHmilltl
Fig. 103
spirals of that machine there is an iron core which, by
being approached to the poles of the large horse-shoe mag-
net, is sometimes magnetised in one and sometimes in the
other direction.
I will not accumulate examples of such relations; in sub-
sequent lectures we shall come across them. Let us review
these examples once more, and recognise in them the law
which is common to all.
A raised weight can produce work, but in doing so it must
necessarily sink from its height, and, when it has fallen as
deep as it can fall, its gravity remains as before, but it
can no longer do work.
A stretched spring can do work, but in so doing it becomes
loose. The velocity of a moving mass can do work, but in
doing so it comes to rest. Heat can perform work; it is
destroyed in the operation. Chemical forces can perform
work, but they exhaust themselves in the effort.
Electrical currents can perform work, but to keep them up
218 HELMHOLTZ
we must consume either chemical or mechanical forces, or
heat.
We may express this generally. It is a universal character
of all known natural forces that their capacity for work is
exhausted in the degree in which they actually perform work.
We have seen, further, that when a weight fell without
performing any work, it either acquired velocity or pro-
duced heat. We might also drive a magneto-electrical ma-
chine by a falling weight; it would then furnish electrical
currents.
We have seen that chemical forces, when they come into
play, produce either heat or electrical currents or mechani-
cal work.
We have seen that heat may be changed into work ; there
are apparatus (thermo-electric batteries) in which electrical
currents are produced by it. Heat can directly separate
chemical compounds; thus, when we burn limestone, it sep-
arates carbonic acid from lime.
Thus, whenever the capacity for work of one natural force
is destroyed, it is transformed into another kind of activity.
Even within the circuit of inorganic natural forces, we can
transform each of them into an active condition by the aid
of any other natural force which is capable of work. The
connections between the various natural forces which mod-
ern physics has revealed, are so extraordinarily numerous
that several entirely different methods may be discovered for
each of these problems.
I have stated how we are accustomed to measure mechani-
cal work, and how the equivalent in work of heat may be
found. The equivalent in work of chemical processes is
again measured by the heat which they produce. By simi-
lar relations, the equivalent in work of the other natural
forces may be expressed in terms of mechanical work.
If, now, a certain quantity of mechanical work is lost,
there is obtained, as experiments made with the object of
determining this point show, an equivalent quantity of heat,
or, instead of this, of chemical force; and, conversely, when
heat is lost, we gain an equivalent quantity of chemical or
mechanical force; and, again, when chemical force disap-
pears, an equivalent of heat or work; so that in all these
ON THE CONSERVATION OF FORCE 219
interchanges between various inorganic natural forces work-
ing force may indeed disappear in one form, but then it
reappears in exactly equivalent quantity in some other form ;
it is thus neither increased nor diminished, but always
remains in exactly the same quantity. We shall subsequently
see that the same law holds good also for processes in
organic nature, so far as the facts have been tested.
It follows thence that the total quantity of all the forces
capable of work in the whole universe remains eternal and
unchanged throughout all their changes. All change in
nature amounts to this, that force can change its form and
locality without its quantity being changed. The universe
possesses, once for all, a store of force which is not altered
by any change of phenomena, can neither be increased nor
diminished, and which maintains any change which takes
place on it.
You see how, starting* from considerations based on the
immediate practical interests of technical work, we have
been led up to a universal natural law, which, as far as all
previous experience extends, rules and embraces all natural
processes; which is no longer restricted to the practical
objects of human utility, but expresses a perfectly general
and particularly characteristic property of all natural forces,
and which, as regards generality, is to be placed by the side
of the laws of the unalterability of mass, and the unaltera-
bility of the chemical elements.
At the same time, it also decides a great practical question
which has been much discussed in the last two centuries, to
the decision of which an infinity of experiments has been
made and an infinity of apparatus constructed — that is, the
question of the possibility of a perpetual motion. By this
was understood a machine which was to work continuously
without the aid of any external driving force. The solution
of this problem promised enormous gains. Such a machine
would have had all the advantages of steam without
requiring the expenditure of fuel. Work is wealth. A
machine which could produce work from nothing was as
good as one which made gold. This problem had thus for
a long time occupied the place of gold making, and had
confused many a pondering brain. That a perpetual motion
220 ON THE CONSERVATION OF FORCE
could not be produced by the aid of the then known mechani-
cal forces could be demonstrated in the last century by the
aid of the mathematical mechanics which had at that time
been developed. But to show also that it is not possible even
if heat, chemical forces, electricity, and magnetism were
made to co-operate, could not be done without a knowledge
of our law in all its generality. The possibility of a per-
petual motion was first finally negatived by the law of the
conservation of force, and this law might also be expressed
in the practical form that no perpetual motion is possible,
that force cannot be produced from nothing ; something must
be consumed.
You will only be ultimately able to estimate the importance
and the scope of our law when you have before your eyes a
series of its applications to individual processes in nature.
What I have to-day mentioned as to the origin of the
moving forces which are at our disposal, directs us to some-
thing beyond the narrow confines of our laboratories and
our manufactories, to the great operations at work in the life
of the earth and of the universe. The force of falling water
can only flow down from the hills when rain and snow bring
it to them. To furnish these, we must have aqueous vapour
in the atmosphere, which can only be effected by the aid of
heat, and this heat comes from the sun. The steam-engine
needs the fuel which the vegetable life yields, whether it be
the still active life of the surrounding vegetation, or the
efxtinct life which has produced the immense coal deposits
in the depths of the earth. The forces of man and ani-
mals must be restored by nourishment; all nourishment
comes ultimately from the vegetable kingdom, and leads us
back to the same source.
You see then that when we inquire into the origin of the
moving forces which we take into our service, we are
thrown back upon the meteorological processes in the earth's
atmosphere, on the life of plants in general, and on the sun.
ICE AND GLACIERS
A LECTURE DELIVERED AT FRANKFORT-ON-
THE-MAIN, AND AT HEIDELBERG, IN
FEBRUARY, 1865.
THE world of ice and of eternal snow, as unfolded to
us on the summits of the neighbouring Alpine chain,
so stern, so solitary, so dangerous, it may be, has yet
its own peculiar charm. Not only does it enchain the atten-
tion of the natural philosopher, who finds in it the most
wonderful disclosures as to the present and past history of
the globe, but every summer it entices thousands of travellers
of all conditions, who find there mental and bodily recrea-
tion. While some content themselves with admiring from
afar the dazzling adornment which the pure, luminous
masses of snowy peaks, interposed between the deeper blue
of the sky and the succulent green of the meadows, lend to
the landscape, others more boldly penetrate into the strange
world, willingly subjecting themselves to the most extreme
degrees of exertion and danger, if only they may fill them-
selves with the aspect of its sublimity.
I will not attempt what has so often been attempted in
vain — to depict in words the beauty and magnificence of
nature, whose aspect delights the Alpine traveller. I may
well presume that it is known to most of you from your own
observation ; or, it is to be hoped, will be so. But I imagine
that the delight and interest in the magnificence of those
scenes will make you the more inclined to lend a willing ear
to the remarkable results of modern investigations on the
more prominent phenomena of the glacial world. There
we see that minute peculiarities of ice, the mere mention
of which might at other times be regarded as a scientific
subtlety, are the causes of the most important changes in
glaciers; shapeless masses of rock begin to relate their his-
221
222 HELMHOLTZ
tories to the attentive observer, histories which often stretch
far beyond the past of the human race into the obscurity of
the primeval world; a peaceful, uniform, and beneficent sway
of enormous natural forces, where at first sight only desert
wastes are seen, either extended indefinitely in cheerless,
desolate solitudes, or full of wild, threatening confusion —
an arena of destructive forces. And thus I think I may
promise that the study of the connection of those phenomena
of which I can! now only give you a very short outline, will
not only afford you some prosaic instruction, but will make
your pleasure in the magnificent scenes of the high moun-
tains more vivid, your interest deeper, and your admiration
more exalted.
Let me first of all recall to your remembrance the chief
features of the external appearance of the snow-fields and
of the glaciers; and let me mention the accurate measure-
ments which have contributed to supplement observation,
before I pass to discuss the casual connection of those
processes.
The higher we ascend the mountains the colder it becomes.
Our atmosphere is like a warm covering spread over the
earth; it is well-nigh entirely transparent for the luminous
darting rays of the sun, and allows them to pass almost
without appreciable change. But it is not equally penetrable
by obscure heat-rays, which, proceeding from heated terres-
trial bodies, struggle to diffuse themselves into space. These
are absorbed by atmospheric air, especially when it is moist;
the mass of air is itself heated thereby, and only radiates
slowly into space the heat which has been gained. The
expenditure of heat is thus retarded as compared with the
supply, and a certain store of heat is retained along the
whole surface of the earth. But on high mountains the
protective coating of the atmosphere is far thinner — the
radiated heat of the ground can escape thence more freely
into space; there, accordingly, the accumulated store of heat
and the temperature are far smaller than at lower levels.
To this must be added another property of air which acts
in the same direction. In a mass of air which expands,
part of its store of heat disappears, it becomes cooler, if it
cannot acquire fresh heat from without. Conversely, by
ICE AND GLACIERS 223
renewed compression of the air, the same quantity of heat is
reproduced which had disappeared during expansion. Thus
if, for instance, south winds drive the warm air of the
Mediterranean towards the north, and compel it to ascend
along the great mountain-wall of the Alps, where the air,
in consequence of the diminished pressure, expands by about
half its volume, it thereby becomes very greatly cooled — for a
mean height of 11,000 feet, by from i8° to 30 C, according
as it is moist or dry — and it thereby deposits the greater
part of its moisture as rain or snow. If the same wind,
passing over to the north side of the mountains as Fohn-
wind, reaches the valleys and plains, it again becomes con-
densed, and is again heated. Thus the same current of air
which is warm in the plains, both on this side of the chain
and on the other, is bitterly cold on the heights, and can
there deposit snow, while in the plain we find it insupportably
hot.
The lower temperature at greater heights, which is due
to both these causes, is, as we know, very marked on the
lower mountain chains of our neighbourhood. In central
Europe it amounts to about i° C. for an ascent of 480 feet;
in winter it is less — 1° for about 720 feet of ascent. In the
Alps the differences of temperature at great heights are
accordingly far more considerable, so that upon the higher
parts of their peaks and slopes the snow which has fallen in
winter no longer melts in summer. This line, above which
snow covers the ground throughout the entire year, is well
known as the snow-line; on the northern side of the Alps it is
about 8,000 feet high, on the southern side about 8,800 feet.
Above the snow-line it may on sunny days be very warm;
the unrestrained radiation of the sun, increased by the light
reflected from the snow, often becomes utterly unbearable,
so that the tourist of sedentary habits, apart from the daz-
zling of his eyes, which he must protect by dark spectacles
or by a veil, usually gets severely sunburnt in the face and
hands, the result of which is an inflammatory swelling of
the skin and great blisters on the surface. More pleasant
testimonies to the power of the sunshine are the vivid colours
and the powerful odour of the small Alpine flowers which
bloom in the sheltered rocky clefts among the snow-fields.
224 HELMHOLTZ
Notwithstanding the powerful radiation of the sun the tem-
perature of the air above the snow-fields only rises to 5 ,
or at most 8° ; this, however, is sufficient to melt a tolerable
amount of the superficial layers of snow. But the warm
hours and days are too short to overpower the great masses
of snow which have fallen during colder times. Hence the
height of the snow-line does not depend merely on the tem-
perature of the mountain slope, but also essentially on the
amount of the yearly snow-fall. It is lower, for instance,
on the moist and warm south slope of the Himalayas than
on the far colder but also far drier north slope of the same
mountain. Corresponding to the moist climate of western
Europe, the snow-fall upon the Alps is very great, and hence
the number and extent of their glaciers are comparatively
considerable, so that few mountains of the earth can be
compared with them in this respect. Such a development
of the glacial world is, as far as we know, met with only on
the Himalayas, favoured by the greater height; in Green-
land and in Northern Norway, owing to the colder climate;
in a few islands in Iceland; and in New Zealand, from the
more abundant moisture.
Places above the snow-line are thus characterised by the
fact that the snow which in the course of the year falls on
its surface does not quite melt away in summer, but remains
to some extent. This snow, which one summer has left, is
protected from the further action of the sun's heat by the
fresh quantities that fall upon it during the next autumn,
winter, and spring. Of this new snow also next summer
leaves some remains, and thus year by year fresh layers of
snow are accumulated one above the other. In those places
where such an accumulation of snow ends in a steep preci-
pice, and its inner structure is thereby exposed, the regularly
stratified yearly layers are easily recognised.
But it is clear that this accumulation of layer upon layer
cannot go on indefinitely, for otherwise the height of the
snow peak would continually increase year by year. But
the more the snow is accumulated the steeper are the slopes,
and the greater the weight which presses upon the lower
and older layers and tries to displace them. Ultimately a
state must be reached in which the snow-slopes are too
ICE AND GLACIERS 225
steep to allow fresh snow to rest upon them, and in which
the burden which presses the lower layers downwards is so
great that these can no longer retain their position on the
sides of the mountain. Thus, part of the snow which had
originally fallen on the higher regions of the mountain
above the snow-line, and had there been protected from
melting, is compelled to leave its original position and seek
a new one, which it of course finds only below the snow-line
on the lower slopes of the mountain, and especially in the
valleys, where, however, being exposed to the influence of
a warmer air, it ultimately melts and flows away as water.
The descent of masses of snow from their original positions
sometimes happens suddenly in avalanches, but it is usually
very gradual in the form of glaciers.
Thus we must discriminate between two distinct parts of
the ice-fields; that is, first, the snow which originally fell —
called firn in Switzerland — above the snow-line, covering the
slopes of the peaks as far as it can hang on to them, and
filling up the upper wide kettle-shaped ends of the valleys
forming widely extending fields of snow or firnmeere.
Secondly, the glaciers, called in the Tyrol fimer, which as
prolongations of the snow-fields often extend to a distance
of from 4,000 to 5,000 feet below the snow-line, and in
which the loose snow of the snow-fields is again found
changed into transparent solid ice. Hence the name glacier,
which is derived from the Latin, glacies; French, glace,
glacier.
The outward appearance of glaciers is very characteristi-
cally described by comparing them, with Goethe, to currents
of ice. They generally stretch from the snow-fields along
the depth of the valleys, filling them throughout their entire
breadth, and often to a considerable height. They thus
follow all the curvatures, windings, contractions, and en-
largements of the valley. Two glaciers frequently meet
the valleys of which unite. The two glacial currents then
join in one common principal current, filling up the valley
common to them both. In some places these ice-currents
present a tolerably level and coherent surface, but they are
usually traversed by crevasses, and both over the surface
and through the crevasses countless small and large water-
hc vol. xxx — H
226
HELMHOLTZ
rills ripple, which carry off the water formed by the melting
of the ice. United, and forming a stream, they burst,
through a vaulted and clear blue gateway of ice, out at the
lower end of the larger glacier.
On the surface of the ice there is a large quantity of blocks
of stone, and of rocky debris, which at the lower end of
Fig. 104
the glacier are heaped up and form immense walls; these
are called the lateral and terminal moraine of the glacier.
Other heaps of rock, the central moraine, stretch along the
surface of the glacier in the direction of its length, form-
ing long regular dark lines. These always start from the
places where two glacier streams coincide and unite. The
central moraines are in such places to be regarded as the con-
tinuations of the united lateral moraines of the two glaciers.
The formation of the central moraine is well represented
in the view above given of the Unteraar Glacier (Fig. 104).
In the background are seen the two glacier currents emerg-
ing from different valleys; on the right from the Shreck-
ICE AND GLACIERS 227
horn, and on the left from the Finsteraarhorn. From
the place where they unite the rocky wall occupying the
middle of the picture descends, constituting the central
moraine. On the left are seen individual large masses of
rock resting on pillars of ice, which are known as glacier
tables.
To exemplify these circumstances still further, I lay
before you in Fig. 105 a map of the Mer de Glace of Cha-
mouni, copied from that of Forbes.
The Mer de Glace in size is well known as the largest
glacier in Switzerland, although in length it is exceeded by
the Aletsch Glacier. It is formed from the snow-fields that
cover the heights directly north of Mont Blanc, several of
which, as the Grande Jorasse, the Aiguille Verte (a, Figs.
105 and 106), the Aiguille du Geant (b), Aiguille du Midi
(c), and the Aiguille du Dru (d), are only 2,000 to 3,000 feet
below that king of the European mountains. The snow-
fields which lie on the slopes and in the basins between these
mountains collect in three principal currents, the Glacier du
Geant, Glacier de Lechaud, and Glacier du Talefre, which,
ultimately united, as represented in the map, form the Mer
de Glace; this stretches as an ice-current 2,600 to 3,000 feet
in breadth down into the valley of Chamouni, where a power-
ful stream, the Arveyron, bursts from its lower end at k,
and plunges into the Arve. The lowest precipice of the
Mer de Glace, which is visible from the valley of Chamouni,
and forms a large cascade of ice, is commonly called Glacier
des Bois, from a small village which lies below.
Most of the visitors at Chamouni only set foot on the low-
est part of the Mer de Glace from the inn at the Montan-
vert, and when they are free from giddiness cross the
glacier at this place to the little house on the opposite side,
the Chapeau (n). Although, as the map shows, only a com-
paratively very small portion of the glacier is thus seen and
crossed, this way shows sufficiently the magnificent scenes,
and also the difficulties of a glacier excursion. Bolder wan-
derers march upwards along the glacier to the Jardin, a
rocky cliff clothed with some vegetation, which divides the
glacial current of the Glacier du Talefre into two branches ;
and bolder still they ascend yet higher, to the Col du Geant
228
HELMHOLTZ
(11,000 feet above the sea), and down the Italian side to the
valley of Aosta.
The surface of the Mer de Glace shows four of the rocky
walls which we have designated as medial moraines. The
Fig. 105
first, nearest the side of the glacier, is formed where
the two arms of the Glacier du Talefre unite at the lower
end of the Jardin; the second proceeds from the union of
the glacier in question with the Glacier de Lechaud; the
third, from the union of the last with the Glacier du Geant;
and the fourth, finally, from the top of the rock-ledge
which stretches from the Aiguille du Geant towards the
cascade (g) of the Glacier du Geant.
To give you an idea of the slope and the fall of the glacier,
I have given in Fig. 106 a longitudinal section of it accord-
ICE AND GLACIERS 229
ing to the levels and measurements taken by Forbes, with
the view of the right bank of the glacier. The letters stand
for the same objects as in Fig. 105; p is the Aiguille de
Lechaud, q the Aiguille Noire, r the Mont Tacul, f is the
Col du Geant, the lowest point in the high wall of rock that
•0000'
Fig. 106
surrounds the upper end of the snow-fields which feed the
Mer de Glace. The base line corresponds to a length of a
little more than nine miles: on the right the heights above
the sea are given in feet. The drawing shows very dis-
tinctly how small in most places is the fall of the glacier.
Only an approximate estimate could be made of the depth,
for hitherto nothing certain has been made out in reference
to it. But that it is very deep is obvious from the following
individual and accidental observations.
At the end of a vertical rock wall of the Tacul, the edge
of the Glacier du Geant is pushed forth, forming an ice
wall 140 feet in height. This would give the depth of one
of the upper arms of the glacier at the edge. In the middle
and after the union of the three glaciers the depth must be
far greater. Somewhat below the junction Tyndall and
Hirst sounded a moulin, that is, a cavity through which the
surface glacier waters escape, to a depth of 160 feet; the
guides alleged that they had sounded a similar aperture to a
depth of 350 feet, and had found no bottom. From the
usually deep trough-shaped or gorge-like form of the bottom
of the valleys, which is constructed solely of rock walls, it
seems improbable that for a breadth of 3,000 feet the mean
depth should only be 350 feet; moreover, from the manner
in which ice moves, there must necessarily be a very thick
coherent mass beneath the crevassed part.
To render these magnitudes more intelligible by reference
to more familiar objects, imagine the valley of Heidelberg
230
HELMHOLTZ
filled with ice up to the Molkencur, or higher, so that the
whole town, with all its steeples and the castle, is buried
deeply beneath it; if, further, you imagine this mass of ice,
gradually extending in height, continued from the mouth
of the valley up to Neckargemiind, that would about corre-
spond to the lower united ice-current of the Mer de Glace.
Or, instead of the Rhine and the Nahe at Bingen, suppose
two ice-currents united which fill the Rhine valley to its
upper border as far as we can see from the river, and then
the united currents stretching downwards to beyond As-
Fig. 107
mannshausen and Burg Rheinstein; such a current would
also about correspond to the size of the Mer de Glace.
Fig. 107, which is a view of the magnificent Gorner Glacier
seen from below, also gives an idea of the size of the masses
of ice of the larger glaciers.
The surface of most glaciers is dirty, from the numerous
pebbles and sand which lie upon it, and which are heaped
together the more the ice under them and among them melts
away. The ice of the surface has been partially destroyed
and rendered crumbly. In the depths of the crevasses ice is
seen of a purity and clearness with which nothing that we
are acquainted with on the plains can be compared. From
its purity it shows a splendid blue, like that of the sky, only
with a greenish hue. Crevasses in which pure ice is visible
in the interior occur of all sizes; in the beginning tfiey form
ICE AND GLACIERS 231
slight cracks in which a knife can scarcely be inserted;
becoming gradually enlarged to chasms, hundreds or even
thousands, of feet in length, and twenty, fifty, and as much
as a hundred feet in breadth, while some of them are im-
measurably deep. Their vertical dark blue walls of crystal
ice, glistening with moisture from the trickling water, form
one of the most splendid spectacles which nature can present
to us ; but, at the same time, a spectacle strongly impregnated
with the excitement of danger, and only enjoyable by the
traveller who feels perfectly free from the slightest tendency
to giddiness. The tourist must know how, with the aid of well-
nailed shoes and a pointed Alpenstock, to stand even on
slippery ice, and at the edge of a vertical precipice the foot
of which is lost in the darkness of night, and at an unknown
depth. Such crevasses cannot always be evaded in crossing
the glacier; at the lower part of the Mer de Glace, for
instance, where it is usually crossed by travellers, we are
compelled to travel along some extent of precipitous banks
of ice which are occasionally only four to six feet in
breadth, and on each side of which is such a blue abyss.
Many a traveller, who has crept along the steep rocky
slopes without fear, there feels his heart sink, and cannot
turn his eyes from the yawning chasm, for he must first
carefully select every step for his feet. And yet these blue
chasms, which lie open and exposed in the daylight, are by
no means the worst dangers of the glacier; though, indeed,
we are so organised that a danger which we perceive, and
which therefore we can safely avoid, frightens us far more
than one which we know to exist, but which is veiled from
our eyes. So also it is with glacier chasms. In the lower
part of the glacier they yawn before us, threatening death
and destruction, and lead us, timidly collecting all our pres-
ence of mind, to shrink from them; thus accidents seldom
occur. On the upper part of the glacier, on the contrary,
the surface is covered with snow; this, when it falls thickly,
soon arches over the narrower crevasses of a breadth of
from four to eight feet, and forms bridges which quite con-
ceal the crevasse, so that the traveller only sees a beautiful
plane snow surface before him. If the snow bridges are thick
enough, they will support a man; but they are not always
232 HELMHOLTZ
so, and these are the places where men, and even chamois
are so often lost. These dangers may readily be guarded
against if two or three men are roped together at intervals
of ten or twelve feet. If then one of them falls into a
crevasse, the two others can hold him, and draw him out
again.
In some places the crevasses may be entered, especially
at the lower end of a glacier. In the well-known glaciers
of Grindelwald, Rosenlaui, and other places, this is facili-
tated by cutting steps and arranging wooden planks. Then
any one who does not fear the perpetually trickling water
may explore these crevasses, and admire the wonderfully
transparent and pure crystal walls of these caverns. The
beautiful blue colour which they exhibit is the natural
colour of perfectly pure water; liquid water as well as ice is
blue, though to an extremely small extent, so that the colour
is only visible in layers of from ten to twelve feet in thick-
ness. The water of the Lake of Geneva and of the Lago di
Garda exhibits the same splendid colour as ice.
The glaciers are not everywhere crevassed; in places
where the ice meets with an obstacle, and in the middle of
great glacier currents the motion of which is uniform, the
surface is perfectly coherent.
Fig. 108 represents one of the more level parts of the Mer
de Glace at the Montanvert, the little house of which is seen
in the background. The Gries Glacier, where it forms the
height of the pass from the Upper Rhone valley to the Tosa
valley, may even be crossed on horseback. We find the
greatest disturbance of the surface of the glacier in those
places where it passes from a slightly inclined part of its
bed to one where the slope is steeper. The ice is there torn
in all directions into a quantity of detached blocks, which by
melting are usually changed into wonderfully shaped sharp
ridges and pyramids, and from time to time fall into the
interjacent crevasses with a loud rumbling noise. Seen from
a distance such a place appears like a wild frozen waterfall,
and is therefore called a cascade ; such a cascade is seen in
the Glacier du Talefre at 1, another is seen in the Glacier
du Geant at g, Fig. iio, while a third forms the lower end of
the Mer de Glace. The latter, already mentioned as the
ICE AND GLACIERS
233
Glacier des Bois, which rises directly from the trough of the
valley at Chamouni to a height of 1,700 feet, the height of
the Konigstuhl at Heidelberg, affords at all times a chief
Fig. 108
object of admiration to the Chamouni tourist. Fig. 109
represents a view of its fantastically rent blocks of ice.
We have hitherto compared the glacier with a current as
regards its outer form and appearance. This similarity,
however, is not merely an external one : the ice of the
glacier does, indeed, move forwards like the water of a
stream, only more slowly. That this must be the case fol-
lows from the considerations by which I have endeavoured
234
HELMHOLTZ
to explain the origin of a glacier. For as the ice is being
constantly diminished at the lower end by melting, it would
Fig. 109
entirely disappear if fresh ice did not continually press
forward from above, which, again, is made up by the
snowfalls on the mountain tops.
But by careful ocular observation we may convince our-
selves that the glacier does actually move. For the inhab-
itants of the valleys, who have the glaciers constantly
before their eyes, often cross them, and in so doing make
use of the larger blocks of stone as sign posts — detect this
motion by the fact that their guide posts gradually descend
in the course of each year. And as the yearly displacement
of the lower half of the Mer de Glace at Chamouni amounts
to no less than from 400 to 600 feet, you can readily con-
ICE AND GLACIERS 235
ceive that such displacements must ultimately be observed,
notwithstanding the slow rate at which they take place, and
in spite of the chaotic confusion of crevasses and rocks
which the glacier exhibits.
Besides rocks and stones, other objects which have acci-
dentally alighted upon the glacier are dragged along. In
1788 the celebrated Genevese Saussure, together with his
son and a company of guides and porters, spent sixteen
days on the Col du Geant. On descending the rocks at the
side of the cascade of the Glacier du Geant, they left behind
them a wooden ladder. This was at the foot of the Aiguille
Noire, where the fourth band of the Mer de Glace begins;
this line thus marks at the same time the direction in which
ice travels from thiscpoint. In the year 1832, that is, forty-
four years after, fragments of this ladder were found by
Forbes and other travellers not far below the junction of
the three glaciers of the Mer de Glace, in the same line (at
s, Fig. i 10), from which it results that these parts of the
glacier must on the average have each year descended 375
feet.
In the year 1827 Hugi had built a hut on the central
moraine of the Unteraar Glacier for the purpose of making
observations; the exact position of this hut was determined
by himself and afterwards by Agassiz, and they found that
each year it had moved downwards. Fourteen years later,
in the year 1841, it was 4,884 feet lower, so that every year
it had on the average moved through 349 feet. Agassiz
afterwards found that his own hut, which he had erected on
the same glacier, had moved to a somewhat smaller extent.
For these observations a long time was necessary. But if
the motion of the glacier be observed by means of accurate
measuring instruments, such as theodolites, it is not neces-
sary to wait for years to observe that ice moves — a single
day is sufficient.
Such observations have in recent times been made by
several observers, especially by Forbes and by Tyndall. They
show that in summer the middle of the Mer de Glace moves
through twenty inches a day, while towards the lower ter-
minal cascade the motion amounts to as much as thirty-five
inches in a day. In winter the velocity is only about half as
236
HELMHOLTZ
great. At the edges and in the lower layers of the glacier,
as in a flow of water, it is considerably smaller than in the
centre of the surface.
The upper sources of the Mer de Glace also have a slower
motion, the Glacier du Geant thirteen inches a day, and
Vlrve
Fig. iio
the Glacier du Lechaud nine inches and a half. In different
glaciers the velocity is in general very various, according
to the size, the inclination, the amount of snow-fall, and
other circumstances.
Such an enormous mass of ice thus gradually and gently
moves on, imperceptibly to the casual observer, about an
inch an hour — the ice of the Col du Geant will take 120
years before it reaches the lower end of the Mer de Glace —
but it moves forward with uncontrollable force, before
which any obstacles that man could oppose to it yield like
straws, and the traces of which are distinctly seen even on
the granite walls of the valley. If, after a series of wet
seasons, and an abundant fall of snow on the heights, the
base of a glacier advances, not merely does it crush dwell-
ICE AND GLACIERS 237
ing houses, and break the trunks of powerful trees, but
the glacier pushes before it the boulder walls which form its
terminal moraine without seeming to experience any resist-
ance. A truly magnificent spectacle is this motion, so gentle
and so continuous, and yet so powerful and so irresistible.
I will mention here that from the way in which the glacier
moves we can easily infer in what places and in what direc-
tions crevasses will be formed. For as all layers of the
glacier do not advance with equal velocity, some points
remain behind others; for instance, the edges as compared
with the middle. Thus if we observe the distance from a
given point at the edge to a given point of the middle, both
of which were originally in the same line, but the latter of
which afterwards descended more rapidly, we shall find that
this distance continually increases; and since the ice cannot
expand to an extent corresponding to the increasing distance,
it breaks up and forms crevasses, as seen along the edge of
the glacier in Fig. hi, which represents the Gorner Glacier
at Zermatt. It would lead me too far if I were here to
attempt to give a detailed explanation of the formation of
the more regular system of crevasses, as they occur in certain
parts of all glaciers ; it may be sufficient to mention that the
conclusions deducible from the considerations above stated
are fully borne out by observation.
I will only draw attention to one point — what extremely
small displacements are sufficient to cause ice to form hun-
dreds of crevasses. The section of the Mer de Glace (Fig.
H2, at g, c, h) shows places where a scarcely perceptible
change in the inclination of the surface of the ice occurs
of from two to four degrees. This is sufficient to produce
a system of cross crevasses on the surface. Tyndall more
especially has urged and confirmed by observation and
measurements, that the mass of ice of the glacier does not
give way in the smallest degree to extension, but when sub-
jected to a pull is invariably torn asunder.
The distribution of the boulders, too, on the surface of the
glacier is readily explained when we take their motion into
account. These boulders are fragments of the mountains
between which the glacier flows. Detached partly by the
weathering of the stone, and partly by the freezing of water
238
HELMHOLTZ
in its crevices, they fall, and for the most part on the edge
of the mass of ice. There they either remain lying on the
surface, or if they have originally burrowed in the snow,
they ultimately reappear in consequence of the melting of
Fig. hi
the superficial layers of ice and snow, and they accumulate
especially at the lower end of the glacier, where more of
the ice between them has been melted. The blocks which are
gradually borne down to the lower end of the glacier are
sometimes quite colossal in size. Solid rocky masses of this
kind are met with in the lateral and terminal moraines,
which are as large as a two-storied house.
The masses of stone move in lines which are always nearly
parallel to each other and to the longitudinal direction of
the glacier. Those, therefore, that are already in the middle
remain in the middle, and those that lie on the edge remain
at the edge. These latter are the more numerous, for during
the entire course of the glacier fresh boulders are constantly
falling on the edge, but cannot fall on the middle. Thus are
formed on the edge of the mass of ice the lateral moraines,
ICE AND GLACIERS 239
the boulders of which partly move along with the ice, partly
glide over its surface, and partly rest on the solid rocky
base near the ice. But when two glacier streams unite,
their coinciding lateral moraines come to lie upon the centre
of the united ice-stream, and then move forward as central
moraines parallel to each other and to the banks of the
isono*
-5CG0'
Fig. 112
stream, and they show, as far as the lower end, the boundary-
line of the ice which originally belonged to one or the other
of the arms of the glacier. They are very remarkable as
displaying in what regular parallel bands the adjacent parts
of the ice-stream glide downwards. A glance at the map
of the Mer de Glace, and its four central moraines, exhibits
this very distinctly.
On the Glacier du Geant and its continuation in the Mer
de Glace, the stones on the surface of the ice delineate, in
alternately greyer and whiter bands, a kind of yearly rings
which were first observed by Forbes. For since in the cas-
cade at g, Fig. 112, more ice slides down in summer than in
winter, the surface of the ice below the cascade forms a
series of terraces as seen in the drawing, and as those slopes
of the terraces which have a northern aspect melt less than
their upper plane surfaces, the former exhibit purer ice than
the latter. This, according to Tyndall, is the probable
origin of these dirt bands. At first they run pretty much
across the glacier, but as afterwards their centre moves
somewhat more rapidly than the ends, they acquire farther
down a curved shape, as represented in the map, Fig. no.
By their curvature they thus show to the observer with
what varying velocity ice advances in the different parts of
its course.
A very peculiar part is played by certain stones which are
imbedded in the lower surface of the mass of ice, and which
240 HELMHOLTZ
have partly fallen there through crevasses, and may partly
have been detached from the bottom of the valley. For
these stones are gradually pushed with the ice along the
base of the valley, and at the same time are pressed against
this base by the enormous weight of the superincumbent ice.
Both the stones imbedded in the ice as well as the rocky
base are equally hard, but by their friction against each
other they are ground to powder with a power compared to
which any human exertion of force is infinitely small. The
product of this friction is an extremely fine powder, which,
swept away by water, appears lower down in the glacier
brook, imparting to it a whitish or yellowish muddy ap-
pearance.
The rocks of the trough of the valley, on the contrary,
on which the glacier exerts year by year its grinding power,
are polished as if in an enormous polishing machine. They
remain as rounded, smoothly polished masses, in which are
occasional scratches produced by individual harder stones.
Thus we see them appear at the edge of existing glaciers,
when after a series of dry and hot seasons the glaciers have
somewhat receded. But we find such polished rocks as
remains of gigantic ancient glaciers to a far greater extent
in the lower parts of many Alpine valleys. In the valley
of the Aar more especially, as far down as Meyringen, the
rock-walls polished to a considerable height are very charac-
teristic. There also we find the celebrated polished plates,
over which the way passes, and which are so smooth that
furrows have had to be hewn into them and rails erected to
enable men and animals to traverse them in safety.
The former enormous extent of glaciers is recognised by
ancient moraine-dykes and by transported blocks of stone,
as well as by these polished rocks. The blocks of stone
which have been carried away by the glacier are distin-
guished from those which water has rolled down, by their
enormous magnitude, by the perfect retention of all their
edges which are not at all rounded off, and finally by their
being deposited on the glacier in exactly the same order in
which the rocks of which they formed part stand in the
mountain ridge; while the stones which currents of water
carry along are completely mixed together.
ICE AND GLACIERS 241
From these indications, geologists have been able to prove
that the glaciers of Chamouni, of Monte Rosa, of the St.
Gotthard, and the Bernese Alps, formerly penetrated
through the valley of the Arve, the Rhone, the Aar, and
the Rhine to the more level part of Switzerland and the
Jura, where they have deposited their boulders at a height
of more than a thousand feet above the present level of the
lake of Neufchatel. Similar traces of ancient glaciers are
found upon the mountains of the British Islands, and upon
the Scandinavian Peninsula.
The drift ice too of the Arctic Sea is glacier ice; it is
pushed down into the sea by the glaciers of Greenland,
becomes detached from the rest of the glacier, and floats
away. In Switzerland we find a similar formation of drift-
ice, though on a far smaller scale, in the little Marjelen See,
into which part of the ice of the great Aletsch Glacier
pushes down. Blocks of stone which lie in drift-ice may
make long voyages over the sea. The vast number of blocks
of granite which are scattered on the North German plains,
and whose granite belongs to the Scandinavian mountains,
has been transported by drift-ice at the time when the
European glaciers had such an enormous extent.
I must unfortunately content myself with these few refer-
ences to the ancient history of glaciers, and revert now to
the processes at present at work in them.
From the facts which I have brought before you it results
that the ice of a glacier flows slowly like the current of a
very viscous substance, such for instance as honey, tar, or
thick magma of clay. The mass of ice does not merely flow
along the ground like a solid which glides over a precipice,
but it bends and twists in itself; and although even while
doing this it moves along the base of the valley, yet the
parts which are in contact with the bottom and the sides
of the valley are perceptibly retarded by the powerful fric-
tion ; the middle of the surface of the glacier, which is most
distant both from the bottom and the sides, moving most
rapidly. Rendu, a Savoyard priest, and the celebrated natural
philosopher Forbes, were the first to suggest the similarity
of a glacier with a current of a viscous substance.
Now you will perhaps inquire with astonishment how it is
242 HELMHOLTZ
possible that ice, which is the most brittle and fragile of
substances, can flow in the glacier like a viscous mass; and
you may perhaps be disposed to regard this as one of the
wildest and most improbable statements that have ever been
made by philosophers. I will at once admit that philoso-
phers themselves were not a little perplexed by these results
of their investigations. But the facts were there, and could
not be got rid of. How this mode of motion originated was
for a long time quite enigmatical, the more so since the
numerous crevasses in glaciers were a sufficient indication
of the well-known brittleness of ice; and as Tyndall cor-
rectly remarked, this constituted an essential difference be-
tween a stream of ice and the flow of lava, of tar, of honey,
or of a current of mud.
The solution of this strange problem was found, as is so
often the case in the natural sciences, in apparently recondite
investigations into the nature of heat, which form one of
the most important conquests of modern physics, and which
constitute what is known as the mechanical theory of heat.
Among a great number of deductions as to the relations of
the diverse natural forces to each other, the principles of
the mechanical theory of heat lead to certain conclusions as
to the dependence of the freezing-point of water on the
pressure to which ice and water are exposed.
Every one knows that we determine that one fixed point
of our thermometer scale which we call the freezing-point
or zero by placing the thermometer in a mixture of pure
water and ice. Water, at any rate when in contact with ice,
cannot be cooled below zero without itself being converted
into ice ; ice cannot be heated above the freezing-point with-
out melting. Ice and water can exist in each other's pres-
ence at only one temperature, the temperature of zero.
Now, if we attempt to heat such a mixture by a flame
beneath it, the ice melts, but the temperature of the mixture
is never raised above that of o° so long as some of the ice
remains unmelted. The heat imparted changes ice at zero
into water at zero, but the thermometer indicates no in-
crease of temperature. Hence physicists say that heat has
become latent, and that water contains a certain quantity of
latent heat beyond that of ice at the same temperature.
ICE AND GLACIERS 243
On the other hand, when we withdraw more heat from
the mixture of ice and water, the water gradually freezes;
but as long as there is still liquid water, the temperature
remains at zero. Water at o° has given up its latent heat,
and has become changed into ice at o°.
Now a glacier is a mass of ice which is everywhere inter-
penetrated by water, and its internal temperature is there-
fore everywhere that of the freezing-point. The deeper
layers, even of the fields of neve, appear on the heights
which occur in our Alpine chain to have everywhere the
same temperature. For, though the freshly fallen snow of
these heights is, for the most part, at a lower temperature
than that of o°, the first hours of warm sunshine melt its
surface and form water, which trickles into the deeper
colder layers, and there freezes, until it has throughout been
brought to the temperature of the freezing-point. This
temperature then remains unchanged. For, though by the
sun's rays the surface of the ice may be melted, it cannot
be raised above zero, and the cold of winter penetrates as
little into the badly conducting masses of snow and ice as
it does into our cellars. Thus the interior of the masses of
neve, as well as of the glacier, remains permanently at the
melting-point.
But the temperature at which water freezes may be altered
by strong pressure. This was first deduced from the me-
chanical theory of heat by James Thomson of Belfast, and
almost simultaneously by Clausius of Zurich; and, indeed,
the amount of this change may be correctly predicted from
the same reasoning. For each increase of a pressure of one
atmosphere the freezing-point is lowered by the i-ii5th part
of a degree Centigrade. The brother of the former, Sir W.
Thomson, the celebrated Glasgow physicist, made an experi-
mental confirmation of this theoretical deduction by com-
pressing in a suitable vessel a mixture of ice and snow.
This mixture became colder and colder as the pressure was
increased, and to the extent required by the mechanical
theory.
Now, if a mixture of ice and water becomes colder when
it is subjected to increased pressure without the withdrawal
of heat, this can only be effected by some free heat be-
244 HELMHOLTZ
coming latent; that is, some ice in the mixture must melt
and be converted into water. In this is found the reason
why mechanical pressure can influence the freezing-point.
You know that ice occupies more space than the water from
which it is formed. When water freezes in closed vessels,
it can burst not only glass vessels, but even iron shells.
Inasmuch, therefore, as in the compressed mixture of ice
and water some of the ice melts and is converted into water,
the volume of the mass diminishes, and the mass can yield
more to the pressure upon it than it could have done without
such an alteration of the freezing-point. Pressure furthers
in this case, as is usual in the interaction of various natural
forces, the occurrence of a change, that is fusion, which is
favourable to the development of its own activity.
In Sir W. Thomson's experiments water and ice were
confined in a closed vessel from which nothing could escape.
The case is somewhat different when, as with glaciers, the
water disseminated in the compressed ice can escape through
fissures. The ice is then compressed, but not the water
which escapes. The compressed ice becomes colder in con-
formity with the lowering of its freezing-point by pressure;
but the freezing-point of water which is not compressed is
not lowered. Thus under these circumstances we have ice
colder than o° in contact with water at o°. The consequence
is that around the compressed ice water continually freezes
and forms new ice, while on the other hand part of the
compressed ice melts.
This occurs, for instance, when only two pieces of ice are
pressed against each other. By the water which freezes at
their surfaces of contact they are firmly joined into one
coherent piece of ice. With powerful pressure, and the chil-
ling therefore great, this is quickly effected ; but even with a
feeble pressure it takes place, if sufficient time be given.
Faraday, who discovered this property, called it the regela-
iion of ice; the explanation of this phenomenon has been
much controverted ; I have detailed to you that which I con-
sider most satisfactory.
This freezing together of two pieces of ice is very readily
effected by pieces of any shape, which must not, however,
be at a lower temperature than o°, and the experiment sue-
ICE AND GLACIERS 245
ceeds best when the pieces are already in the act of melt-
ing. 1 They need only be strongly pressed together for a few
minutes to make them adhere. The more plane are the
surfaces in contact, the more complete is their union. But
a very slight pressure is sufficient if the two pieces are left
in contact for some time. 2
This property of melting ice is also utilised by boys in
making snow-balls and snow-men. It is well known that
this only succeeds either when the snow is already melting, or
at any rate is only so much lower than o° that the warmth of
the hand is sufficient to raise it to this temperature. Very cold
snow is a dry loose powder which does not stick together.
The process which children carry out on a small scale in
making snow-balls takes place in glaciers on the very largest
scale. The deeper layers of what was originally fine loose
neve are compressed by the huge masses resting on them,
often amounting to several hundred feet, and under this
pressure they cohere with an ever firmer and closer struc-
ture. The freshly fallen snow originally consisted of delicate
microscopically fine ice-spicules, united and forming delicate
six-rayed, feathery stars of extreme beauty. As often as
the upper layers of the snow-fields are exposed to the sun's
rays, some of the snow melts; water permeates the mass,
and on reaching the lower layers of still colder snow, it
again freezes; thus it is that the firn first becomes granular
and acquires the temperature of the freezing-point. But as
the weight of the superincumbent masses of snow con-
tinually increases by the firmer adherence of its individual
granules, it ultimately changes into a dense and perfectly
hard mass.
This transformation of snow into ice may be artificially
effected by using a corresponding pressure.
We have here (Fig. 113) a cylindrical cast iron vessel,
A A; the base, B B, is held by three screws, and can be
detached, so as to remove the cylinder of ice which is
formed. After the vessel has lain for a while in ice-water,
so as to reduce it to the temperature of o°, it is packed full
1 In the Lecture a series of small cylinders of ice, which had been pre-
pared by a method to be afterwards described, were pressed with their plane
ends against each other, and thus a cylindrical bar of ice produced.
2 Vide the additions at the end of this Lecture.
246
HELMHOLTZ
of snow, and then the cylindrical plug, C C, which fits the
inner aperture, but moves in it with gentle friction, is
forced in with the aid of an
hydraulic press. The press
used was such that the pres-
sure to which the snow was
exposed could be increased to
fifty atmospheres. Of course
the looser snow contracts to a
very small volume under such
a powerful pressure. The
pressure is removed, the
cylindrical plug taken out,
the hollow again filled up with
snow, and the process re-
peated until the entire form
is filled with the mass of ice,
which no longer gives way
to pressure. The compressed
snow which I now take out,
you will see, has been trans-
formed into a hard, angu-
lar, and translucent cylinder
of ice; and how hard it is
appears from the crash which
ensues when I throw it to the ground. Just as the loose
snow in the glaciers is pressed together to solid ice, so also
in many places ready-formed irregular pieces of ice are
joined and form clear and compact ice. This is most re-
markable at the base of the glacier cascades. These are
glacier falls where the upper part of the glacier ends at a
steep rocky wall, and blocks of ice shoot down as avalanches
over the edge of this wall. The heap of shattered blocks of
ice which accumulate become joined at the foot of the rock-
wall to a compact, dense mass, which then continues its way
downwards as glacier. More frequent than such cascades,
where the glacier-stream is quite dissevered, are places
where the base of the valley has a steeper slope, as for in-
stance, the places in the Mer de Glace (Fig. 105), at g, of
the Cascade of the Glacier du Geant, and at i and h of the
Fig. 113
ICE AND GLACIERS 247
great terminal cascade of the Glacier des Bois. The ice
splits there into thousands of banks and cliffs, which then
recombine towards the bottom of the steeper slope and form
a coherent mass.
This also we may imitate in our ice-mould. Instead of
the snow I take irregular pieces of ice, press them together ;
add new pieces of ice, press them again, and so on, until
the mould is full. When the mass is taken out it forms a
compact coherent cylinder of tolerably clear ice, which has
a perfectly sharp edge, and is an accurate copy of the
mould.
This experiment, which was first made by Tyndall, shows
that a block of ice may be pressed into any mould just like
a piece of wax. It might, perhaps, be thought that such a
block had, by the pressure in the interior, been first reduced
to powder so fine that it readily penetrated every crevice of
the mould, and then that this powdered ice, like snow, was
again combined by freezing. This suggests itself the more
readily, since while the press is being worked a continual
creaking and cracking is heard in the interior of the mould.
Yet the mere aspect of the cylinders pressed from blocks of
ice shows us that it has not been formed in this manner ; for
they are generally clearer than the ice which is produced
from snow, and the individual larger pieces of ice which
have been used to produce them are recognised, though they
are somewhat changed and flattened. This is most beautiful
when clear pieces of ice are laid in the form and the rest
of the space stuffed full of snow. The cylinder is then seen
to consist of alternate layers of clear and opaque ice, the
former arising from the pieces of ice, and the latter from
the snow; but here also the pieces of ice seem pressed into
flat discs.
These observations teach, then, that ice need not be com-
pletely smashed to fit into the prescribed mould, but that
it may give way without losing its coherence. This can be
still more completely proved, and we can acquire a still
better insight into the cause of the pliability of ice, if we
press the ice between two plane wooden boards, instead of
in the mould, into which we cannot see.
I place first an irregular cylindrical piece of natural ice,
248
HELMHOLTZ
taken from the frozen surface of the river, with its two
plane terminal surfaces between the plates of the press. If
I begin to work, the block is broken by pressure; every
Fig. 114
Fig. 115
crack which forms extends through the entire mass of the
block; this splits into a heap of larger fragments, which
again crack and are broken the more the press is worked.
If the pressure is relaxed, all these fragments are, indeed,
reunited by freezing, but the aspect of the whole indicates
that the shape of the block has resulted less from pliability
than from fracture, and that the individual fragments have
completely altered their mutual positions.
The case is quite different when one of the cylinders
which we have formed from snow or ice is placed between
the plates of the press. As the press is worked the creak-
ing and cracking is heard, but it does not break ; it gradually
changes its shape, becomes lower and at the same time
thicker ; and only when it has been changed into a tolerably
flat circular disc does it begin to give way at the edges
and form cracks, like crevasses on a small scale. Fig. 114
shows the height and diameter of such a cylinder in its
original condition; Fig. 115 represents its appearance after
the action of the press.
A still stronger proof of the pliability of ice is afforded
when one of our cylinders is forced through a narrow
ICE AND GLACIERS
249
Fig. 116
aperture. With this view I place a base on the previously-
described mould, which has a conical perforation, the ex-
ternal aperture of which is
only two thirds the diameter
of the cylindrical aperture of
the form. Fig. 116 gives a
section of the whole. If now
I insert into this one of
the compressed cylinders of
ice, and force down the plug
a, the ice is forced through
the narrow aperture in the
base. It at first emerges as
a solid cylinder of the same
diameter as the aperture; but
as the ice follows more rap-
idly in the centre than at
the edges, the free terminal surface of the cylinder becomes
curved, the end thickens, so that it could not be brought
back through the aperture, and it ultimately splits off. Fig.
117 exhibits a series of shapes which have resulted in
this manner. 3
Here also the cracks in the emerging cylinder of ice
exhibit a surprising similarity with the longitudinal rifts
which divide a glacier current where it presses through a
narrow rocky pass into a wider valley.
In the cases which we have described we see the change
in shape of the ice taking place before our eyes, whereby
the block of ice retains its coherence without breaking into
individual pieces. The brittle mass of ice seems rather to
yield like a piece of wax.
A closer inspection of a clear cylinder of ice compressed
from clear pieces of ice, while the pressure is being applied,
shows us what takes place in the interior; for we then see
an innumerable quantity of extremely fine radiating cracks
shoot through it like a turbid cloud, which mostly disappear,
8 In this experiment the lower temperature of the compressed ice some-
times extended so far through the iron form, that the water in the slit
between the base plate and the cylinder froze and formed a thin sheet of
ice, although the pieces of ice as well as the iron mould had previously laid
in. ice-water, and could not be colder than o°.
250 HELMHOLTZ
though not completely, the moment the pressure is suspended.
Such a compressed block is distinctly more opaque imme-
diately after the experiment than it was before; and the
turbidity arises, as may easily be observed by means of a
lens, from a great number of whitish capillary lines crossing
Fig. 117
the interior of the mass of what is otherwise clear. These
lines are the optical expression of extremely fine cracks 4
which interpenetrate the mass of the ice. Hence we may
conclude that the compressed block is traversed by a great
number of fine cracks and fissures which render it- pliable;
that its particles become a little dispersed, and are therefore
withdrawn from pressure, and that immediately afterwards
the greater part of the fissures disappear, owing to their
sides freezing. Only in those places in which the surfaces
of the small displaced particles do not accurately fit to each
other some fissured spaces remain open, and are discovered
as white lines and surfaces by the reflection of the light.
These cracks and laminae also become more perceptible
when the ice — which, as I before mentioned, is below zero
immediately after pressure has been applied — is again raised
to this temperature and begins to melt. The crevices then
fill with water, and such ice then consists of a quantity of
minute granules from the size of a pin's head to that of a
pea, which are closely pushed into one another at the edges
and projections, and in part have coalesced, while the nar-
row fissures between them are full of water. A block of
ice thus formed of ice-granules adheres firmly together ; but
4 These cracks are probably quite empty and free from air, for they are
also formed when perfectly clear and air-free pieces of ice are pressed in
the form which has been previously rilled with water, and where, therefore,
no air could gain access to the pieces of ice. That such air-free crevices
occur in glacier ice has been a1 r eady demonstrated by Tyndall. When the
compressed ice afterwards melts, these crevices fill up with water, no air
being left. They are then, however, far less visible, and the whole block
is therefore clearer. And just for this reason they could not originally have
been filled with water.
ICE AND GLACIERS 251
if particles be detached from its corners they are seen to
consist of these angular granules. Glacier ice, when it
begins to melt, is seen to possess the same structure, except
that the pieces of which it consists are mostly larger than
in artificial ice, attaining the size of a pigeon's egg.
Glacier ice and compressed ice are thus seen to be sub-
stances of a granular structure, in opposition to regularly
crystallised ice, such as is formed on the surface of still
water. We here meet with the same differences as between
calcareous spar and marble, both of which consist of carbon-
ate of lime ; but while the former is in large, regular crystals,
the latter is made up of irregularly agglomerated crystalline
grains. In calcareous spar, as well as in crystallised ice, the
cracks produced by inserting the point of a knife extend
through the mass, while in granular ice a crack which arises
in one of the bodies where it must yield does not necessarily
spread beyond the limits of the granule.
Ice which has been compressed from snow, and has thus
from the outset consisted of innumerable very fine crystal-
line needles, is seen to be particularly plastic. Yet in ap-
pearance it materially differs from glacier ice, for it is very
opaque, owing to the great quantity of air which was origi-
nally inclosed in the flaky mass of snow, and which remains
there as extremely minute bubbles. It can be made clearer
by pressing a cylinder of such ice between wooden boards;
the air-bubbles appear then on the top of the cylinder as a
light foam. If the discs are again broken, placed in the
mould, and pressed into a cylinder, the air may gradually
be more and more eliminated, and the ice be made clearer.
No doubt in glaciers the originally whitish mass of neve is
thus gradually transformed into the clear, transparent ice
of the glacier.
Lastly, when streaked cylinders of ice formed from pieces
of snow and ice are pressed into discs, they become finely
streaked, for both their clear and their opaque layers are
uniformly extended.
Ice thus striated occurs in numerous glaciers, and is no
doubt caused, as Tyndall maintains, by snow falling between
the blocks of ice; this mixture of snow and clear ice is
again compressed in the subsequent path of the glacier, and
252 HELMHOLTZ
gradually stretched by the motion of the mass: a process
quite analogous to the artificial one which we have demon-
strated.
Thus to the eye of the natural philosopher the glacier,
with its wildly heaped ice-blocks, its desolate, stony, and
muddy surface, and its threatening crevasses, has become
a majestic stream whose peaceful and regular flow has
no parallel; which according to fixed and definite laws,
narrows, expands, is heaped up, or, broken and shattered,
falls down precipitous heights. If we trace it beyond its
termination we see its waters, uniting to a copious brook,
burst through its icy gate and flow away. Such a brook, on
emerging from the glacier, seems dirty and turbid enough,
for it carries away as powder the stone which the glacier
has ground. We are disenchanted at seeing the wondrously
beautiful and transparent ice converted into such muddy
water. But the water of the glacier streams is as pure and
beautiful as the ice, though its beauty is for the moment con-
cealed and invisible. We must search for these waters after
they have passed through a lake in which they have de-
posited this powdered stone. The Lakes of Geneva, of
Thun, of Lucerne, of Constance, the Lago Maggiore, the
Lake of Como, and the Lago di Garda are chiefly fed with
glacier waters; their clearness and their wonderfully beau-
tiful blue or blue-green colour are the delight of all
travellers.
Yet, leaving aside the beauty of these waters, and consid-
ering only their utility, we shall have still more reason for
admiration. The unsightly mud which the glacier streams
wash away forms a highly fertile soil in the places where it
is deposited ; for its state of mechanical division is extremely
fine, and it is moreover an utterly unexhausted virgin soil,
rich in the mineral food of plants. The fruitful layers of
fine loam which extend along the whole Rhine plain as far
as Belgium, and are known as Loess, are nothing more than
the dust of ancient glaciers.
Then, again, the irrigation of a district, which is effected
by the snow-fields and glaciers of the mountains, is distin-
guished from that of other places by its comparatively
greater abundancy, for the moist air which is driven over
ICE AND GLACIERS 253
the cold mountain peaks deposits there most of the water it
contains in the form of snow. In the second place, the snow
melts most rapidly in summer, and thus the springs which
flow from the snow-fields are most abundant in that season
of the year in which they are most needed.
Thus we ultimately get to know the wild, dead ice-wastes
from another point of view. From them trickles in thou-
sands of rills, springs, and brooks the fructifying moisture
which enables the industrious dwellers of the Alps to pro-
cure succulent vegetation and abundance of nourishment
from the wild mountain slopes. On the comparatively small
surface of the Alpine chain they produce the mighty streams
the Rhine, the Rhone, the Po, the Adige, the Inn, which for
hundreds of miles form broad, rich river-valleys, extending
through Europe to the German Ocean, the Mediterranean,
the Adriatic, and the Black Sea. Let us call to mind how
magnificently Goethe, in " Mahomet's Song," has depicted the
course of the rocky spring, from its origin beyond the clouds
to its union with Father Ocean. It would be presumptuous
after him to give such a picture in other than his own
words : —
And along, in triumph rolling,
Names he gives to regions ; cities
Grow amain beneath his feet.
On and ever on he rushes ;
Spire and turret fiery crested
Marble palaces, the creatures
Of his wealth, he leaves behind.
Pine-built houses bears the Atlas
On his giant shoulders. O'er his
Head a thousand pennons rustle,
Floating far upon the breezes,
Tokens of his majesty.
And so beareth he his brothers,
And his treasures, and his children,
To their primal sire expectant,
All his bosom throbbing, heaving,
With a wild tumultuous joy.
Theodore Martin's Translation,
ADDITIONS
The theory of the regelation of ice has led to scientific dis-
cussions between Faraday and Tyndall on the one hand,
and James and Sir W. Thomson on the other. In the text
I have adopted the theory of the latter, and must now ac-
cordingly defend it.
Faraday's experiments show that a very slight pressure,
not more than that produced by the capillarity of the layer
of water between two pieces of ice, is sufficient to freeze
them together. James Thomson observed that in Faraday's
experiments pressure which could freeze them together was
not utterly wanting. I have satisfied myself by my own
experiments that only very slight pressure is necessary. It
must, however, be remembered that the smaller the pressure
the longer will be the time required to freeze the two pieces,
and that then the junction will be very narrow and very
fragile. Both these points are readily explicable on Thom-
son's theory. For under a feeble pressure the difference in
temperature between ice and water will be very small, and
the latent heat will only be slowly abstracted from the
layers of water in contact with the pressed parts of the
ice, so that a long time is necessary before they freeze. We
must further take into account that we cannot in general
consider that the two surfaces are quite in contact; under
a feeble pressure which does not appreciably alter their
shape, they will only touch in what are practically three
points. A feeble total pressure on the pieces of ice con-
centrated on such narrow surfaces will always produce a
tolerably great local pressure under the influence of which
some ice will melt, and the water thus formed will freeze.
But the bridge which joins them will never be otherwise
than narrow.
Under stronger pressure, which may more completely
alter the shape of the pieces of ice, and fit them against
each other, and which will melt more of the surfaces that
254
ICE AND GLACIERS 255
are first in contact, there will be a greater difference between
the temperature of the ice and water, and the bridges will
be more rapidly formed, and be of greater extent.
In order to show the slow action of the small differences
of temperature which here come into play, I made the
following experiments.
A glass flask with a drawn-out neck was half filled with
water, which was boiled until all the air in the flask was driven
out. The neck of the flask was then hermetically sealed.
When cooled, the flask was void of air, and the water within
it freed from the pressure of the atmosphere. As the
water thus prepared can be cooled considerably below o° C.
before the first ice is formed, while when ice is in the flask it
freezes at o° C, the flask was in the first instance placed in a
freezing mixture until the water was changed into ice. It
was afterwards permitted to melt slowly in a place the temper-
ature of which was + 2° C, until the half of it was liquefied.
The flask thus half filled with water, having a disc of ice
swimming upon it, was placed in a mixture of ice and water,
being quite surrounded by the mixture. After an hour, the
disc within the flask was frozen to the glass. By shaking
the flask the disc was liberated, but it froze again. This
occurred as often as the shaking was repeated.
The flask was permitted to remain for eight days in the
mixture, which was kept throughout at a temperature of o°
C. During this time a number of very regular and sharply
defined ice-crystals were formed, and augmented very slowly
in size. This is perhaps the best method of obtaining beauti-
fully formed crystals of ice.
While, therefore, the outer ice which had to support the
pressure of the atmosphere slowly melted, the water within
the flask, whose freezing-point, on account of a defect of
pressure, was 0.0075 C. higher, deposited crystals of ice.
The heat abstracted from the water in this operation had,
moreover, to pass through the glass of the flask, which,
together with the small difference of temperature, explains
the slowness of the freezing process.
Now as the pressure of one atmosphere on a square
millimetre amounts to about ten grammes, a piece of ice
weighing ten grammes, which lies upon another and touches
256 HELMHOLTZ
it in three places, the total surface of which is a square
millimetre, will produce on these surfaces a pressure of an
atmosphere. Ice will therefore be formed more rapidly in
the surrounding water than it was in the flask, where the
side of the glass was interposed between the ice and the
water. Even with a much smaller weight the same result
will follow in the course of an hour. The broader the
bridges become, owing to the freshly formed ice, the greater
will be the surfaces over which the pressure exerted by
the upper piece of ice is distributed, and the feebler it will
become; so that with such feeble pressure the bridges can
only slowly increase, and therefore they will be readily
broken when we try to separate the pieces.
It cannot, moreover, be doubted that in Faraday's ex-
periments, in which two perforated discs of ice were
placed in contact on a horizontal glass rod, so that grav-
ity exerted no pressure, capillary attraction is sufficient to
produce a pressure of some grammes between the plates, and
the preceding discussions show that such a pressure, if ade-
quate time be given, can form bridges between the plates.
If, on the other hand, two of the above-described cylinders
of ice are powerfully pressed together by the hands, they
adhere in a few minutes so firmly that they can only be
detached by the exertion of a considerable force, for which
indeed that of the hands is sometimes inadequate.
In my experiments I found that the force and rapidity
with which the pieces of ice united were so entirely pro-
portional to the pressure that I cannot but assign this as the
actual and sufficient cause of their union.
In Faraday's explanation, according to which regelation
is due to a contact action of ice and water, I find a theo-
retical difficulty. By the water freezing, a considerable
quantity of latent heat must be set free, and it is not clear
what becomes of this.
Finally, if ice in its change into water passes through
an intermediate viscous condition, a mixture of ice and water
which was kept for days at a temperature of o° must ulti-
mately assume this condition in its entire mass, provided
its temperature was uniform throughout; this however is
never the case.
ICE AND GLACIERS 257
As regards what is called the plasticity of ice, James
Thomson has given an explanation of it in which the form-
ation of cracks in the interior is not presupposed. No doubt
when a mass of ice in different parts of the interior is ex-
posed to different pressures, a portion of the more power-
fully compressed ice will melt ; and the latent heat necessary
for this will be supplied by the ice which is less strongly
compressed, and by the water in contact with it. Thus ice
would melt at the compressed places, and water would
freeze in those which are not pressed: ice would thus be
gradually transformed and yield to pressure. It is also clear
that, owing to the very small conductivity for heat which ice
possesses, a process of this kind must be extremely slow,
if the compressed and colder layers of ice, as in glaciers,
are at considerable distances from the less compressed ones,
and from the water which furnishes the heat for melting.
To test this hypothesis, I placed in a cylindrical vessel,
between two discs of ice of three inches in diameter, a
smaller cylindrical piece of an inch in diameter. On the
uppermost disc I placed a wooden disc, and this I loaded
with a weight of twenty pounds. The section of the narrow
piece was thus exposed to a pressure of more than an
atmosphere. The whole vessel was packed between pieces
of ice, and left for five days in a room the temperature of
which was a few degrees above the freezing-point. Under
these circumstances the ice in the vessel, which was exposed
to the pressure of the weight, should melt, and it might be
expected that the narrow cylinder on which the pressure
was most powerful should have been most melted. Some
water was indeed formed in the vessel, but mostly at the
expense of the larger discs at the top and bottom, which
being nearest the outside mixture of ice and water, could
acquire heat through the sides of the vessel. A small welt,
too, of ice, was formed round the surface of contact of the
narrower with the lower broad piece, which showed that
the water, which had been formed in consequence of the
pressure, had again frozen in places in which the pressure
ceased. Yet under these circumstances there was no ap-
preciable alteration in the shape of the middle piece which
was most compressed.
hc vol. xxx — I
258 HELMHOLTZ
This experiment shows that although changes in the
shape of the pieces of ice must take place in the course of
time in accordance with J. Thomson's explanation, by which
the more strongly compressed parts melt, and new ice is
formed at the places which are freed from pressure, these
changes must be extremely slow when the thickness of the
pieces of ice through which the heat is conducted is at all
considerable. Any marked change in shape by melting in
a medium the temperature of which is everywhere o°, could
not occur without access of external heat, or from the un-
compressed ice and water; and with the small differences in
temperature which here come into play, and from the
badly conducting power of ice, it must be extremely slow.
That on the other hand, especially in granular ice, the
formation of cracks, and the displacement of the surfaces
of those cracks, render such a change of form possible, is
shown by the above-described experiments on pressure; and
that in glacier ice changes of form thus occur, follows
from the banded structure, and the granular aggregation
which is manifest on melting, and also from the manner in
which the layers change their position when moved, and
so forth. Hence, I doubt not that Tyndall has discovered
the essential and principal cause of the motion of glaciers,
in referring it to the formation of cracks and to regelation.
I would at the same time observe that a quantity of heat,
which is far from inconsiderable, must be produced by
friction in the larger glaciers. It may be easily shown by
calculation that when a mass of firn moves from the Col
du Geant to the source of the Arveyron, the heat due to the
mechanical work would be sufficient to melt a fourteenth
part of the mass. And as the friction must be greatest in
those places that are most compressed, it will at any rate
be sufficient to remove just those parts of the ice which
offer most resistance to motion.
I will add, in conclusion, that the above-described granular
structure of ice is beautifully shown in polarised light. If
a small clear piece is pressed in the iron mould, so as to
form a disc of about five inches in thickness, this is suffi-
ciently transparent for investigation. Viewed in the polar-
ising apparatus, a great number of variously coloured small
ICE AND GLACIERS 259
bands and rings are seen in the interior; and by the ar-
rangement of their colours it is easy to recognise the limits
of the ice-granules, which, heaped on one another in irregu-
lar order of their optical axes, constitute the plate. The
appearance is essentially the same when the plate has just
been taken out of the press, and the cracks appear in it
as whitish lines, as afterwards when these crevices have
been filled up in consequence of the ice beginning to melt.
In order to explain the continued coherence of the piece
of ice during its change of form, it is to be observed that
in general the cracks in the granular ice are only superficial,
and do not extend throughout its entire mass. This is
directly seen during the pressing of the ice. The crevices
form and extend in different directions, like cracks produced
by a heated wire in a glass tube. Ice possesses a certain de-
gree of elasticity, as may be seen in a thin flexible plate. A
fissured block of ice of this kind will be able to undergo a
displacement at the two sides which form the crack, even
when these continue to adhere in the unfissured part of the
block. If then part of the fissure at first formed is closed
by regelation, the fissure can extend in the opposite direction
without the continuity of the block being at any time dis-
turbed. It seems to me doubtful, too, whether in compressed
ice and in glacier ice, which apparently consists of interlaced
polyhedral granules, these granules, before any attempt is
made to separate them, are completely detached from each
other, and are not rather connected by ice-bridges which
readily give way ; and whether these latter do not produce the
comparatively firm coherence of the apparent heap of granules.
The properties of ice here described are interesting from
a physical point of view, for they enable us to follow so
closely the transition from a crystalline body to a granular
one; and they give the causes of the alteration of its proper-
ties better than in any other well-known example. Most
natural substances show no regular crystalline structure;
our theoretical ideas refer almost exclusively to crystallised
and perfectly elastic bodies. It is precisely in this relation-
ship that the transition from fragile and elastic crystalline
ice into plastic granular ice is so very instructive.
THE WAVE THEORY OF LIGHT
THE TIDES
BY
SIR WILLIAM THOMSON
(LORD KELVIN)
INTRODUCTORY NOTE
William Thomson, Baron Kelvin of Largs, was born in
Belfast, Ireland, June 24, 1824. He was the son of the pro-
fessor of mathematics at Glasgow University, and himself en-
tered that institution at the age of eleven. By the time he was
twenty-one he graduated from Cambridge as Second Wrangler,
and, after studying in Paris, he returned to Scotland to become,
as professor of natural philosophy, the colleague of his father
and elder brother. The story of his life thenceforth is the
record of amazingly brilliant and fruitful scientific work,
recognized by the award of almost all the honors appropriate
to such service, from learned societies, universities, and govern-
ments at home and abroad. His part in laying the Atlantic
Cable was the occasion of his receiving knighthood, and in 1892
he was raised to the peerage. He held his professorship at
Glasgow for fifty-three years, and later was chosen its Chan-
cellor. He died on December 17, 1907, and was buried in
Westminster Abbey.
Lord Kelvin's activities were remarkable for both profundity
and range. A large number of his results are to be appreciated
only by the highly skilled mathematician and physicist; but his
speculations on the ultimate constitution of matter; his state-
ment of the principle of the dissipation of energy, with its
bearing upon the age of life on the earth; his calculations as
to the age of the earth itself, and much more, are of great
general interest. His fertility in practical invention was no less
notable. He contrived a large number of instruments, his ser-
vices to navigation and ocean telegraphy being especially valu-
able. Long before his death he was recognized as the most
distinguished man of science of his time and country, and he
was also the most loved.
The lectures which follow are favorable examples of his
power of exposition in subjects in which he had no superior.
262
THE WAVE THEORY OF
LIGHT
[A LECTURE DELIVERED AT THE ACADEMY
OF MUSIC, PHILADELPHIA, UNDER THE
AUSPICES OF THE FRANKLIN INSTI-
TUTE, SEPTEMBER 29th, 1884]
THE subject upon which I am to speak to you this even-
ing is happily for me not new in Philadelphia. The
beautiful lectures on light which were given several
years ago by President Morton, of the Stevens' Institute, and
the succession of lectures on the same subject so admirably
illustrated by Professor Tyndall, which many now pres-
ent have heard, have fully prepared you for anything I
can tell you this evening in respect to the wave theory
of light.
It is indeed my humble part to bring before you only
some mathematical and dynamical details of this great theory.
I cannot have the pleasure of illustrating them to you by
anything comparable with the splendid and instructive ex-
periments which many of you have already seen. It is
satisfactory to me to know that so many of you, now
present, are so thoroughly prepared to understand anything
I can say, that those who have seen the experiments will not
feel their absence at thisi time. At the same time I wish to
make them intelligible to those who have not had the advan-
tages to be gained by a systematic course of lectures. I must
say, in the first place, without further preface, as time is
short and the subject is long, simply that sound and light
are both due to vibrations propagated in the manner of
waves; and I shall endeavour in the first place to define
the manner of propagation and the mode of motion that
263
264 KELVIN
constitute those two subjects of our senses, the sense 6l
sound and the sense of light.
Each is due to vibrations, but the vibrations of light differ
widely from the vibrations of sound. Something that I
can tell you more easily than anything in the way of dy-
namics or mathematics respecting the two classes of vibra-
tions is, that there is a great difference in the frequency of
the vibrations of light when compared with the frequency
of the vibrations of sound. The term " frequency " applied
to vibrations is a convenient term, applied by Lord Rayleigh
in his book on sound to a definite number of full vibrations
of a vibrating body per unit of time. Consider, then, in
respect to sound, the frequency of the vibrations of notes,
which you all know in music represented by letters, and
by the syllables for singing, the do, re, mi, &c. The notes
of the modern scale correspond to different frequencies of
vibrations. A certain note and the octave above it, cor-
respond to a certain number of vibrations per second, and
double that number.
I may conveniently explain in the first place the note
called ' C ' ; I mean the middle ' C ' ; I believe it is the C
of the tenor voice, that most nearly approaches the
tones used in speaking. That note corresponds to two
hundred and fifty-six full vibrations per second — two hun-
dred and fifty-six times to and fro per second of time.
Think of one vibration per second of time. The seconds
pendulum of the clock performs one vibration in two sec-
onds, or a half vibration in one direction per second. Take
a ten-inch pendulum of a drawing-room clock, which vibrates
twice as fast as the pendulum of an ordinary eight-day
clock, and it gives a vibration of one per second, a full
period of one per second to and fro. Now think of three
vibrations per second. I can move my hand three times
per second easily, and by a violent effort I can move it
to and fro five times per second. With four times as
great force, if I could apply it, I could move it twice
five times per second.
Let us think, then, of an exceedingly muscular arm that
would cause it to vibrate ten times per second, that is, ten
times to the left and ten times to the right. Think of twice
THE WAVE THEORY OF LIGHT 265
ten times, that is, twenty times per second, which would
require four times as much force; three times ten, or
thirty times a second, would require nine times as much
force. If a person were nine times as strong as the most
muscular arm can be, he could vibrate his hand to and
fro thirty times per second, and without any other musi-
cal instrument could make a musical note by the move-
ment of his hand which would correspond to one of the
pedal notes of an organ.
If you want to know the length of a pedal pipe, you can
calculate it in this way. There are some numbers you must
remember, and one of them is this. You, in this coun-
try, are subjected to the British insularity in weights and
measures ; you use the foot and inch and yard. I am obliged
to use that system, but I apologise to you for doing so,
because it is so inconvenient, and I hope all Americans
will do everything in their power to introduce the French
metrical system. I hope the evil action performed by
an English minister whose name I need not mention, be-
cause I do not wish to throw obloquy on any one, may
be remedied. He abrogated a useful rule, which for a
short time was followed, and which I hope will soon be
again enjoined, that the French metrical system be taught
in all our national schools. 1 do not know how it is in Amer-
ica. The school system seems to be very admirable, and
I hope the teaching of the metrical system will not be let
slip in the American schools any more than the use of
the globes. I say this seriously: I do not think any one
knows how seriously I speak of it. I look upon our English
system as a wickedly brain-destroying piece of bondage
under which we suffer. The reason why we continue to
use it is the imaginary difficulty of making a change, and
nothing else; but I do not think in America that any such
difficulty should stand in the way of adopting so splendidly
useful a reform.
I know the velocity of sound in feet per second. If
I remember rightly, it is 1089 feet per second in dry air
at the freezing temperature, and 11 15 feet per second in
air of what we would call moderate temperature, 59 or 60 de-
grees — (I do not know whether that temperature is ever
266 KELVIN
attained in Philadelphia or not; I have had no experience
of it, but people tell me it is sometimes 59 or 60 degrees
in Philadelphia, and I believe them) — in round numbers
let us call the speed 1000 feet per second. Sometimes we
call it a thousand musical feet per second, it saves trouble
in calculating the length of organ pipes; the time of vibra-
tion in an organ pipe is the time it takes a vibration
to run from one end to the other and back. In an
organ pipe 500 feet long the period would be one per
second; in an organ pipe ten feet long the period would
be 50 per second ; in an organ pipe twenty feet long the period
would be 25 per second at the same rate. Thus 25 per second,
and 50 per second of frequencies correspond to the periods
of organ pipes of 20 feet and 10 feet.
The period of vibration of an organ pipe, open at both
ends, is approximately the time it takes sound to travel
from one end to the other and back. You remember that
the velocity in dry air in a pipe 10 feet long is a little
more than 50 periods per second; going up to 256 periods
per second, the vibrations correspond to those of a pipe
two feet long. Let us take 512 periods per second; that
corresponds to a pipe about a foot long. In a flute, open
at both ends, the holes are so arranged that the length
of the sound-wave is about one foot, for one of the chief
"open notes." Higher musical notes correspond to greater
and greater frequency of vibration, viz., 1,000, 2,000, 4,000,
vibrations per second ; 4,000 vibrations per second correspond
to a piccolo flute of exceedingly small length; it would
be but one and a half inches long. Think of a note from
a little dog-call, or other whistle, one and a half inches
long, open at both ends, or from a little key having a tube
three quarters of an inch long, closed at one end; you
will then have 4,000 vibrations per second.
A wave length of sound is the distance traversed in
the period of vibration. I will illustrate what the vibra-
tions of sound are by this condensation travelling along
our picture on the screen. Alternate condensations and
rarefactions of the air are made continuously by a sound-
ing body. When I pass my hand vigorously in one direc-
tion, the air before it becomes dense, and the air on the
THE WAVE THEORY OF LIGHT 267
other side becomes rarefied. When I move it in the other
direction these things become reversed; there is a spread-
ing out of condensation from the place where my hand
moves in one direction and then in the reverse. Each
condensation is succeeded by a rarefaction. Rarefaction
succeeds condensation at an interval of one-half what we
call " wave-lengths." Condensation succeeds condensation
at the full interval of a wave-length.
We have here these luminous particles on this scale, 1 rep-
resenting portions of air close together, more dense; a
little higher up, portions of air less dense. I now slowly
turn the handle of the apparatus in the lantern, and you
see the luminous sectors showing condensation travelling
slowly upwards on the screen ; now you have another
condensation making one wave-length.
This picture or chart represents a wave-length of four
feet. It represents a wave of sound four feet long. The
fourth part of a thousand is 250. What we see now of
the scale represents the lower note C of the tenor voice.
The air from the mouth of a singer is alternately con-
densed and rarefied just as you see here. But that proc-
ess shoots forward at the rate of about one thousand
feet per second; the exact period of the motion being 256
vibrations per second for the actual case before you.
Follow one particle of the air forming part of a sound
wave, as represented by these moving spots of light on the
screen; now it goes down, then another portion goes
down rapidly; now it stops going down; now it begins
to go up ; now it goes down and up again. As the maximum
of condensation is approached it is going up with dimin-
ishing maximum velocity. The maximum of rarefaction
has now reached it, and the particle stops going up and
begins to move down. When it is of mean density the par-
ticles are moving with maximum velocity, one way or
the other. You can easily follow these motions, and you
will see that each particle moves to and fro and the thing
that we call condensation travels along.
I shall show the distinction between these vibrations
1 Alluding to a moving diagram of wave motion of sound produced by a
working slide for lantern projection.
268 KELVIN
and the vibrations of light. Here is the fixed appearance
of the particles when displaced but not in motion. You
can imagine particles of something, the thing whose mo-
tion constitutes light. This thing we call the luminiferous
ether. That is the only substance we are confident of in
dynamics. One thing we are sure of, and that is the
reality and substantiality of the luminiferous ether. This
instrument is merely a method of giving motion to a dia-
gram designed for the purpose of illustrating wave motion
of light. I will show you the same thing in a fixed
diagram, but this arrangement shows the mode of motion.
Now follow the motion of each particle. This repre-
sents a particle of the luminiferous ether, moving at the
greatest speed when it is at the middle position.
You see the two modes of vibrations, 2 sound and light
now moving together; the travelling of the wave of con-
densation and rarefaction, and the travelling of the wave
of transverse displacement. Note the direction of propa-
gation. Here it is from your left to your right, as you
look at it. Look at the motion when made faster. We
have now the direction reversed. The propagation of the
wave is from right to left, again the propagation of the
wave is from left to right; each particle moves perpen-
dicularly to the line of propagation.
1 have given you an illustration of the vibration of sound
waves, but I must tell you that the movement illustrating
the condensation and rarefaction represented in that mov-
ing diagram are necessarily very much exaggerated, to
let the motion be perceptible, whereas the greatest con-
densation in actual sound motion is not more than one
or two per cent, or a small fraction of a per cent. Except
that the amount of condensation was exaggerated in the
diagram for sound, you have in the chart a correct rep-
resentation of what actually takes place in sounding the
low note C.
On the other hand, in the moving diagram representing
light waves what had we? We had a great exaggeration
of the inclination of the line of particles. You must first
2 Showing two moving diagrams, simultaneously, on the screen, one
depicting a wave motion of light, the other a sound vibration.
THE WAVE THEORY OF LIGHT
269
imagine a line of particles in a straight line, and then you
must imagine them disturbed into a wave-curve, the shape
of the curve corresponding to the disturbance. Having
seen what the propagation of the wave is, look at this
diagram and then look at that one. This, in light, cor-
responds to the different sounds I spoke of at first. The
wave-length of light is the distance from crest to crest
of the wave, or from hollow to hollow. I speak of crests
and hollows, because we have a diagram of ups and downs
as the diagram is placed.
Here, then, you have a wave-length. 8 In this lower dia-
gram (Fig. 119) you have a wave-length of violet light.
It is but one-half the length of the upper wave of red
Fig. 118 — Waves of Red Light
Fig. 119 — Waves of Violet Light
light; the period of vibration is but half as long. Now
there, on an enormous scale, exaggerated not only as to
slope, but immensely magnified as to wave-length, we have
an illustration of the waves of violet light. The drawing
marked "red" (Fig. 118) corresponds to red light, and this
lower diagram corresponds to violet light. The upper
curve really corresponds to something a little below the red
ray of light in the spectrum, and the lower curve to something
beyond the violet light. The variation in wave-length be-
tween the most extreme rays is in the proportion of four and
a half of red to eight of the violet, instead of four and
eight ; the red waves are nearly as one to two of the violet.
To make a comparison between the number of vibra-
tions for each wave of sound and the number of vibrations
3 Exhibiting a large drawing, or chart, representing a red and a violet
wave of light (reproduced in Figs. 118 and 119).
270 KELVIN
constituting light waves, I may say that 30 vibrations
per second is about the smallest number which will pro-
duce a musical sound; 50 per second gives one of the
grave pedal notes of an organ, 100 or 200 per second give
the low notes of the bass voice, higher notes with 250 per
second, 300 per second, 1,000, 4,000 up to 8,000 per sec-
ond give about the shrillest notes audible to the human
ear.
Instead of the numbers, which we have, say in the most
commonly used part of the musical scale, i. e. x from 200
or 300 to 600 or 700 per second, we have millions of millions
of vibrations per second in light waves: that is to say,
400 per second, instead of 400 million million per second,
which is the number of vibrations performed when we have
red light produced.
An exhibition of red light travelling through space from
the remotest star is due to propagation by waves or
vibrations, in which each individual particle of the trans-
mitting medium vibrates to and fro 400 million million
times in a second.
Some people say they cannot understand a million million.
Those people cannot understand that twice two makes four.
That is the way I put it to people who talk to me about
the incomprehensibility of such large numbers. I say fini-
tude is incomprehensible, the infinite in the universe is com-
prehensible. Now apply a little logic to this. Is the
negation of infinitude incomprehensible? What would you
think of a universe in which you could travel one, ten,
or a thousand miles, or even to California, and then find
it come to an end? Can you suppose an end of matter
or an end of space? The idea is incomprehensible. Even
if you were to go millions and millions of miles the idea
of coming to an end is incomprehensible. You can under-
stand one thousand per second as easily as you can under-
stand one per second. You can go from one to ten, and
ten times ten and then to a thousand without taxing your
understanding and then you can go on to a thousand million
and a million million. You can all understand it.
Now 400 million million vibrations per second is the kind
of thing that exists as a factor in the illumination
THE WAVE THEORY OF LIGHT 271
by red light. Violet light, after what we have seen and have
had illustrated by that curve (Fig. 119), I need not tell
you corresponds to vibrations of about 800 million million
per second. There are recognisable qualities of light caused
by vibrations of much greater frequency and much less fre-
quency than this. You may imagine vibrations having about
twice the frequency of violet light, and others having about
one-fifteenth the frequency of red light, and still you do not
pass the limit of the range of continuous phenomena, only a
part of which constitutes visible light.
When we go below visible red light what have we ? We
have something we do not see with the eye, something that
the ordinary photographer does not bring out on his pho-
tographically sensitive plates. It is light, but we do not see
it. It is something so closely continuous with visible light,
that we may define it by the name of invisible light. It
is commonly called radiant heat; invisible radiant heat.
Perhaps, in this thorny path of logic, with hard words fly-
ing in our faces, the least troublesome way of speaking of
it is to call it radiant heat. The heat effect you experience
when you go near a bright hot coal fire, or a hot steam boiler ;
or when you go near, but not over, a set of hot water pipes
used for heating a house ; the thing we perceive in our faces
and hands when we go near a boiling pot and hold the hand
on a level with it, is radiant heat; the heat of the hands and
face caused by a hot fire, or by a hot kettle when held under
the kettle, is also radiant heat.
You might readily make the experiment with an earthen
teapot; it radiates heat better than polished silver. Hold
your hands below the teapot and you perceive a sense of
heat; above it you get more heat; either way you perceive
heat. If held over the teapot you readily understand that
there is a little current of hot air rising; if you put your
hand under the teapot you find cold air rising, and the upper
side of your hand is heated by radiation while the lower
side is fanned and is actually cooled by virtue of the heated
kettle above it.
That perception by the sense of heat, is the perception of
something actually continuous with light. We have knowl-
edge of rays of radiant heat perceptible down to (in round
272 KELVIN
numbers) about four times the wave-length, or one-fourth
the period, of visible or red light. Let us take red light at
400 million million vibrations per second, then the lowest
radiant heat, as yet investigated, is about 100 million million
per second of frequency of vibration.
I had hoped to be able to give you a lower figure. Pro-
fessor Langley has made splendid experiments on the top of
Mount Whitney, at the height of 15,000 feet above the sea-
level, with his "Bolometer," and has made actual measure-
ments of the wave-length of radiant heat down to exceedingly
low figures. I will read you one of the figures; I have not
got it by heart yet, because I am expecting more from him. 4
I learned a year and a half ago that the lowest radiant heat
observed by the diffraction method of Professor Langley
corresponds to 28 one hundred thousandths of a centimetre of
wave-length, 28 as compared with red light, which is 7.3;
or nearly four-fold. Thus wave-lengths of four times the
amplitude, or one-fourth the frequency per second of red
light, have been experimented on by Professor Langley and
recognised as radiant heat.
Everybody knows the "photographer's light," and has
heard of invisible light producing visible effects upon the
chemically prepared plate in the camera. Speaking in round
numbers, I may say that, in going up to about twice the fre-
quency I have mentioned for violet light, you have gone to
the extreme end of the range of known light of the highest
rates of vibration ; I mean to say that you have reached the
greatest frequency that has yet been observed. Photo-
graphic, or actinic light, as far as our knowledge extends at
present, takes us to a little less than one-half the wave-length
of violet light.
You will thus see that while our acquaintance with wave
motion below the red extends down to one quarter of the
slowest rate which affects the eye, our knowledge of vibra-
tions at the other end of the scale only comprehends those
4 Since my lecture I have heard from Professor Langley that he has
measured the refrangibility by a rock salt prism, and inferred the wave-
length of heat rays from a "Leslie cube" (a metal vessel filled with hot
water and radiating heat from a blackened side). The greatest wave-
length he has thus found is one-thousandth of a centimetre, which is sev-
enteen times that of sodium light — the corresponding period being about
thirty million million per second.
November, 1884.— W. T.
THE WAVE THEORY OF LIGHT 273
having twice the frequency of violet light. In round num-
bers we have 4 octaves of light, corresponding to 4 octaves
of sound in music. In music the octave has a range to a
note of double frequency. In light we have one octave of
visible light, one octave above the visible range and two
octaves below the visible range. We have 100 per second,
200 per second, 400 per second (million million understood)
for invisible radiant heat; 800 per second for visible light,
and 1,600 per second for invisible or actinic light.
One thing common to the whole is the heat effect. It is
extremely small in moonlight, so small that until recently
nobody knew there was any heat in the moon's rays.
Herschel thought it was perceptible in our atmosphere by
noticing that it dissolved away very light clouds, an effect
which seemed to show in full moonlight more than when we
have less than full moon. Herschel, however, pointed this
out as doubtful; but now, instead of its being a doubtful
question, we have Professor Langley giving as a fact that
the light from the moon drives the indicator of his sensi-
tive instrument clear across the scale, showing a compara-
tively prodigious heating effect !
I must tell you that if any of you want to experiment with
the heat of the moonlight, you must measure the heat by
means of apparatus which comes within the influence of the
moon's rays only. This is a very necessary precaution; if,
for instance, you should take your Bolometer or other heat
detector from a comparatively warm room into the night
air, you would obtain an indication of a fall in temperature
owing to this change. You must be sure that your apparatus
is in thermal equilibrium with the surrounding air, then
take your burning-glass, and first point it to the moon and
then to space in the sky beside the moon; you thus get a
differential measurement in which you compare the radiation
of the moon with the radiation of the sky. You will then
see that the moon has a distinctly heating effect.
To continue our study of visible light, that is undulations
extending from red to violet in the spectrum (which I am
going to show you presently), I would first point out on this
chart (Fig. 120) that in the section from letter A to letter
D we have visual effect and heating effect only ; but no ordi-
274 KELVIN
nary chemical or photographic effect. Photographers can
leave their usual sensitive chemically prepared plates ex-
posed to yellow light and red light without experiencing
any sensible effect; but when you get toward the blue end
J> E6 # G
Fig. 120 — The Solar Spectrum
of the spectrum the photographic effect begins to tell, and
more and more strongly as you get towards the violet end.
When you get beyond the violet there is the invisible light
known chiefly by its chemical action. From yellow to violet
we have visual effect, heating effect, and chemical effect, all
three ; above the violet only chemical and heating effects, and
so little of the heating effect that it is scarcely perceptible.
The prismatic spectrum is Newton's discovery of the
composition of white light. White light consists of every
variety of colour from red to violet. Here, now, we have
Newton's prismatic spectrum, produced by a prism. I will
illustrate a little in regard to the nature of colour by putting
something before the light which is like coloured glass ; it is
coloured gelatin. I will put in a plate of red gelatin which
is carefully prepared of chemical materials and see what that
will do. Of all the light passing to it from violet to red it
only lets through the red and orange, giving a mixed reddish
colour. Here is a plate of green gelatin: the green absorbs
all the red, giving only green. Here is a plate absorbing
something from each portion of the spectrum, taking away
a great deal of the violet and giving a yellow or orange ap-
pearance to the light. Here is another absorbing the green
and all the violet, leaving red, orange, and a very little faint
green.
When the spectrum is very carefully produced, far more
carefully than Newton knew how to show it, we have a
homogeneous spectrum. It must be noticed that Newton did
not understand what we call a homogeneous spectrum ; he did
not produce it, and does not point out in his writings the
conditions for producing it. With an exceedingly fine line of
light we can bring it out as in sunlight, like this upper picture
THE WAVE THEORY OF LIGHT 275
— red, orange, yellow, green, blue, indigo, and violet, accord-
ing to Newton's nomenclature. Newton never used a narrow
beam of light, and so could not have had a homogeneous
spectrum.
This is a diagram painted on glass and showing the colours
as we know them. It would take two or three hours if I
were to explain the subject of spectrum analysis to-night.
We must tear ourselves away from it. I will just read out
to you the wave-lengths corresponding to the different posi-
tions of the sun's spectrum of certain dark lines commonly
called " Fraunhofer's lines/' I will take as a unit the one
hundred thousandth of a centimetre. A centimetre is .4 of
an inch; it is a rather small half an inch. I take the
thousandth of a centimetre and the hundredth of that as a
unit. At the red end of the spectrum the light in the neigh-
bourhood of that black line A (Fig. 120) has for its wave-
length 7.6; B has 6.87; D has 5.89; the " frequency" for A
is 3.9 times 100 million million, the frequency of D light is 5.1
times 100 million million per second.
Now what force is concerned in those vibrations as com-
pared with sound at the rate of 400 vibrations per second?
Suppose for a moment the same matter was to move to and
fro through the same range but 400 million million times per
second. The force required is as the square of the number
expressing the frequency. Double frequency would require
quadruple force for the vibration of the same body. Sup-
pose I vibrate my hand again, as I did before. If I move it
once per second a moderate force is required; for it to
vibrate ten times per second 100 times as much force is re-
quired; for 400 vibrations per second 160,000 times as much
force. If I move my hand once per second through a space
of a quarter of an inch a very small force is required; it
would require very considerable force to move it ten times
a second, even through so small a range; but think of the
force required to move a tuning-fork 400 times a second, and
compare that with the force required for a motion of 400
million million times a second. If the mass moved is the
same, and the range of motion is the same, then the force
would be one million million million million times as great as
the force required to move the prongs of the tuning-fork — it
276 KELVIN
is as easy to understand that number as any number like
2, 3, or 4. Consider now what that number means and what
we are to infer from it. What force is there in the space be-
tween my eye and that light ? What forces are there in the
space between our eyes and the sun, and our eyes and the re-
motest visible star? There is matter and there is motion, but
what magnitude of force may there be?
I move through this " luminiferous ether " as if it were
nothing. But were there vibrations with such frequency
in a medium of steel or brass, they would be measured by
millions and millions and millions of tons' action on a square
inch of matter. There are no such forces in our air. Comets
make a disturbance in the air, and perhaps the luminiferous
ether is split up by the motion of a comet through it. So
when we explain the nature of electricity, we explain it by
a motion of the luminiferous ether. We cannot say that
it is electricity. What can this luminiferous ether be? It
is something that the planets move through with the greatest
ease. It permeates our air; it is nearly in the same con-
dition, so far as our means of judging are concerned, in
our air and in the inter-planetary space. The air dis-
turbs it but little; you may reduce air by air-pumps to
the hundred thousandth of its density, and you make little
effect in the transmission of light through it. The lumi-
niferous ether is an elastic solid, for which the nearest
analogy I can give you is this jelly which you see, 5 and the
nearest analogy to the waves of light is the motion, which
you can imagine, of this elastic jelly, with a ball of
wood floating in the middle of it. Look there, when with
my hand I vibrate the little red ball up and down, or when I
turn it quickly round the vertical diameter, alternately in
opposite directions ; — that is the nearest representation I can
give you of the vibrations of luminiferous ether.
Another illustration is Scottish shoemakers' wax or Bur-
gundy pitch, but I know Scottish shoemakers' wax better.
It is heavier than water, and absolutely answers my purpose.
I take a large slab of the wax, place it in a glass jar filled
with water, place a number of corks on the lower side and
8 Exhibiting a large bowl of clear jelly with a small red wooden ball embedded
in the surface near the centre.
THE WAVE THEORY OF LIGHT 277
bullets on the upper side. It is brittle like the Trinidad
pitch or Burgundy pitch which I have in my hand — you
can see how hard it is — but when left to itself it flows like
a fluid. The shoemakers' wax breaks with a brittle fracture,
but it is viscous and gradually yields.
What we know of the luminiferous ether is that it has the
rigidity of a solid and gradually yields. Whether or not it
is brittle and cracks we cannot yet tell, but I believe the dis-
coveries in electricity and the motions of comets and the
marvellous spurts of light from them, tend to show cracks
in the luminiferous ether — show a correspondence between
the electric flash and the aurora borealis and cracks in the
luminiferous ether. Do not take this as an assertion, it is
hardly more than a vague scientific dream: but you may re-
gard the existence of the luminiferous ether as a reality
of science ; that is, we have an all-pervading medium, an
elastic solid, with a great degree of rigidity — a rigidity so pro-
digious in proportion to its density that the vibrations of light
in it have the frequencies I have mentioned, with the wave-
lengths I have mentioned. The fundamental question as to
whether or not luminiferous ether has gravity has not been
answered. We have no knowledge that the luminiferous
ether is attracted by gravity ; it is sometimes called im-
ponderable because some people vainly imagine that it has
no weight: I call it matter with the same kind of rigidity
that this elastic jelly has.
Here are two tourmalines; if you look through them to-
ward the light you see the white light all round, i.e. they are
transparent. If I turn round one of these tourmalines the
light is extinguished, it is absolutely black, as though the
tourmalines were opaque. This is an illustration of what
is called polarisation of light. I cannot speak to you about
qualities of light without speaking of the polarisation of
light. I want to show you a most beautiful effect of polaris-
ing light, before illustrating a little further by means of
this large mechanical illustration which you have in the bowl
of jelly. What you saw first were two plates of the crystal
tourmaline (which came from Brazil, I believe) having
the property of letting light pass when both plates are placed
in one particular direction as regards their axes of crystal-
278 KELVIN
lisation, and extinguishing it when it passes through them
with one of the plates held in another direction. Now I
put in the lantern an instrument called a " Nicol prism,"
which also gives rays of polarised light. A Nicol prism
is a piece of Iceland spar, cut in two and turned one part
relatively to the other in a very ingenious way, and put
together again and cemented into one by Canada balsam.
The Nicol prism takes advantage of the property which the
spar has of double refraction, and produces the phenom-
enon which I now show you. I turn one prism round in a
certain direction and you get light — a maximum of light.
I turn it through a right angle and you get blackness. I
turn it one quarter round again, and get maximum light;
one quarter more, maximum blackness ; one quarter more,
and bright light. We rarely have a grand specimen of
a Nicol prism as this.
There is another way of producing polarised light. I
stand before that light and look at its reflection in a plate
of glass on the table through one of the Nicol prisms, which
I turn round, so. Now if I incline that plate of glass at a
particular angle — rather more than fifty-five degrees — I find
a particular position in which, if I look at it and then turn
the prism round in the hand, the effect is absolutely to
extinguish the light in one position of the prism and to give
it maximum brightness in another position. I use the term
" absolute " somewhat rashly. It is only a reduction to a
very small quantity of light, not an absolute annulment as
we have in the case of the two Nicol prisms used conjointly.
As to the mechanics of the thing, those of you who have
never heard of this before would not know what I was talk-
ing about; it could only be explained to you by a course of
lectures in physical optics. The thing is this, vibrations of
light must be in a definite direction relatively to the line in
which the light travels.
Look at this diagram, the light goes from left to right;
we have vibrations perpendicular to the line of transmis-
sion. There is a line up and down which is the line of
vibration. Imagine here a source of light, violet light, and
here in front of it is the line of propagation. Sound-vibra-
tions are to and fro in, this is transverse to, the line of prop-
THE WAVE THEORY OF LIGHT
279
agation. Here is another, perpendicular to the diagram,
still following the law of transverse vibration; here is an-
other, circular vibration. Imagine a long rope, you whirl
one end of it and you see a screw-like motion running along,
and you can get this circular motion in one direction or in
the opposite.
Plane-polarised light is light with the vibrations all in a
single plane, perpendicular to the plane through the ray
which is technically called the "plane of polarisation."
Circularly polarised light consists of undulations of lumi-
niferous ether having a circular motion. Elliptically polar-
ised light is something between the two, not in a straight line,
and not in a circular line; the course of vibration is an
ellipse. Polarised light is light that performs its motions
continually in one mode or direction. If in a straight line it
is plane-polarised; if in a circular direction it is circularly
polarised light; when elliptical it is elliptically polarised
light.
With Iceland spar, one unpolarised ray of light divides on
entering it into two rays of polarised light, by reason of its
power of double refraction, and the vibrations are perpen-
dicular to one another in the two emerging rays. Light
is always polarised when it is reflected from a plate of un-
silvered glass, or from water, at a certain definite angle of
fifty-six degrees for glass, fifty-two degrees for water,
the angle being reckoned in each case from a perpendicular
to the surface. The angle for water is the angle whose tan-
gent is 1.4. I wish you to look at the polarisation with your
own eyes. Light from glass at fifty-six degrees and from
water at fifty-two degrees goes away vibrating perpen-
dicularly to the plane of incidence and plane of reflection.
We can distinguish it without the aid of an instrument.
There is a phenomenon well known in physical optics as
" Haidinger's Brushes." The discoverer is well known in
Philadelphia as a mineralogist, and the phenomenon I speak
of goes by his name. Look at the sky in a direction of ninety
degrees from the sun, and you will see a yellow and blue
cross, with the yellow toward the sun, and from the sun,
spreading out like two foxes' tails with blue between, and
then two red brushes in the space at right angles to the blue.
280 KELVIN
If you do not see it, it is because your eyes are not sensitive
enough, but a little training will give them the needed sen-
sitiveness. If you cannot see it in this way try another
method. Look into a pail of water with a black bottom; or
take a clear glass dish of water, rest it on a black cloth, and
look down at the surface of the water on a day with a white
cloudy sky (if there is such a thing ever to be seen in Phila-
delphia). You will see the white sky reflected in the basin
of water at an angle of about fifty degrees. Look at it
with the head tipped on one side and then again with the
head tipped to the other side, keeping your eyes on the water,
and you will see Haidinger's brushes. Do not do it fast
or you will make yourself giddy. The explanation of this
is the refreshing of the sensibility of the retina. The
Haidinger's brush is always there, but you do not see it be-
cause your eye is not sensitive enough. After once seeing
it you always see it; it does not thrust itself inconveniently
before you when you do not want to see it. You can also
readily see it in a piece of glass with a dark cloth below it,
or in a basin of water.
I am going to conclude by telling you how we know the
wave-lengths of light, and how we know the frequency of
the vibrations, and we shall actually make a measurement
of the wave-length of yellow light. I am now going to show
you the diffraction spectrum.
You see on the screen, 6 on each side of a central white bar
of light, a set of bars of light, of variegated colours, the first
one on each side showing blue or indigo colour, about four
inches from the central white bar, and red about four inches
farther, with vivid green between the blue and the red. That
effect is produced by a grating with 400 lines to the centi-
metre, engraved on glass, which I now hold in my hand. The
next grating that we shall try has 3,000 lines on a Paris inch.
You see the central space and on each side a large number
of spectrums, blue at one end and red at the other. The
fact that, in the first spectrum, red is about twice as far
from the centre as the blue, proves that a wave-length of
red light is double that of blue light.
6 Showing the chromatic bands thrown upon the screen from a diffrac-
tion grating.
THE WAVE THEORY OF LIGHT 281
I will now show you the operation of measuring the length
of a wave of sodium light, that is a light like that marked
D on the spectrum (Fig. 120), a light produced by a spirit-
lamp with salt in it. The sodium vapour is heated up to
several thousand degrees, when it becomes self-luminous and
gives such a light as we get by throwing salt upon a spirit-
lamp in the game of snap-dragon.
I hold in my hand a beautiful grating of glass silvered by
Liebig's process with metallic silver, a grating with 6,480
lines to the inch, belonging to my friend Professor Barker,
which he has kindly brought here for us this evening. You
will see the brilliancy of colour as I turn the light reflected
from the grating toward you and pass the beam round the
room. You have now seen directly with your own eyes these
brilliant colours reflected from the grating, and you have also
seen them thrown upon the screen from a grating placed
in the lantern. Now with a grating of 17,000 lines per inch
— a much greater number than the other — you will see how
much further from the central bright space the first spectrum
is; how much more this grating changes the direction, or
diffraction, of the beam of light. Here is the centre of the
grating, and there is the first spectrum. You will note that
the violet light is least diffracted and the red light is most
diffracted. This diffraction of light first proved to us
definitely the reality of the undulatory theory of light.
You ask why does not light go round a corner as sound
does. Light does go round a corner in these diffraction
spectrums; and it is shown going round a corner, since it
passes through these bars and is turned round an angle of
thirty degrees. The phenomena of light going round a cor-
ner seen by means of instruments' adapted to show the
result and to measure the angles through which it is turned,
is called the diffraction of light.
I can show you an instrument which will measure the
wave-lengths of light. Without proving the formula, let
me tell it to you. A spirit-lamp with salt sprinkled on the
wick gives very nearly homogeneous light, that is to say,
light of one v/ave-length, or all of the same period. I have
here a little grating which I take in my hand. I look through
this grating and see that candle before me. Close behind
282 KELVIN
it you see a blackened slip of wood with two white marks on
it ten inches asunder. The line on which they are marked
is placed perpendicular to the line at which I shall go
from it. When I look at this salted spirit-lamp I see a series of
spectrums of yellow light. As I am somewhat short-sighted
I am making my eye see with this eye-glass and the natural
lenses of the eye what a long-sighted person would make
out without an eye-glass. On that screen you saw a suc-
cession of spectrums. I now look direct at the candle and
what do I see? I see a succession of five or six brilliantly
coloured spectrums on each side of the candle. But when I
look at the salted spirit-lamp, now I see ten spectrums on
one side and ten on the other, each of which is a monochro-
matic band of light.
I will measure the wave-length of the light thus. I walk
away to a considerable distance and look at the spirit-lamp
and marks. I see a set of spectrums. The first white line
is exactly behind the flame. I want the first spectrum to
the right of that white line to fall exactly on the other white
line, which is ten inches from the first. As I walk away
from it I see it is now very near it ; it is now on it. Now the
distance from my eye is to be measured, and the problem
is again to reduce feet to inches. The distance from the
spectrum of the flame to my eye is thirty- four feet nine
inches. Mr. President, how many inches is that? 417
inches, in round numbers 420 inches. Then we have the
proportion, as 420 is to 10 so is the length from bar to bar
of the grating to the wave-length of sodium light. That
is to say as forty-two is to one. The distance from bar to
bar is the four hundredth of a centimetre: therefore the
42nd part of the four hundredth of a centimetre is the wave-
length according to our simple, and easy and hasty ex-
periment. The true wave-length of sodium light, accord-
ing to the most accurate measurement, is about a 17,000th
of a centimetre, which differs by scarcely more than one
per cent, from our result !
The only apparatus you see is this little grating — a piece
of glass having a space four-tenths of an inch wide ruled
with 400 fine lines. Any of you who will take the trouble
to buy one may measure the wave-length of a candle
THE WAVE THEORY OF LIGHT 283
flame himself. I hope some of you will be induced to
make the experiment for yourselves.
If I put salt on the flame of a spirit-lamp, what do I
see through this grating? I see merely a sharply defined
yellow light, constituting the spectrum of vaporised so-
dium, while from the candle flame I see an exquisitely
coloured spectrum, far more beautiful than that I showed
you on the screen. I see in fact a series of spectrums
on the two sides with the blue toward the candle flame
and the red further out. I cannot get one definite thing
to measure from in the spectrum from the candle flame, as
I can with the flame of a spirit-lamp with the salt thrown
on it, which gives as I have said a simple yellow light.
The highest blue light I see in the candle flame is now
exactly on the line. Now measure to my eye, it is
forty-four feet four inches, or 532 inches. The length
of this wave then is the 532d part of the four hundredth
of a centimetre which would be the 21,280th of a centi-
metre, say the 21,000th of a centimetre. Then measure
for the red and you will find something like the 11, oooth
for the lowest of the red light.
Lastly, how do we know the frequency of vibration?
Why, by the velocity of light. How do we know that?
We know it in a number of different ways, which I can-
not explain now because time forbids, and I can now only
tell you shortly that the frequency of vibration for any par-
ticular ray is equal to the velocity of light divided by
the wave-length for that ray. The velocity of light is
about 187,000 British statute miles per second, but it
is much better to take the kilometre — which is about six-
tenths of a mile — for the unit, when we find the velocity
is very accurately 300,000 kilometres, or 30,000,000,000
centimetres, per second. Take now the wave-length of
sodium light, as we have just measured it by means of the
salted spirit-lamp, to be one 17,000th of a centimetre, and
we find the frequency of vibration of the sodium light
to be 510 million million per second. There, then, you
have a calculation of the frequency from a simple obser-
vation which you all can make for yourselves.
Lastly, I must tell you about the colour of the blue
284 KELVIN
sky which is illustrated by this spherule imbedded in an
elastic solid (Fig. 121). I want to explain to you in
two minutes the mode of vibration. Take the simplest
plane-polarised light. Here is a spherule which is pro-
ducing it in an elastic solid. Imagine the solid to extend
miles horizontally and miles up and down, and imagine
this spherule to vibrate up and down. It is quite clear
that it will make transverse vibrations similarly in all
horizontal directions. The plane of polarisation is defined
as a plane perpendicular to the line of vibration. Thus,
light produced by a molecule vibrating up and down,
as this red globe in the jelly before you, is polarised in a
horizontal plane because the vibrations are vertical.
Here is another mode of vibration. Let me twist this
spherule in the jelly as I am now doing, and that will
///
III
Fig. 121 — Vibrating Spherule Imbedded in an Elastic Solid
produce vibrations, also spreading out equally in all horizon-
tal directions. When I twist this globe round it draws
the jelly round with it; twist it rapidly back and the
jelly flies back. By the inertia of the jelly the vibrations
spread in all directions and the lines of vibration are
horizontal all through the jelly. Everywhere, miles away
that solid is placed in vibration. You do not see the
vibrations, but you must understand that they are there.
If it flies back it makes vibration, and we have waves
of horizontal vibrations travelling out in all directions
from the exciting molecule.
I am now causing the red globe to vibrate to and fro
horizontally. That will cause vibrations to be produced
which will be parallel to the line of motion at all places
of the plane perpendicular to the range of the exciting
molecule. What makes the blue sky? These are exactly
the motions that make the blue light of the sky, which is
THE WAVE THEORY OF LIGHT 285
due to spherules in the luminiferous ether, but little modi-
fied by the air. Think of the sun near the horizon, think
of the light of the sun streaming through and giving you
the azure blue and violet overhead. Think first of any
one particle and think of it moving in such a way as to
give horizontal and vertical vibrations and circular and
elliptic vibrations.
You see the blue sky in high pressure steam blown into
the air; you see it in the experiment of Tyndall's blue
sky in which a delicate condensation of vapour gives rise
to exactly the azure blue of the sky.
Now the motion of the luminiferous ether relatively
to the spherule gives rise to the same effect as would
an opposite motion impressed upon the spherule quite
independently by an independent force. So you may think
of the blue colour coming from the sky as being produced
by to and fro vibrations of matter in the air, which vibrates
much as this little globe vibrates imbedded in the jelly.
The result in a general way is this: The light coming
from the blue sky is polarised in a plane through the
sun, but the blue light of the sky is complicated by a great
number of circumstances and one of them is this, that
the air is illuminated not only by the sun but by the
earth. If we could get the earth covered by a black cloth
then we could study the polarised light of the sky with
a simplicity which we cannot do now. There are, in
nature, reflections from the seas and rocks and hills and
waters in an infinitely complicated manner.
Let observers observe the blue sky not only in winter
when the earth is covered with snow, but in summer when
it is covered with dark green foliage. This will help to
unravel the complicated phenomena in question. But the
azure blue of the sky is light produced by the reaction on the
vibrating ether of little spherules of water, of perhaps
a fifty thousandth or a hundred thousandth of a centi-
metre diameter, or perhaps little motes, or lumps, or crystals
of common salt, or particles of dust, or germs of vege-
table or animal species wafted about in the air. Now
what is the luminiferous ether? It is matter prodigiously
less dense than air — millions and millions and millions of
286 KELVIN
times less dense than air. We can form some sort of
idea of its limitations. We believe it is a real thing,
with great rigidity in comparison with its density: it
may be made to vibrate 400 million million times per sec-
ond; and yet be of such density as not to produce the
slightest resistance to any body going through it.
Going back to the illustration of the shoemakers' wax;
if a cork will, in the course of a year, push its way up
through a plate of that wax when placed under water,
and if a lead bullet will penetrate downwards to the bot-
tom, what is the law of the resistance? It clearly depends
on time. The cork slowly in the course of a year works
its way up through two inches of that substance ; give it one
or two thousand years to do it and the resistance will
be enormously less; thus the motion of a cork or bullet,
at the rate of one inch in 2,000 years, may be compared
with that of the earth, moving at the rate of six times
ninety-three million miles a year, or nineteen miles per
second, through the luminiferous ether; but when we can
have actually before us a thing elastic like jelly and
yielding like pitch, surely we have a large and solid
ground for our faith in the speculative hypothesis of an
elastic luminiferous ether, which constitutes the wave theory
of light.
THE TIDES
[EVENING LECTURE TO THE BRITISH ASSOCIA-
TION AT THE SOUTHAMPTON MEETING,
FRIDAY, AUGUST 25, 1882]
THE subject on which I have to speak this evening is
the tides, and at the outset I feel in a curiously diffi-
cult position. If I were asked to tell what I mean by
the Tides I should feel it exceedingly difficult to answer
the question. The tides have something to do with mo-
tion of the sea. Rise and fall of the sea is sometimes
called a tide ; but I see, in the Admiralty Chart of the Firth
of Clyde, the whole space between Ailsa Craig and the
Ayrshire coast marked " very little tide here." Now, we find
there a good ten feet rise and fall, and yet we are authori-
tatively told there is very little tide. The truth is, the word
" tide " as used by sailors at sea means horizontal motion of
the water; but when used by landsmen or sailors in port,
it means vertical motion of the water. I hope my friend
Sir Frederick Evans will allow me to say that we must
take the designation in the chart, to which I have referred,
as limited to the instruction of sailors navigating that part
of the sea, and to say that there is a very considerable
landsman's tide there — a rise and fall of the surface of the
water relatively to the land — though there is exceedingly
little current.
One of the most interesting points of tidal theory is the
determination of the currents by which the rise and fall is
produced, and so far the sailor's idea of what is most note-
worthy as to tidal motion is correct: because before there
can be a rise and fall of the water anywhere it must come
from some other place, and the water cannot pass from
place to place without moving horizontally, or nearly hori-
287
288 KELVIN
zontally, through a great distance. Thus the primary
phenomenon of the tides is after all the tidal current; and
it is the tidal currents that are referred to on charts
where we have arrow-heads marked with the statement
that we have "very little tide here," or that we have "strong
tides" there.
One instance of great interest is near Portland. We hear
of the "race of Portland" which is produced by an exceed-
ingly strong tidal current; but in Portland harbour there is
exceedingly little rise and fall, and that little is much con-
fused, as if the water did not know which way it was go-
ing to move. Sometimes the water rises, sinks, seems to
think a little while about it, and then rises again. The
rise of the tide at Portland is interesting to the inhabitants
of Souhampton in this, that whereas here, at Southampton,
there is a double high water, there, at Portland, there is a
double low water. The double high water seems to extend
across the Channel. At Havre, and on the bar off the en-
trance to Havre, there is a double high water very useful
to navigation; but Southampton I believe is pre-eminent
above all the ports, in the British Islands with respect to
this convenience. There is here (at Southampton) a good
three hours of high water ; — a little dip after the first high
water, and then the water rises again a very little more
for an hour and a half or two hours, before it begins to
fall to low water.
I shall endeavour to refer to this subject again. It is not
merely the Isle of Wight that gives rise to the phenom-
enon. The influence extends to the east as far as Christ-
church, and is reversed at Portland, and we have the double
or the prolonged high water also over at Havre; therefore,
it is clearly not, as it has been supposed to be, due to the
Isle of Wight.
But now I must come back to the question, What are the
"Tides"? Is a "tidal wave" a tide? What is called in the
newspapers a "tidal wave" rises sometimes in a few min-
utes, does great destruction, and goes down again, some-
what less rapidly. There are frequent instances in all parts
of the world of the occurrence of that phenomenon. Such
motions of the water, however, are not tides; they are
THE TIDES
289
usually caused by earthquakes. But we are apt to call any
not very short-time rise and fall of the water a tide, as
when standing on the coast of a slanting shore where there
are long ocean waves, we see the gradual sinkings and ris-
ings produced by them, and say that it is a wave we see,
not a tide, till one comes which is exceptionally slow, and
then we say " that is liker a tide than a wave." The fact is,
there is something perfectly continuous in the species of
Fig. 122 — Wave forms
motion called wave, from the smallest ripple in a musical
glass, whose period may be a thousandth of a second to a
"lop of water" in the Solent, whose period is one or two
seconds, and thence on to the great ocean wave with a
period of from fifteen to twenty seconds, where end the
phenomenon which we commonly call waves (Fig. 122), and
not tides. But any rise and fall which is manifestly of
longer period, or slower in its rise from lowest to highest,
than a wind wave, we are apt to call a tide; and some
of the phenomena that are analysed for, and worked out
in this very tidal analysis that I am going to explain,
are in point of fact more properly wind waves than true
tides.
Leaving these complicated questions, however, I will make
a short cut, and assuming the cause without proving it,
define the thing by the cause. I shall therefore define tides
thus: Tides are motions of water on the earth, due to the
attractions of the sun and of the moon. I cannot say tides
are motions due to the actions of the sun and of the moon ;
for so I would include, under the designation of tide, every
ripple that stirs a puddle or a millpond, and waves in the
Solent or in the English Channel, and the long Atlantic
HC VOL. XXX — J
290
KELVIN
wind waves, and the great swell of the ocean from one hem-
isphere to the other and back again (under the name which
I find in the harmonic reduction of tidal observations),
proved to take place once a year, and which I can only ex-
plain as the result of the sun's heat.
But while the action of the sun's heat by means of the
wind produces ripples and waves of every size, it also pro-
duces a heaping-up of the water as illustrated by this dia-
Fig. 123— Showing the heaping-up of water produced by wind
gram (Fig. 123). Suppose we have wind blowing across
one side of a sheet of water, the wind ruffles the surface,
the waves break if the wind is strong, and the result is a
strong tangential force exerted by the wind on the surface
water. If a ship is sailing over the water there is strong
tangential force ; thus the water is found going fast to lee-
ward for a long distance astern of a great ship sailing with
a side wind: and, just as the sails of a ship standing high
above the sea give a large area for the wind to act upon,
every wave standing up gives a surface, and we have hori-
zontal tangential force over the whole surface of a troubled
sea. The result is that water is dragged along the surface
from one side of the ocean to the other — from one side of
the Atlantic to the other — and is heaped up on the side
towards which the wind is blowing. To understand the
dynamics of this phenomenon, think of a long straight canal
with the wind blowing lengthwise along it. In virtue of
the tangential force exerted on the surface of the water by
the wind, and which increases with the speed of the wind,
the water will become heaped up at one end of the canal, as
shown in the diagram (Fig. 123), while the surface water
throughout the whole length will be observed moving in the
THE TIDES 291
direction of the wind — say in the direction of the two
arrows near to the surface of the water above and be-
low it. But to re-establish the disturbed hydrostatic
equilibrium, the water so heaped up will tend to flow
back to the end from which it has been displaced, and as
the wind prevents this taking place by a surface current,
there will be set up a return current along the bottom of
the canal, in a direction opposite to that of the winds, as indi-
cated by the lowermost arrow in the diagram (Fig. 123).
The return current in the ocean, however, is not always an
under current, such as I have indicated in the diagram,
but may sometimes be a lateral current. Thus a gale of
wind blowing over ten degrees of latitude will cause a drag
of water at the surface, but the return current may be not
an under current but a current on one side or the other
of the area affected by the wind. Suppose, for instance,
in the Mediterranean there is a strong east wind blowing
along the African coast, the result will be a current from
east to west along that coast, and return current along
the northern coasts of the Mediterranean.
The rise and fall of the water due to these motions are
almost inextricably mixed up with the true tidal rise and
fall.
There is another rise and fall, also connected with the
heating effect of the sun, that I do not call a true tide, and
that is a rise and fall due to change of atmospheric pres-
sure. When the barometer is high over a large area of
ocean, then, there and in neighboring places, the tendency
to hydrostatic equilibrium causes the surface of the water
to be lower, where it is pushed down by the greater weight
of air, and to be higher where there is less weight over it.
It does not follow that in every case it is lower, because
there may not be time to produce the effect, but there is this
tendency. It is very well known that two or three days of
low barometer make higher tides on our coast. In Scot-
land and England and Ireland, two or three days of low
barometer generally produce all round the shore higher
water than when the barometer is high; and this effect is
chiefly noticed at the time of tidal high water, because
people take less notice of low water — as at Portland where
292 KELVIN
they think nothing of the double low water. Hence we hear
continually of very high tides — very high water noticed at
the time of high tides — when the barometer is low. We
have not always, however, in this effect of barometric
pressure really great tidal rise and fall. On the contrary
we have the curious phenomenon that sometimes when the
barometer is very low, and there are gales in the neighbour-
hood, there is very little rise and fall, as the water is kept
heaped up and does not sink by anything like its usual
amount from the extra high level that it has at high water.
But I fear I have got into questions which are leading me
away from my subject, and as I cannot get through them I
must just turn back.
Now think of the definition which I gave of the "tides,"
and think of the sun alone. The action of the sun cannot
be denned as the cause of the solar tides. Solar tides are
due to action of the sun, but all risings and fallings of the
water due to the action of the sun are not tides. We
want the quantification of the predicate here very badly.
We have a true tide depending on the sun, the mean solar
diurnal tide, having for its period twenty-four solar hours,
which is inextricably mixed up with those meteorological
tides that I have just been speaking of — tides depending on
the sun's heat, and on the variation of the direction of the
wind, and on the variation of barometric pressure according
to the time of day. The consequence is that in tidal analy-
sis, when we come to the solar tides, we cannot know how
much of the analysed result is due to attraction, and how
much to heating effect directly or indirectly, whether on
water, or on air, or on water as affected by air. As to
the lunar tides we are quite sure of them ; — they are gravi-
tational, and nothing but gravitational; but I hope to speak
later of the supposed relation of the moon to the weather,
and the relation that has to the tides.
I have defined the tides as motions of water on the earth
due to the attractions of the sun and of the moon. How
are we to find out whether an observed motion of the water
is a tide or is not a tide as thus defined? Only by the com-
bination of theory and observation : if these afford sufficient
reason for believing that the motion is due to attraction of
THE TIDES 293
the sun or of the moon, or of both, then we must call it a
tide.
It is curious to look back on the knowledge of the tides
possessed in ancient times, and to find as early as two hun-
dred years before the Christian era a very clear account
given of the tides at Cadiz. But the Romans generally,
knowing only the Mediterranean, had not much clear knowl-
edge of the tides. At a much later time than that, we hear
from the ancient Greek writers and explorers — Posidonius,
Stra^-\ and others — that in certain remote parts of the world,
in Thule, in Britain, in Gaul, and on the distant coasts of
Spain, there were motions of the sea — a rising and falling of
the water — which depended in some way on the moon. Julius
Caesar came to know something about it; but it is certain
the Roman Admiralty did not supply Julius Caesar's cap-
tains with tide tables when he sailed from the Mediter-
ranean wi^h his expeditionary force, destined to put down
anarchy in Britain. He says, referring to the fourth
day after his first landing in Britain — "That night it hap-
pened to be full moon, which time is accustomed to give the
greatest risings of water in the ocean, though our people
did not know it." It has been supposed however that
some of his people did know it — some of his quarter-
masters had been in England before and did know — but
that the discipline in the Roman navy was so good that they
had no right to obtrude their knowledge; and so, although
a storm was raging at the time, he was not told that the
water would rise in the night higher than usual, and noth-
ing was done to make his transports secure higher up on
the shore while he was fighting the Britons. After the
accident Caesar was no doubt told — " Oh, we knew that be-
fore, but it might have been ill taken if we had said so."
Strabo says — "Soon after moonrise the sea begins to
swell up and flow over the earth till the moon reaches
mid heaven. As she descends thence the sea recedes till
about moonset, when the water is lowest. It then rises
again as the moon, below the horizon, sinks to mid heaven
under the earth." It is interesting here to find the tides
described simply with reference to the moon. But there is
something more in this ancient account of Strabo; he says,
294 KELVIN
quoting Posidonius — "This is the daily circuit of the sea.
Moreover, there is a regular monthly course, according to
which the greatest rise and fall takes place about new
moon, then diminishing rise and fall till half moon, and
again increasing till full moon." And lastly he refers to a
hearsay report of the Gaditani (Cadizians) regarding an
annual period in the amount of the daily rise and fall of
the sea, which seems to be not altogether right, and is con-
fessedly in part conjectural. He gave no theory, of course,
and he avoided the complication of referring to the sun. But
the mere mention of an annual period is interesting in the
history of tidal theory, as suggesting that the rises and falls
are due not to the moon alone but to the sun also. The ac-
count given by Posidonius is fairly descriptive of what
occurs at the present day at Cadiz. Exactly the opposite
would be true at many places; but at Cadiz the time of
high water at new and full moon is nearly twelve o'clock.
Still, I say we have only definition to keep us clear of am-
biguities and errors; and yet, to say that those motions of
the sea which we call tides depend on the moon, was con-
sidered, even by Galileo, to be a lamentable piece of mysti-
cism which he read with regret in the writings of so re-
nowned an author as Kepler.
It is indeed impossible to avoid theorising. The first who
gave a theory was Newton ; and I shall now attempt to speak
of it sufficiently to allow us to have it as a foundation for
estimating the forces with which we are concerned, in deal-
ing with some of the very perplexing questions which tidal
phenomena present.
We are to imagine the moon as attracting the earth, sub-
ject to the forces that the different bodies exert upon each
other. We are not to take Hegel's theory — that the Earth
and the Planets do not move like stones, but move along
like blessed gods, each an independent being. If Hegel had
any grain of philosophy in his ideas of the solar system,
Newton is all wrong in his theory of the tides. Newton
considered the attraction of the sun upon the earth and the
moon, of the earth upon the moon, and the mutual attrac-
tions of different parts of the earth ; and left it for Caven-
dish to complete the discovery of gravitation, by exhibiting
THE TIDES 295
the mutual attraction of two pieces of lead in his balance.
Tidal theory is one strong link in the grand philosophic
chain of the Newtonian theory of gravitation. In explain-
ing the tide-generating force we are brought face to face
with some of the subtleties, and with some of the mere
elements, of physical astronomy. I will not enter into de-
tails, as it would be useless for those who already under-
stand the tidal theory, and unintelligible to those who do not.
I may just say that the moon attracts a piece of matter,
for example a pound-weight, here on the earth, with a force
which we compare with the earth's attraction thus. Her
mass is §V of the earth's, and she is sixty times as far
away from the earth's centre as we are here. Newton's
theory of gravitation shows, that when you get outside
the mass of the earth the resultant attraction of the earth
on the pound weight, is the same as if the whole mass of the
earth were collected at the centre, and that it varies inversely
as the square of the distance from the centre. The same law
is inferred regarding the moon's attraction from the general
theory. The moon's attraction on this pound weight is
therefore "80" , or * of the attraction of the earth
j — — - 200,000
60x60
on the same mass. But that is not the tide-generating
force. The moon attracts any mass at the nearest parts of
the earth's surface with greater force than an equal mass
near the centre; and attracts a mass belonging to the re-
moter parts with less force. Imagine a point where the
moon is overhead, and imagine another point on the sur-
face of the earth at the other end of a diameter passing
through the first point and the centre of the earth (illus-
trated by B and A of Fig. 124, p. 298). The moon attracts
the nearest point (B) with a force which is greater than that
with which it attracts the farthest point (A) in the ratio of
the square of 59 to the square of 61. Hence the moon's at-
traction on equal masses at the nearest and farthest points
differs by one fifteenth part of her attraction on an equal
mass at the earth's centre, or about a 4,320,000th, or roughly,
a four-millionth, of the earth's attraction on an equal mass at
its surface. Consequently the water tends to protrude
towards the moon and from the moon. If the moon and earth
296 KELVIN
were held together by a rigid bar the water would be drawn
to the side nearest to the moon — drawn to a prodigious height
of several hundred feet. But the earth and moon are not so
connected. We may imagine the earth as falling towards the
moon, and the moon as falling towards the earth, but never
coming nearer; the bodies, in reality, revolving round their
common centre of gravity. A point nearest to the moon is
as it were dragged away from the earth, and thus the result
is that apparent gravity differs by about one four-millionth at
the points nearest to and farthest from the moon. At the in-
termediate points of the circle C, D (Fig. 124, p. 298), there
is a somewhat complicated action according to which gravita-
tion is increased by about one 17-millionth, and its direction
altered by about one 17-millionth, so that a pendulum 17,000
feet long, a plummet rather longer than from the top of Mont
Blanc to sea level, would, if showing truly the lunar disturb-
ing force, be deflected through a space of one thousandth of a
foot. It seems quite hopeless by a plummet to exhibit the
lunar disturbance of gravity. A spring balance to show the al-
teration of magnitude, and a plummet to show the change
of direction are conceivable ; but we can scarcely believe that
either can ever be produced, with sufficient delicacy and
consistency and accuracy to indicate these results.
A most earnest and persevering effort has been made by
Mr. George Darwin and Mr. Horace Darwin to detect vari-
ations in gravity due to lunar disturbance, and they have
made apparatus which, notwithstanding the prodigious small-
ness of the effect to be observed, is in point of delicacy
and consistency capable of showing it; but when they had
got their delicate pendulum — their delicate plummet about
the length of an ordinary seconds' pendulum — and their
delicate multiplying gear to multiply the motion of its lower
end by about a million times, and to show the result on a
scale by the reflection of a ray of light, they found the
little image incessantly moving backward and forward on
the scale with no consistency or regularity; and they have
come to the conclusion that there are continual local varia-
tions of apparent gravity taking place for which we know
no rule, and which are considerably greater than the lunar
disturbance for which they were seeking. That which they
THE TIDES 297
found — continual motions of the surface of the earth, and
which was not the primary object of their investigation —
is in some respects more interesting than what they sought
and did not find. The delicate investigation thus opened
up promises a rich harvest of knowledge. These disturb-
ances are connected with earthquakes such as have been
observed in a very scientific and accurate manner by Milne,
Thomas Gray, and Ewing in Japan, and in Italy by many
accurate observers. All such observations agree in show-
ing continual tremor and palpitation of the earth in every
part.
One other phenomenon that I may just refer to now as
coming out from tide-gauge observations, is a phenome-
non called seiches by Forel, and described by him as having
been observed in the lakes of Geneva and Constance. He
attributes them to differences of barometric pressure at the
ends of the lake, and it is probable that part of the phe-
nomenon is due to such differences. I think it is certain,
however, that the whole is not due to such differences. The
Portland tide curve and those of many other places, notably
the tide curve for Malta, taken about ten years ago by Sir
Cooper Key, and observations on the Atlantic coasts and
in many other parts of the world, show something of these
phenomena; a ripple or roughness on the curve traced by
the tide gauge, which, when carefully looked to, indicates a
variation not regular but in some such period as twenty or
twenty-five minutes. It has been suggested that they are
caused by electric action ! Whenever the cause of a thing
is not known it is immediately put down as electrical !
I would like to explain to you the equilibrium theory, and
the kinetic theory, of the tides, but I am afraid I must
merely say there are such things; and that Laplace in his
great work, his Mecaniqae Celeste, first showed that the
equilibrium theory was utterly insufficient to account for
the phenomena, and gave the true principles of the dynamic
action on which they depend. The resultant effect of the
tide-generating force is to cause the water to tend to become
protuberant towards the moon and the sun and from them,
when they are in the same straight line, and to take a
regular spheroidal form, in which the difference between
298
KELVIN
the greatest and the least semi-diameter is about 2 feet for
lunar action alone, and 1 foot for the action of the sun
alone — that is a tide which amounts to 3 feet when the sun
and moon act together (Figs. 124 and 125), and to 1 foot
only when they act at cross purposes (Figs. 126 and 127),
so as to produce opposite effects. These diagrams, Figs. 124
to 127, illustrate spring and neap tides: the dark shading
around the globe, E, representing a water envelope sur-
rounding the earth. There has been much discussion on
the origin of the word neap. It seems to be an Anglo-Saxon
word meaning scanty. Spring seems to be the same as
when we speak of plants springing up. I well remember at
Fig. 124 — Spring Tides
Fig. 125 — Spring Tides
the meeting of the British Association at Edinburgh a
French member who, meaning spring tides, spoke of the
grandes marees du printemps. Now you laugh at this ; and
yet, though he did not mean it, he was quite right, for the
spring tides in the spring time are greater on the whole
than those at other times, and we have the greatest spring
tides in the spring of the year. But there the analogy ceases,
for we have also very high spring tides in autumn. Still
the meaning of the two words is the same etymologically.
Neap tides are scanty tides, and spring tides are tides which
spring up to remarkably great heights.
The equilibrium theory of the tides is a way of putting
tidal phenomena. We say the tides would be so and so if
THE TIDES
299
Fig. 126 — Neap Tides
Fig. 127 — Neap Tides
300 KELVIN
the water took the figure of equilibrium. Now the water does
not cover the whole earth, as we have assumed in the dia-
grams (Figs. 152 to 155), but the surface of the water may be
imagined as taking the same figure, so far as there is water,
that it would take if there were water over the whole sur-
face of the earth. But here a difficult question comes in—
namely, the attraction of the water for parts of itself. If we
consider the water flowing over the whole earth this attrac-
tion must be taken into account. If we imagine the water
of exceedingly small density so that its attraction on itself is
insensible compared with that of the earth, we have thus to
think of the equilibrium theory. But, on the other hand, if
the water had the same density as the earth, the result
would be that the solid nucleus would be almost ready to
float ; and now imagine that the water is denser than the
earth, and we put the tides out of consideration altogether.
Think of the earth covered over with mercury instead of
water — a layer of mercury a foot deep. The solid earth
would tend to float, and would float, and the result would be
that the denser liquid would run to, and cover one side up to
a certain depth, and the earth would be as it were floating
out of the sea. That explains one curious result that
Laplace seems to have been much struck with: the stability
of the ocean requires that the density of the water should
be less than that of the solid earth. But take the sea as
having the specific gravity of water, the mean density of
the earth is only 5.6 times that of water, and this is not
enough to prevent the attraction of water for water from
being sensible. Owing therefore to the attraction of the
water for parts of itself the tidal phenomena are somewhat
larger than they would be without it, but neglecing this, and
neglecting the deformation of the solid earth, we have the
ordinary equilibrium theory.
Why does the water not follow the equilibrium theory?
Why have we tides of 20 feet or 30 feet or 40 feet in
some places, and only of 2 or 3 feet in others? Because
the water has not time in the course of 12 hours to take
the equilibrium figure, and because after tending towards
it, the water runs beyond it.
I ask you to think of the oscillations of water in a trough.
THE TIDES
301
Look at this diagram (Fig. 128), which will help you to
understand how the tidal effect is prodigiously magnified by
a dynamical action due to the inertia of the water. The
tendency of water in motion to keep its motion prevents it
from taking the figure of equilibrium. [A chart showing
Fig. 128 — Oscillations of Water in a Trough
the tides of the English Channel was exhibited, from which
it was seen that while at Dover there were tides of 21
feet, there was at Portland very little rise and fall.]
Imagine a canal instead of the English Channel, a canal
stopped at the Straits of Dover and at the opposite end at
Land's End, and imagine somehow a disturbing force caus-
ing the water to be heaped up at one end. There would be
a swing of water from one end to the other, and if the
period of the disturbing force approximately agreed with
the period of free oscillation, the effect would be that the
rise and fall would go vastly above and below the range
due to equilibrium action. Hence it is we have the 21
feet rise and fall at Dover. The very little rise and
fall at Portland is also illustrated in the uppermost figure
of this diagram (Fig. 128). Thus high water at Dover is
low water at Land's End, and the water seesaws as it were
about a line going across from Portland to Havre (repre-
sented by N in the figure) ; not a line going directly across,
however, for on the other side of the Channel there is a
curious complication.
At the time of high water at Dover there is hardly any
current in the Channel. As soon as the water begins to fall
302 KELVIN
at Dover the current begins to flow west through the whole
of the Channel. When it is mid-tide at Dover the tide is
flowing fastest in the Channel. This was first brought to
light by Admiral Beechey.
I wish I had time to show the similar theory as to the
tides in the Irish Channel. The water runs up the English
Channel to Dover, and up the Irish Channel to fill up the
basin round the Isle of Man. Take the northern mouth of
the Irish Channel between the Mull of Cantire and the north-
east coast of Ireland. The water rushes in through the
straits between Cantire and Rathlin Island, to fill up the
Bay of Liverpool and the great area of water round the Isle
of Man. This tidal wave entering from the north, running
southward through the Channel, meets in the Liverpool
basin with the tidal stream coming from the south entrance,
and causes the time of high water at Liverpool to be within
half-an-hour of the time of no currents in the northern and
southern parts of the Channel.
I would like to read you the late Astronomer-Royal's ap-
preciation of Laplace's splendid work on the tides.
Airy says of Laplace: "If now, putting from our
thoughts the details of the investigation, we consider its
general plan and objects, we must allow it to be one of the
most splendid works of the greatest mathematician of the
past age. To appreciate this, the reader must consider, first,
the boldness of the writer, who, having a clear understand-
ing of the gross imperfections in the methods of his pred-
ecessors, had also the courage deliberately to take up the
problem on grounds fundamentally correct (however it
might be limited by suppositions afterwards introduced) ;
secondly, the general difficulty of treating the motion of
fluids; thirdly, the peculiar difficulty of treating the motions
when the fluids cover an area which is not plane but con-
vex; and fourthly, the sagacity of perceiving that it was
necessary to consider the earth as a revolving body, and the
skill of correctly introducing this consideration. This last
point alone, in our opinion, gives a greater claim for reputa-
tion than the boasted explanation of the long inequality of
Jupiter and Saturn."
Tidal theory must be carried on along with tidal ob-
THE TIDES 303
servations. Instruments for measuring and recording the
height of the water at any time give us results of observa-
Fig. 129 — Tide Gauge
tions. 1 Here is such an instrument — a tide gauge (Fig.
1 The various instruments and tide-curves referred to in this lecture
are fully described and illustrated in a paper on " The Tide Gauge, Tidal
Harmonic Analyser, and Tide Predicter," read before the Institution of
Civil Engineers, on 1st March, 1881, and published in their Proceedings
for that date.
304
KELVIN
129). The floater is made of thin sheet copper, and is
suspended by a fine platinum wire. The vertical motion of
the floater, as the water rises and falls, is transmitted,
in a reduced proportion by a single pinion and wheel,
to this frame or marker, which carries a small marking
pencil. The paper on which the pencil marks the recording
curve, is stretched on this cylinder, which, by means of
the clockwork, is caused to make one revolution every
twenty-four hours. The leaning-tower-of-Pisa arrangement
Fig. 130 — Facsimile of weekly sheet of curves traced by Tide Gauge
of the paper-cylinder, and the extreme simplicity of the
connection between marker and floater, constitute the chief
novelty. This tide gauge is similar to one now in actual
use, recording the rise and fall of the water in the River
Clyde, at the entrance to the Queen's Dock, Glasgow. A
sheet bearing the curves (Fig. 130) traced by that machine
during a week is exhibited.
After the observations have been taken, the next thing
is to make use of them. Hitherto this has been done by
laborious arithmetical calculation. I hold in my hand
the Reports of the late Tidal Committee of the British
Association with the results of the harmonic analysis —
about eight years' work carried on with great labour,
and by aid of successive grants from the British Associ-
ation. The Indian Government has continued the harmonic
analysis for the seaports of India. The Tide Tables for
Indian Ports for the Year 1882, issued under the authority
of the Indian Government, show this analysis as in progress
for the following ports, viz. : Aden, Kurrachee, Okha Point
THE TIDES 305
and Beyt Harbour at the entrance to the Gulf of Cutch,
Bombay, Karwar, Beypore, Paumben Pass, Madras, Vizaga-
patam, Diamond Harbour, Fort Gloster and Kidderpore
on the River Hooghly, Rangoon, Moulmein,. and Port
Blair. Mr. Roberts, who was first employed as calculator
by the Committee of the British Association, has been
asked to carry on the work for the Indian Government,
and latterly, in India, native calculators under Major Baird,
have worked by the methods and forms by which Mr.
Roberts had worked in England for the British Associ-
ation. 2 The object is to find the values of the different
tidal constituents. We want to separate out from the whole
rise and fall of the ocean the part due to the sun, the
part due to the moon, the part due to one portion of
the moon's effect, and the part due to another. There are
complications depending on the moon's position — declina-
tional tides according as the moon is or is not in the
plane of the earth's equator — and also on that of the sun.
Thus we have the diurnal declinational tides. When the
moon is in the north declination (Fig. 131) we have (in
the equilibrium theory) higher water at lunar noon than
at lunar midnight. That difference in the height of high
water, and the corresponding solar noon tides and solar
midnight tides, due to the sun not being in the earth's
equator, constitute the lunar and solar diurnal declina-
tional tides. In summer the noon high water might be
expected to be higher than the midnight high water, be-
cause the sun is nearer overhead to us than to our Anti-
podes.
By kind permission of Sir Frederick Evans, I am able
to place before you these diagrams of curves drawn by
Captain Harris, R.N., of the Hydrographic Department
of the Admiralty, exhibiting the rise and fall of tides in
Princess Royal Harbour, King George Sound, Western
Australia, from January 1st to December 31st, 1877, and
in Broad Sound, Queensland, Australia, from July 15th,
2 Note of September 17, 1887. On the subject of Tidal Harmonic
Analysis see " Manual of Instructions for Tidal Observation," by Major
Baird, published by Messrs. Taylor and Francis, London, 1886; also the
Reports of the British Association Committee " On Harmonic Analysis of
Tidal Observations."— W. T.
306 KELVIN
1877, to July 23rd, 1878. Look at this one 01 these dia-
grams, a diagram of the tides at the north-east corner
of Australia. For several days high water always at
noon. When the tides are noticeable at all we have
Fig. 131 — Declinational Tide
high water at noon, and when the tides are not at noon
they are so small that they are not taken notice of at all.
It thus appears as if the tides were irrespective of the
moon, but they are not really so. When we look more
closely, it is a full moon if we have a great tide at noon;
or else it is new moon. It is at half moon that we have
the small tides, and when they are smallest we have
high water at six. There is also a great difference be-
tween day and night high water; the difference between
them is called the diurnal tide. A similar phenomenon
is shown on a smaller scale in this curve, drawn by the
first tide-predicting machine. At a certain time the two
high waters become equalised, and the two low waters
very unequal (see p. 304 for real examples).
The object of the harmonic analysis is to analyse out
from the complicated curve traced by the tide gauge the
simplest harmonic elements. A simple harmonic motion
may be imagined as that of a body which moves simply
up and down in a straight line,, keeping level with the
end of a clock hand, moving uniformly round. The ex-
ceedingly complicated motion that we have in the tides
is analysed into a series of simple harmonic motions in
different periods and with different amplitudes or ranges;
THE TIDES
307
*i
«j
u
W
c
*jj
V^
and these simple harmonic con-
stituents added together give the
complicated tides.
All the work hitherto done
has been accomplished by sheer
calculation ; 'but calculation of
so methodical a kind that a
machine ought to be found to
do it. The Tidal Harmonic
Analyser consists of an applica-
tion of Professor James Thom-
son's disk-globe-and-cylinder in-
tegrator to the evaluation of the
integrals required for the har-
monic analysis. The principle
of the machine and the essential
details are fully described and
explained in papers communi-
cated by Professor James Thom-
son and the author to the Royal
Society, in 1876 and 1878, and
published in the Proceedings for
those years; 3 also reprinted, with
a postscript dated April, 1879, in
Thomson and Tait's Natural
Philosophy, Second Edition, Ap-
pendix B. It remains now to
describe and explain the actual
machine referred to in the last
of these communications, which
is the only tidal harmonic an-
alyser hitherto made. It may be
mentioned, however, in passing,
that a similar instrument, with
the simpler construction wanted
for the simpler harmonic analy-
sis of ordinary meteorological
phenomena, has been constructed
for the Meteorological Com-
3 Vide vol. xxiv. p. 262, and vol. xxvii.
P. 37i.
308
KELVIN
mittee, and is now regularly at work at their office har-
monically analysing the results of meteorological observa-
tions, under the superintendence of Mr. R. H. Scott.
Fig. 132 represents the tidal harmonic analyser, con-
structed under the author's direction, with the assistance
of a grant from the Government Grant Fund of the Royal
Society. The eleven cranks of this instrument are allotted
as follows: —
Cranks.
Object
Dis-
tinguishing
Letter.
Speed.
1 and 2
To find the mean lunar semi-diurnal tide . . .
M
2(r-»)
3 »> 4
i>
mean solar „ >» . . .
S
2(Y-7l)
5 » 6
i»
luni-solar declinational diurnal tide
K,
y
7 „ 8
»i
slower lunar „ „
O
(y-2»)
9 n 10
i*
slower solar „ „
P
(r-*v)
11
11
mean water level
A,
...
The general arrangement of the several parts may be
seen from Fig. 132. The large circle at the back,
near the center, is merely a counter to count the days,
months, and years for four years, being the leap year
period. It is driven by a worm carried on an intermediate
shaft, with a toothed wheel geared on another on the
solar shaft. In front of the centre is the paper drum,
which is on the solar shaft, and goes round in the period
corresponding to twelve mean solar hours. On the ex-
treme left, the first pair of disks, with globes and cylinders,
and crank shafts with cranks at right angles between
them, driving their two cross-heads, corresponds to the Ki, or
luni-solar diurnal tide. The next pair of disk-globe-and-
cylinders corresponds to M, or the mean lunar semi-diurnal
tide, the chief of all the tides. The next pair lie on the
two sides of the main shaft carrying the paper drum, and
correspond to S, the mean solar semi-diurnal tide. The
first pair on the right correspond to O, or the lunar
diurnal tide. The second pair on the right correspond to
THE TIDES 309
P, the solar diurnal tide. The last disk on the extreme
right is simply Professor James Thomson's disk-globe-and-
cylinder integrator, applied to measure the area of the
cmrve as it passes through the machine.
The idle shafts for the M and the O tides are seen in
front respectively on the left and right of the centre.
The two other longer idle shafts for the K and the P tides
are behind, and therefore not seen. That for the P tide
serves also for the simple integrator on the extreme
right.
The large hollow square brass bar, stretching from end
to end along the top of the instrument, and carrying the
eleven forks rigidly attached to it, projecting downwards,
is moved to and fro through the requisite range by a rack
and pinion, worked by a handle and crank in front above
the paper cylinder, a little to the right of its centre. Each
of these eleven forks moves one of the eleven globes of
the eleven disk-globe-and-cylinder integrators of which
the machine is composed. The other handle and crank
in front, lower down and a little to the left of the centre,
drives by a worm, at a conveniently slow speed, the solar
shaft, and through it, and the four idle shafts, the four
other tidal shafts.
To work the machine the operator turns with his left
hand the driving crank, and with his right hand the trac-
ing crank, by which the fork-bar is moved. His left
hand he turns always in one direction, and at as nearly
constant a speed as is convenient to allow his right hand,
alternately in contrary directions, to trace exactly with
the steel pointer the tidal curve on the paper, which is car-
ried across the line of to-and-fro motion of the pointer by
the revolution of the paper drum, of which the speed is in
simple proportion to the speed of the operators left hand.
The eleven little counters of the cylinders in front of the
disks are to be set each at zero at the commencement of an
operation, and to be read off from time to time during the
operation, so as to give the value of the eleven integrals for
as many particular values of the time as it is desired to have
them.
A first working model harmonic analyser, which served
310
KELVIN
for model and for the meteorological analyser, now at work
in the Meteorological Office, is here before you. It has five
disk-globe-and-cylinders, and shafting geared for the ratio
i : 2. Thus it serves to determine, from the deviation curve,
the celebrated "A B C D E" of the Admiralty Compass
Manual, this is to say, the coefficients in the harmonic ex-
pression
A + B sin 6 -f C cos 6 + D sin 2 6 -f E cos 2 0.
for the deviation of the compass in an iron ship.
The first instrument which I designed and constructed
for use as a Tide Predictor was described in the Catalogue
of the Loan Collection of Scientific Apparatus at South
Kensington in 1876; and the instrument itself was presented
Fig. 133 — Tide Predictor
by the British Association to the South Kensington Museum,
where it now is. The second instrument constructed on the
same principle is in London, and is being worked under the
direction of Mr. Roberts, analysing the tides for the Indian
ports. The result of this work is these books {Tide Tables
for Indian Ports) in which we have, for the first time,
THE TIDES 311
tables of the times and heights of high water and low water
for fourteen of the Indian ports.
To predict the tides for the India and China Seas and
Australia we have a much more difficult thing to do than for
the British ports. The Admiralty Tide Tables give all
that is necessary for the British ports, practically speaking;
but for other parts of the world generally the diurnal tide
comes so much into play that we have exceedingly compli-
cated action. The most complete thing would be a table
showing the height of the water every hour of the twenty-
four. No one has yet ventured to do that generally for all
parts of the world; but for the comparatively complicated
tides of the India Seas, the curves traced by the Tide Pre-
dictor from which is obtained the information given in these
Indian tide tables, do actually tell the height of the water
for every instant of the twenty-four hours.
The mechanical method which I have utilised in this
machine is primarily due to the Rev. F. Bashforth who, in
1845, when he was a Bachelor of Arts and Fellow of St.
John's College, Cambridge, described it to Section A of the
1845 (Cambridge) meeting of the British Association in a
communication entitled "A Description of a Machine for
finding the Numerical Roots of Equations and tracing a
Variety of Useful Curves," of which a short notice ap-
pears in the British Association Report for that year. The
same subject was taken up by Mr. Russell in a communica-
tion to the Royal Society in 1869, " On the Mechanical
Description of Curves,"* which contains a drawing showing
mechanism substantially the same as that of the Tide Pre-
dictor. Here is the principal as embodied in No. 3 Tide
Predictor (represented in Fig. 133, p. 310), now actually
before you: —
A long cord of which one end is held fixed passes over
one pulley, under another, and so on. These eleven pulleys
are all moved up and down by cranks, and each pulley takes
in or lets out cord according to the direction in which it
moves. These cranks are all moved by trains of wheels
gearing into the eleven wheels fixed on this driving shaft.
The greatest number of teeth on any wheel is 802 engaging
*Proc. Royal Society, June 17, 1869; (vol. xviii. p. 72).
312
KELVIN
with another of 423. All the other wheels have compara-
tively small numbers of teeth. The machine is finished
now, except a cast-iron sole and cast-iron back. A fly-
wheel of great inertia enables me to turn the machine fast,
without jerking the pulleys, and so to run off a year's curve
in about twenty-five minutes. This machine is arranged for
fifteen constituents in all and besides that there is an ar-
rangement for analysing out the long period tides.
The following table shows how close an approximation to
astronomical accuracy is given by the numbers chosen for
the teeth of the several wheels. These numbers I have found
by the ordinary arithmetical progress of converging frac-
tions.
Tidal
Con-
stituents
Speed in Degrees per Mean Solar Hour.
Losses of Angle in
Machine.
Accurate.
As given by Machine.
Per Mean
Solar Hour.
Per
Half
Year.
M 2
Ki
P
N
L
¥
MS
M
X
Q
28*9841042
i5*04io686
* 3* 9430356
'4*'95893H
28'*4397296
29**5284788
28 0, 5i2583o
58 'o84io42
27* 9682084
29° 4556254
i3 ,, 39866o9
48s
x 5 x 2T1 = * 8 *'984o630
366
15 X 36s s i 5 '04x0959
15 X ^ 9 = i3**943o8Q4
364
15 x 36s = t 4 # 9589040
802
*5 X ~ = 26'-4397i6 3
'5^| = a9°-528 3 ox8
15 x ^ = 28-5123966
230 271
15 x 1T1 x xTx = 58 9836600
468
'5 X — =s 27-9681275
487
15 X 2^8 = *9 9 '455645«
15 * 459 = ' 3 * 3986928
4- o'oooo4X2
- 0* 0000267
- o* '0000538
4 0" '0000242
4- 0* '0000133
4- o*'oooi77
•f 0* '0001864
4- 0* 000444
4- o* '0000809
- 0**0000197
- o* , oooo3i8
o'i8o
o°'xi7
o'*237
o"'ii9
0*059
0*78
0*82
»-95
0*36
o e 'o87
o-x 4
1 . 1 i.l
THE TIDES 313
To-day (Aug. 25, 1882), a committee, consisting of only
two members, Mr. George Darwin and Professor Adams
of Cambridge, have been appointed, and one of their chief
objects will be to examine the long period tides.
There is one very interesting point I said I would endeav-
our to speak of if I had time; I have not time, but still I
must speak of it — the influence of the moon on the weather.
" We almost laugh when we hear of the influence of the
moon on the weather," Sir F. Evans said to me, " but there
is an influence/' Gales of wind are remarkably prevalent
in Torres Straits and the neighbourhood about the time of
new and full moon. This was noticed by Dr. Rattray, a
surgeon in the navy, in connection with observations made
by the surveying ship, Fly, during the three years 1841-44.
Dr. Rattray noticed that at those times there was a large
area of coral reef uncovered at the very low water of the
spring tides, extending out some sixty or seventy miles from
land. This large area becomes highly heated, and the great
heating of that large portion of land gives rise to a tend-
ency to gales at the full and change, that is at the new and
full moon. This indirect effect of the moon upon the weather
through the tides is exceedingly interesting; but it does not
at all invalidate the scientific conclusion that there is no
direct influence, and the general effect of the moon on the
weather — the changes in the moon and the changes in the
weather, and their supposed connection — remains a mere
chimaera.
The subject of elastic tides in which the yielding of the
solid earth is taken into account is to be one of the primary
objects of Mr. G. Darwin's committee. The tide-generating
force which tends to pull the water to and from the moon,
tends to pull the earth also. Imagine the earth made of india-
rubber and pulled out to and from the moon. It will be
made prolate (Fig. 134). If the earth were of india-rubber
the tides would be nothing, the rise and fall of the water
relatively to the solid would be practically nil. If the earth
(as has long been a favourite hypothesis of geologists) had
a thin shell 20 or 30 miles thick with liquid inside, there
would be no such thing as tides of water rising and fall-
ing relatively to land, or sea-bottom. The earth's crust
314
KELVIN
would yield to and from the moon, and the water would
not move at all relatively to the crust. If the earth were
even as rigid as glass all through, calculation shows that the
solid would yield so much that the tides could only be about
one third of what they would be if the earth were perfectly
rigid. Again, if the earth were two or three times as rigid
as glass, about as rigid as a solid globe of steel, it would
Fig. 134 — Elastic Tides
still, considering its great dimensions, yield two or three
feet to that great force, which elastic yielding would be
enough to make the tides only two thirds of what they
would be if the earth were perfectly rigid. Mr. G. Darwin
has made the investigation by means of the lunar fortnightly
tides, and the general conclusion, subject to verification, is
that the earth does seem to yield somewhat, and may have
something like the rigidity of a solid globe of steel.
APPENDIX A
[Extracts from a Lecture on " The Tides/' given to the
Glasgow Science Lectures Association, not hitherto pub-
lished, and now included as explaining in greater detail
certain paragraphs of the preceding Lecture.]
(i) Gravitation. — The great theory of gravitation put
before us by Newton asserts that every portion of matter in
the universe attracts every other portion ; and that the force
depends on the masses of the two portions considered, and
on the distance between them. Now, the first great point
of Newton's theory is, that bodies which have equal masses
are equally attracted by any other body, a body of double
mass experiencing double force. This may seem only what
is to be expected. It would take more time than we have to
spare were I to point out all that is included in this statement ;
but let me first explain to you how the motions of different
kinds of matter depend on a property called inertia. I
might show you a mass of iron as here. Consider that if
I apply force to it, it gets into a state of motion; greater
force applied to it, during the same time, gives it increased
velocity, and so on. Now, instead of a mass of iron, I
might hang up a mass of lead, or a mass of wood, to test
the equality of the mass by the equality of the motion which
is produced in the same time by the action of the same
force, or in equal times by the action of equal forces. Thus,
quite irrespectively of the kind of matter concerned, we
have a test of the quantity of matter. You might weigh
a pound of tea against a pound of brass without ever
putting them into the balance at all. You might hang up
one body by a proper suspension, and you might, by a spring,
measure the force applied, first to the one body, and then to
the other. If the one body is found to acquire equal velocity
under the influence of equal force for equal times as com-
315
316 KELVIN
pared with the other body, then the mass of the one is said
to be equal to the mass of the other.
I have spoken of mutual forces between any two masses.
Let us consider the weight or heaviness of a body on the
earth's surface. Newton explained that the attraction of the
whole earth upon a body — for example, this 56 pounds mass
of iron — causes its heaviness or weight. Well, now, take
56 pounds of iron here, and take a mass of lead, which,
when put in the balance, is found to be of equal weight. You
see we have quite a new idea here. You weigh this mass
of iron against a mass of lead, or to weigh out a commodity
for sale; as, for instance, to weigh out pounds of tea, to
weigh them with brass weights is to compare their gravita-
tions towards the earth — to compare the heavinesses of the
different bodies. But the first subject that I asked you to
think of had nothing to do with heaviness. The first subject
was the mass of the different bodies as tested by their resist-
ance to force tending to set them in motion. I may just
say that the property of resistance against being set into
motion, and again against resistance to being stopped when in
motion, is the property of matter called inertia.
The first great point in Newton's discovery shows, then,
that if the property of inertia is possessed to an equal degree
by two different substances, they have equal heaviness.
One of his proofs was founded on the celebrated guinea
and feather experiment, showing that the guinea and feather
fall at the same rate when the resistance of the air is re-
moved. Another was founded upon making pendulums of
different substances — lead, iron, and wood — to vibrate, and
observing their times of vibration. Newton thus discovered
that bodies which have equal heaviness have equal inertia.
The other point of the law of gravitation is, that the
force between any two bodies diminishes as the distance
increases, according to the law of the inverse square of the
distance. That law expresses that, with double distance, the
force is reduced one quarter, at treble distance the force is
reduced to one-ninth part. Suppose we compare forces at
the distance of one million miles, then again at the distance
of two and a half million miles, we have to square the one
number, then square the other, and find the proportion of the
THE TIDES 317
square of the one number to the square of the other. The
forces are inversely as the squares of the distance, that
is the most commonly quoted part of the law of gravitation;
but the law is incomplete without the first part, which estab-
lishes the relation between two apparently different prop-
erties of matter. Newton founded this law upon a great
variety of different natural phenomena. The motion of the
planets round the sun, and the moon round the earth, proved
that for each planet the force varies inversely as the square
of its distance from the sun; and that from planet to planet
the forces on equal portions of their masses are inversely
as the squares of their distances. The last link in the great
chain of this theory is the tides.
(2) Tide-Generating Force. — And now we are nearly
ready to complete the theory of tide-generating force. The
first rough view of the case, which is not always incorrect,
is that the moon attracts the waters of the earth towards
herself and heaps them up, therefore, on one side of the
earth. It is not so. It would be so if the earth and moon
were at rest and prevented from falling together by a rigid
bar or column. If the earth and moon were stuck on the two
ends of a strong bar, and put at rest in space, then the
attraction of the moon would draw the waters of the earth
to the side of the earth next to the moon. But in reality
things are very different from that supposition. There is
no rigid bar connecting the moon and the earth. Why then
does not the moon fall towards the earth? According to
Newton's theory, the moon is always falling towards the
earth. Newton compared the fall of the moon, in his cele-
brated statement, with the fall of a stone at the earth's sur-
face, as he recounted, after the fall of an apple from the
tree, which he perceived when sitting in his garden musing
on his great theory. The moon is falling towards the earth,
and falls in an hour as far as a stone falls in a second.
It chances that the number 60 is nearly enough, as I have
said before, a numerical expression for the distance of the
moon from the earth in terms of the earth's radius. It is
only by that chance that the comparison between the second
and hour can be here introduced. Since there are 60 times
60 seconds in an hour, and about 60 radii of the earth in
318 KELVIN
the distance from the moon, we are led to the comparison
now indicated, but I am inverting the direction of Newton's
comparison. He found by observation that the moon falls
as far in an hour as a stone falls in a second, and hence
inferred that the force on the moon is a 6oth of the 6oth
of the force per equal mass on the earth's surface. Then
he learned from accurate observations, and from the earth's
dimensions, what I have mentioned as the moon's distance,
and perceived the law of variation between the weight of
a body at the earth's surface and the force that keeps the
moon in her orbit. The moon in Newton's theory was al-
ways falling towards the earth. Why does it not come
down? Can it be always falling and never come down?
That seems impossible. It is always falling, but it has also
a motion perpendicular to the direction in which it is fall-
ing, and the result of that continual falling is simply a
change of direction of this motion.
It would occupy too much of our time to go into this
theory. It is simply the dynamical theory of centrifugal
force. There is a continual falling away from the line of
motion, as illustrated in a stone thrown from the hand
describing an ordinary curve. You know that if a stone is
thrown horizontally it describes a parabola — the stone fall-
ing away from the line in which it was thrown. The moon
is continually falling away from the line in which it moves
at any instant, falling away towards the point of the earth's
centre, and falling away towards that point in the varying
direction from itself. You can see it may be always falling,
now from the present direction, now from the altered direc-
tion, now from the farther altered direction in a further
altered line ; and so it may be always falling and never com-
ing down. The parts of the moon nearest to the earth tend
to fall most rapidly, the parts furthest from the earth, least
rapidly; in its own circle, each is falling away and the
result is as if we had the moon falling directly.
But while the moon is always falling towards the earth,
the earth is always falling towards the moon; and each
preserves a constant distance, or very nearly a constant
distance from the common centre of gravity of the two.
The parts of the earth nearest to the moon are drawn
THE TIDES 319
towards the moon with more force than an equal mass at
the average distance; the most distant parts are drawn
towards the moon with less force than corresponds to the
average distance. The solid mass of the earth, as a whole,
experiences, according to its mass, a force depending on the
average distance; while each portion of the water on the
surface of the earth experiences an attractive force due
to its own distance from the moon. The result clearly is,
then, a tendency to protuberance towards the moon and from
the moon ; and thus, in a necessarily most imperfect manner,
I have explained to you how it is that the waters are not
heaped up on the side next the moon, but are drawn up
towards the moon and left away from the moon so as to
tend to form an oval figure. The diagram (Fig. 125, p. 298)
shows the protuberance of water towards and from the
moon. It shows also the sun on the far side, I need
scarcely say, with an enormous distortion of proportions,
because without that it would be impossible in a diagram to
show the three bodies. This illustrates the tendency of the
tide-generating forces.
(3) Elastic Tides. — But another question arises. This
great force of gravity operating in different directions, pull-
ing at one place, pressing in at another, will it not squeeze
the earth out of shape? I perceive signs of incredu-
lity; you think it impossible it can produce any sensible
effect. Well, I will just tell you that instead of being im-
possible, instead of it not producing any such effect, we have
to suppose the earth to be of exceedingly rigid material, in
order that the effect of these distorting influences on it may
not mask the phenomenon of the tides altogether.
There is a very favourite geological hypothesis which
I have no doubt many here present have heard, which per-
haps till this moment many here present have believed, but
which I hope no one will go out of this room believing, and
that is that the earth is a mere crust, a solid shell thirty,
or forty, or fifty miles thick at the most, and that it is
filled with molten liquid lava. This is not a supposition to
be dismissed as absurd, as ludicrous, as absolutely unfounded
and unreasonable. It is a theory based on hypothesis which
requires most careful weighing. But it has been carefully
320 KELVIN
weighed and found wanting in conformity to the truth. On
a great many different essential points it has been found at
variance with the truth. One of these points is, that unless
the material of this supposed shell were preternaturally
rigid, were scores of times more rigid than steel, the shell
would yield so freely to the tide-generating forces that it
would take the figure of equilibrium, and there would be
no rise and fall of the water, relatively to the solid land,
left to show us the phenomena of the tides.
Imagine that this (Fig. 134, p. 314) represents a solid shell
with water outside, you can understand if the solid shell
yields with sufficiently great freedom, there will be exceed-
ingly little tidal yielding left for the water to show. It
may seem strange when I say that hard steel would yield so
freely. But consider the great hardness of steel and the
smaller hardness of india-rubber. Consider the greatness of
the earth, and think of a little hollow india-rubber ball,
how freely it yields to the pressure of the hand, or even
to its own weight when laid on a table. Now, take a great
body like the earth: the greater the mass the more it is
disposed to yield to the attraction of distorting forces when
these forces increase with the whole mass. I cannot just
now fully demonstrate to you this conclusion; but I say
that a careful calculation of the forces shows that in virtue
of the greatness of the mass it would require an enormously
increased rigidity in order to keep in shape. So that if we
take the actual dimensions of the earth at forty-two million
feet diameter, and the crust at fifty miles thick, or two hun-
dred and fifty thousand feet, and with these proportions
make the calculation, we find that something scores of times
more rigid than steel would be required to keep the shape so
well as to leave any appreciable degree of difference from
the shape of hydrostatic equilibrium, and allow the water
to indicate, by relative displacement, its tendency to take
the figure of equilibrium; that is to say, give to us the
phenomena of tides. The geological inference from this con-
clusion is, that not only must we deny the fluidity of the
earth and the assertion that it is encased by a thin shell,
but we must say that the earth has, on the whole, a rigidity
greater than that of a solid globe of glass of the same
THE TIDES 321
dimensions ; and perhaps greater than that of a globe of
steel of the same dimensions. But that it cannot be less
rigid than a globe of glass, we are assured. It is not to be
denied that there may be a very large space occupied by
liquid. We know there are large spaces occupied by lava ; but
we do not know how large they may be, although we can cer-
tainly say that there are no such spaces, as can in volume be
compared with the supposed hollow shell, occupied by liquid
constituting the interior of the earth. The earth as a whole
must be rigid, and perhaps exceedingly rigid, probably ren-
dered more rigid than it is at the surface strata by the
greater pressure in the greater depths.
The phenomena of underground temperature, which led
geologists to that supposition, are explained otherwise than
by their assumption of a thin shell full of liquid; and
further, every view we can take of underground temperature,
in the past history of the earth, confirms the statement that
we have no right to assume interior fluidity.
HG VOL. XXX— K
THE EXTENT OF THE UNIVERSE
BY
SIMON NEWCOMB
INTRODUCTORY NOTE
In spite of the fertility of America in mechanical invention
and applied science, there are few branches of pure science in
which she can be regarded as among the leading nations. Her
nearest approach to preeminence has probably been in astron-
omy ; and in this field Simon Newcomb was, at his death, the
most distinguished figure.
Newcomb was born in the village of Wallace, Nova Scotia,
March 12, 1835. His father, who was a teacher, gave him his ele-
mentary education; and at the age of eighteen we find him
teaching a country school in Maryland. Two years later, a
position as computer on the ( Nautical Almanac" brought him
to Cambridge, Mass., where he studied in Harvard University
till 1861, when he was appointed professor of mathematics in
the United States Navy. He remained in the government ser-
vice till he was retired as a rear admiral in 1897, having
served besides as professor of mathematics and astronomy in
Johns Hopkins University, Baltimore, from 1884.
Newcomb's chief labors were in the department of mathe-
matical astronomy, and were directed toward the explanation
of the observed movements of the heavenly bodies. The diffi-
culty and complexity of the calculations involved are beyond
the conception of the layman; and the achievements which
brought Newcomb honors from the learned of almost all civi-
lized countries have to be taken on trust by the general. He
had, nevertheless, an admirable power of clear exposition
of those parts of his subject which were capable of populariza-
tion; and the accompanying paper is a good example of the
simple treatment of a large subject.
NewcomVs interests extended beyond his special field, and he
wrote with vigor and originality on finance and economics, and
played a leading part in the general intellectual life of his time.
When he died in the midst of his labors on July 11, 1909, he
left a place at the head of American science that will not easily
be filled.
324
THE EXTENT OF THE
UNIVERSE
WE cannot expect that the wisest men of our
remotest posterity, who can base their conclusions
upon thousands of years of accurate observation,
will reach a decision on this subject without some measure
of reserve. Such being the case, it might appear the dictate
of wisdom to leave its consideration to some future age,
when it may be taken up with better means of information
than we now possess. But the question is one which will
refuse to be postponed so long as the propensity to think of
the possibilities of creation is characteristic of our race. The
issue is not whether we shall ignore the question altogether,
like Eve in the presence of Raphael ; but whether in studying
it we shall confine our speculations within the limits set
by sound scientific reasoning. Essaying to do this, I invite
the reader's attention to what science may suggest, admit-
ting in advance that the sphere of exact knowledge is small
compared with the possibilities of creation, and that outside
this sphere we can state only more or less probable con-
clusions.
The reader who desires to approach this subject in the
most receptive spirit should begin his study by betaking him-
self on a clear, moonless evening, when he has no earthly
concern to disturb the serenity of his thoughts, to some point
where he can lie on his back on bench or roof, and scan
the whole vault of heaven at one view. He can do this
with the greatest pleasure and profit in late summer or
autumn — winter would do equally well were it possible for
the mind to rise so far above bodily conditions that the
Copyright, 1906, by Harper & Brothers.
325
326 NEWCOMB
question of temperature should not enter. The thinking
man who does this under circumstances most favorable for
calm thought will form a new conception of the wonder of
the universe. If summer or autumn be chosen, the stupen-
dous arch of the Milky Way will pass near the zenith, and
the constellation Lyra, led by its beautiful blue Vega of
the first magnitude, may be not very far from that point.
South of it will be seen the constellation Aquila, marked by
the bright Altair, between two smaller but conspicuous stars.
The bright Arcturus will be somewhere in the west, and, if
the observation is not made too early in the season, Alde-
baran will be seen somewhere in the east. When attention
is concentrated on the scene the thousands of stars on each
side of the Milky Way will fill the mind with the conscious-
ness of a stupendous and all-embracing frame, beside which
all human affairs sink into insignificance. A new idea will
be formed of such a well-known fact of astronomy as the
motion of the solar system in space, by reflecting that, dur-
ing all human history, the sun, carrying the earth with it,
has been flying towards a region in or just south of the
constellation Lyra, with a speed beyond all that art can pro-
duce on earth, without producing any change apparent to
ordinary vision in the aspect of the constellation. Not only
Lyra and Aquila, but every one of the thousand stars which
form the framework of the sky, were seen by our earliest
ancestors just as we see them now. Bodily rest may be
obtained at any time by ceasing from our labors, and weary
systems may find nerve rest at any summer resort; but I
know of no way in which complete rest can be obtained for
the weary soul — in which the mind can be so entirely re-
lieved of the burden of all human anxiety — as by the con-
templation of the spectacle presented by the starry heavens
under the conditions just described. As we make a feeble
attempt to learn what science can tell us about the structure
of this starry frame, I hope the reader will allow me to at
least fancy him contemplating it in this way.
The first question which may suggest itself to the inquiring
reader is: How is it possible by any methods of observa-
tion yet known to the astronomer to learn anything about
the universe as a whole? We may commence by answering
THE EXTENT OF THE UNIVERSE 327
this question in a somewhat comprehensive way. It is pos-
sible only because the universe, vast though it is, shows cer-
tain characteristics of a unified and bounded whole. It is
not a chaos, it is not even a collection of things, each of
which came into existence in its own separate way. If it
were, there would be nothing in common between two widely
separate regions of the universe. But, as a matter of fact,
science shows unity in the whole structure, and diversity
only in details. The Milky Way itself will be seen by the
most ordinary observer to form a single structure. This
structure is, in some sort, the foundation on which the uni-
verse is built It is a girdle which seems to span the whole
of creation, so far as our telescopes have yet enabled us to
determine what creation is; and yet it has elements of simi-
larity in all its parts. What has yet more significance, it is
in some respects unlike those parts of the universe which
lie without it, and even unlike those which lie in that cen-
tral region within it where our system is now situated. The
minute stars, individually far beyond the limits of visibility
to the naked eye, which form its cloudlike agglomerations,
are found to be mostly bluer in color, from one extreme to
the other, than the general average of the stars which
make up the rest of the universe.
In the preceding essay on the structure of the universe,
we have pointed out several features of the universe showing
the unity of the whole. We shall now bring together these
and other features with a view of showing their relation
to the question of the extent of the universe.
The Milky Way being in a certain sense the foundation
on which the whole system is constructed, we have first to
notice the symmetry of the whole. This is seen in the fact
that a certain resemblance is found in any two opposite
regions of the sky, no matter where w T e choose them. If we
take them in the Milky Way, the stars are more numerous
than elsewhere; if we take opposite regions in or near the
Milky Way, we shall find more stars in both of them than
elsewhere; if we take them in the region anywhere around
the poles of the Milky Way, we shall find fewer stars, but
they will be equally numerous in each of the two regions.
We infer from this that whatever cause determined the
328 NEWCOMB
number of the stars in space was of the same nature in
every two antipodal regions of the heavens.
Another unity marked with yet more precision is seen in
the chemical elements of which stars are composed. We
know that the sun is composed of the same elements which
we find on the earth and into which we resolve compounds
in our laboratories. These same elements are found in the
most distant stars. It is true that some of these bodies
seem to contain elements which we do not find on earth.
But as these unknown elements are scattered from one
extreme of the universe to the other, they only serve still
further to enforce the unity which runs through the whole.
The nebulae are composed, in part at least, of forms of
matter dissimilar to any with which we are acquainted.
But, different though they may be, they are alike in their
general character throughout the whole field we are con-
sidering. Even in such a feature as the proper motions of
the stars, the same unity is seen. The reader doubtless
knows that each of these objects is flying through space on
its own course with a speed comparable with that of the
earth around the sun. These speeds range from the smallest
limit up to more than one hundred miles a second. Such
diversity might seem to detract from the unity of the whole;
but when we seek to learn something definite by taking their
average, we find this average to be, so far as can yet be
determined, much the same in opposite regions of the uni-
verse. Quite recently it has become probable that a certain
class of very bright stars known as Orion stars — because
there are many of them in the most brilliant of our con-
stellations — which are scattered along the whole course of
the Milky Way, have one and all, in the general average,
slower motions than other stars. Here again we have a
definable characteristic extending through the universe. In
drawing attention to these points of similarity throughout
the whole universe, it must not be supposed that we base our
conclusions directly upon them. The point they bring out is
that the universe is in the nature of an organized system;
and it is upon the fact of its being such a system that we
are able, by other facts, to reach conclusions as to its struc-
ture, extent, and other characteristics.
THE EXTENT OF THE UNIVERSE 329
One of the great problems connected with the universe is
that of its possible extent. How far away are the stars?
One of the unities which we have described leads at once to
the conclusion that the stars must be at very different
distances from us; probably the more distant ones are a
thousand times as far as the nearest ; possibly even farther
than this. This conclusion may, in the first place, be based on
the fact that the stars seem to be scattered equally through-
out those regions of the universe which are not connected
with the Milky Way. To illustrate the principle, suppose a
farmer to sow a wheat-field of entirely unknown extent with
ten bushels of wheat. We visit the field and wish to have
some idea of its acreage. We may do this if we know how
many grains of wheat there are in the ten bushels. Then
we examine a space two or three feet square in any part of
the field and count the number of grains in that space. If
the wheat is equally scattered over the whole field, we find
its extent by the simple rule that the size of the field bears
the same proportion to the size of the space in which the
count was made that the whole number of grains in the ten
bushels sown bears to the number of grains counted. If
we find ten grains in a square foot, we know that the number
of square feet in the whole field is one-tenth that of the
number of grains sown. So it is with the universe of stars.
If the latter are sown equally through space, the extent of
the space occupied must be proportional to the number of
stars which it contains.
But this consideration does not tell us anything about the
actual distance of the stars or how thickly they may be scat-
tered. To do this we must be able to determine the distance
of a certain number of stars, just as we suppose the farmer
to count the grains in a certain small extent of his wheat-
field. There is only one way in which we can make a defi-
nite measure of the distance of any one star. As the earth
swings through its vast annual circuit round the sun,
the direction of the stars must appear to be a little different
when seen from one extremity of the circuit than when seen
from the other. This difference is called the parallax of the
stars; and the problem of measuring it is one of the most
delicate and difficult in the whole field of practical astronomy.
330 NEWCOMB
The nineteenth century was well on its way before the
instruments of the astronomer were brought to such perfec-
tion as to admit of the measurement. From the time of
Copernicus to that of Bessel many attempts had been made
to measure the parallax of the stars, and more than once
had some eager astronomer thought himself successful. But
subsequent investigation always showed that he had been
mistaken, and that what he thought was the effect of paral-
lax was due to some other cause, perhaps the imperfections
of his instrument, perhaps the effect of heat and cold upon
it or upon the atmosphere through which he was obliged to
observe the star, or upon the going of his clock. Thus
things went on until 1837, when Bessel announced that
measures with a heliometer — the most refined instrument
that has ever been used in measurement — showed that a
certain star in the constellation Cygnus had a parallax of
one-third of a second. It may be interesting to give an idea
of this quantity. Suppose one's self in a house on top of a
mountain looking out of a window one foot square, at a
house on another mountain one hundred miles away. One
is allowed to look at that distant house through one edge of
the pane of glass and then through the opposite edge ; and
he has to determine the change in the direction of the dis-
tant house produced by this change of one foot in his own
position. From this he is to estimate how far off the other
mountain is. To do this, one would have to measure just
about the amount of parallax that Bessel found in his
star. And yet this star is among the few nearest to our
system. The nearest star of all, Alpha Centauri, visible only
in latitudes south of our middle ones, is perhaps half as far
as Bessel's star, while Sirius and one or two others are
nearly at the same distance. About 100 stars, all told, have
had their parallax measured with a greater or less degree
of probability. The work is going on from year to year,
each successive astronomer who takes it up being able, as a
general rule, to avail himself of better instruments or to
use a better method. But, after all, the distances of even
some of the 100 stars carefully measured must still remain
quite doubtful.
Let us now return to the idea of dividing the space in
THE EXTENT OF THE UNIVERSE 331
which the universe is situated into concentric spheres drawn
at various distances around our system as a centre. Here
we shall take as our standard a distance 400,000 times that
of the sun from the earth. Regarding this as a unit, we
imagine ourselves to measure out in any direction a dis-
tance twice as great as this — then another equal distance,
making one three times as great, and so indefinitely. We
then have successive spheres of which we take the nearer
one as the unit. The total space filled by the second sphere
will be 8 times the unit ; that of the third space 27 times, and
so on, as the cube of each distance. Since each sphere
includes all those within it, the volume of space between
each two spheres will be proportional to the difference
of these numbers — that is, to 1, 7, 19, etc. Comparing
these volumes with the number of stars probably within
them, the general result up to the present time is that
the number of stars in any of these spheres will be about
equal to the units of volume which they comprise, when
we take for this unit the smallest and innermost of the
spheres, having a radius 400,000 times the sun's distance.
We are thus enabled to form some general idea of how
thickly the stars are sown through space. We cannot
claim any numerical exactness for this idea, but in the
absence of better methods it does afford us some basis
for reasoning.
Now we can carry on our computation as we supposed
the farmer to measure the extent of his wheat-field. Let us
suppose that there are 125,000,000 stars in the heavens. This
is an exceedingly rough estimate, but let us make the suppo-
sition for the time being. Accepting the view that they are
nearly equally scattered throughout space, it will follow
that they must be contained within a volume equal to 125,-
000,000 times the sphere we have taken as our unit. We
find the distance of the surface of this sphere by extracting
the cube root of this number, which gives us 500. We may,
therefore, say, as the result of a very rough estimate, that
the number of stars we have supposed would be contained
within a distance found by multiplying 400,000 times the dis-
tance of the sun by 500; that is, that they are contained
within a region whose boundary is 200,000,000 times the
332 NEWCOMB
distance of the sun. This is a distance through which light
would travel in about 3300 years.
It is not impossible that the number of stars is much
greater than that we have supposed. Let us grant that there
are eight times as many, or 1,000,000,000. Then we should
have to extend the boundary of our universe twice as far,
carrying it to a distance which light would require 6600
years to travel.
There is another method of estimating the thickness with
which stars are sown through space, and hence the extent of
the universe, the result of which will be of interest. It is
based on the proper motion of the stars. One of the
greatest triumphs of astronomy of our time has been the
measurement of the actual speed at which many of the stars
are moving to or from us in space. These measures are
made with the spectroscope. Unfortunately, they can be
best made only on the brighter stars — becoming very diffi-
cult in the case of stars not plainly visible to the naked eye.
Still the motions of several hundreds have been measured and
the number is constantly increasing.
A general result of all these measures and of other esti-
mates may be summed up by saying that there is a certain
average speed with which the individual stars move in space ;
and that this average is about twenty miles per second.
We are also able to form an estimate as to what proportion
of the stars move with each rate of speed from the lowest
up to a limit which is probably as high as 150 miles per
second. Knowing these proportions we have, by obser-
vation of the proper motions of the stars, another method of
estimating how thickly they are scattered in space ; in other
words, what is the volume of space which, on the average,
contains a single star. This method gives a thickness of
the stars greater by about twenty-five per cent, than that
derived from the measures of parallax. That is to say, a
sphere like the second we have proposed, having a radius
800,000 times the distance of the sun, and therefore a di-
ameter 1,600,000 times this distance, would, judging by the
proper motions, have ten or twelve stars contained within it,
while the measures of parallax only show eight stars within
the sphere of this diameter having the sun as its centre.
THE EXTENT OF THE UNIVERSE 333
The probabilities are in favor of the result giving the
greater thickness of the stars. But, after all, the discrep-
ancy does not change the general conclusion as to the limits
of the visible universe. If we cannot estimate its extent
with the same certainty that we can determine the size of
the earth, we can still form a general idea of it.
The estimates we have made are based on the supposition
that the stars are equally scattered in space. We have good
reason to believe that this is true of all the stars except
those of the Milky Way. But, after all, the latter prob-
ably includes half the whole number of stars visible with
a telescope, and the question may arise whether our results
are seriously wrong from this cause. This question can
best be solved by yet another method of estimating the
average distance of certain classes of stars.
The parallaxes of which we have heretofore spoken con-
sist in the change in the direction of a star produced by the
swing of the earth from one side of its orbit to the other.
But we have already remarked that our solar system, with
the earth as one of its bodies, has been journeying straight-
forward through space during all historic times. It follows,
therefore, that we are continually changing the position
from which we view the stars, and that, if the latter were
at rest, we could, by measuring the apparent speed with
which they are moving in the opposite direction from that
of the earth, determine their distance. But since every star
has its own motion, it is impossible, in any one case, to
determine how much of the apparent motion is due to the
star itself, and how much to the motion of the solar system
through space. Yet, by taking general averages among
groups of stars, most of which are probably near each
other, it is possible to estimate the average distance by
this method. When an attempt is made to apply it, so as
to obtain a definite result, the astronomer finds that the
data now available for the purpose are very deficient. The
proper motion of a star can be determined only by comparing
its observed position in the heavens at two widely separate
epochs. Observations of sufficient precision for this purpose
were commenced about 1750 at the Greenwich Observatory,
by Bradley, then Astronomer Royal of England. But out
334 NEWCOMB
of 3000 stars which he determined, only a few are available
for the purpose. Even since his time, the determinations
made by each generation of astronomers have not been suf-
ficiently complete and systematic to furnish the material for
anything like a precise determination of the proper motions
of stars. To determine a single position of any one star
involves a good deal of computation, and if we reflect that,
in order to attack the problem in question in a satisfactory
way, we should have observations of 1,000,000 of these
bodies made at intervals of at least a considerable fraction
of a century, we see what an enormous task the astronomers
dealing with this problem have before them, and how
imperfect must be any determination of the distance
of the stars based on our motion through space. So
far as an estimate can be made, it seems to agree
fairly well with the results obtained by the other methods.
Speaking roughly, we have reason, from the data so far
available, to believe that the stars of the Milky Way are
situated at a distance between 100,000,000 and 200,000,000
times the distance of the sun. At distances less than this it
seems likely that the stars are distributed through space with
some approach to uniformity. We may state as a general
conclusion, indicated by several methods of making the
estimate, that nearly all the stars which we can see with our
telescopes are contained within a sphere not likely to be much
more than 200,000,000 times the distance of the sun.
The inquiring reader may here ask another question.
Granting that all the stars we can see are contained within
this limit, may there not be any number of stars outside
the limit which are invisible only because they are too far
away to be seen?
This question may be answered quite definitely if we
grant that light from the most distant stars meets with no
obstruction in reaching us. The most conclusive answer
is afforded by the measure of starlight. If the stars ex-
tended out indefinitely, then the number of those of each
order of magnitude would be nearly four times that of the
magnitude next brighter. For example, we should have
nearly four times as many stars of the sixth magnitude as
of the fifth; nearly four times as many of the seventh
THE EXTENT OF THE UNIVERSE 335
as of the sixth, and so on indefinitely. Now, it is actually
found that while this ratio of increase is true for the
brighter stars, it is not so for the fainter ones, and that the
increase in the number of the latter rapidly falls off when
we make counts of the fainter telescopic stars. In fact, it
has long been known that, were the universe infinite in
extent, and the stars equally scattered through all space, the
whole heavens would blaze with the light of countless
millions of distant stars separately invisible even with the
telescope.
The only way in which this conclusion can be invalidated
is by the possibility that the light of the stars is in some
way extinguished or obstructed in its passage through space.
A theory to this effect was propounded by Struve nearly a
century ago, but it has since been found that the facts as he
set them forth do not justify the conclusion, which was, in
fact, rather hypothetical. The theories of modern science
converge towards the view that, in the pure ether of space,
no single ray of light can ever be lost, no matter how far
it may travel. But there is another possible cause for the
extinction of light. During the last few years discoveries
of dark and therefore invisible stars have been made by
means of the spectroscope with a success which would have
been quite incredible a very few years ago, and which, even
to-day, must excite wonder and admiration. The general
conclusion is that, besides the shining stars which exist
in space, there may be any number of dark ones, forever
invisible in our telescopes. May it not be that these bodies
are so numerous as to cut off the light which we would
otherwise receive from the more distant bodies of the uni-
verse? It is, of course, impossible to answer this question
in a positive way, but the probable conclusion is a negative
one. We may say with certainty that dark stars are not so
numerous as to cut off any important part of the light from
the stars of the Milky Way, because, if they did, the latter
would not be so clearly seen as it is. Since we have reason
to believe that the Milky Way comprises the more distant
stars of our system, we may feel fairly confident that not
much light can be cut off by dark bodies from the most dis-
tant region to which our telescopes can penetrate. Up to this
336 NEWCOMB ,
distance we see the stars just as they are. Even within the
limit of the universe as we understand it, it is likely that
more than one-half the stars which actually exist are too
faint to be seen by human vision, even when armed with
the most powerful telescopes. But their invisibility is due
only to their distance and the faintness of their intrinsic
light, and not to any obstructing agency.
The possibility of dark stars, therefore, does not invalidate
the general conclusions at which our survey of the subject
points. The universe, so far as we can see it, is a bounded
whole. It is surrounded by an immense girdle of stars,
which, to our vision, appears as the Milky Way. While we
cannot set exact limits to its distance, we may yet con-
fidently say that it is bounded. It has uniformities running
through its vast extent. Could we fly out to distances equal
to that of the Milky Way, we should find comparatively few
stars beyond the limits of that girdle. It is true that we
cannot set any definite limit and say that beyond this nothing
exists. What we can say is that the region containing
the visible stars has some approximation to a boundary. We
may fairly anticipate that each successive generation of
astronomers, through coming centuries, will obtain a little
more light on the subject — will be enabled to make more
definite the boundaries of our system of stars, and to draw
more and more probable conclusions as to the existence
or non-existence of any object outside of it. The wise in-
vestigator of to-day will leave to them the task of putting
the problem into a more positive shape.
GEOGRAPHICAL EVOLUTION
BY
SIR ARCHIBALD GEIKIE
INTRODUCTORY NOTE
Sir Archibald Geikie was born in Edinburgh, Scotland, in
i835, an & educated at the university in that city. He became
a member of the Geological Survey of Scotland, of which, in
1867, he was made director; and from 1871 to 1882 was Mur-
chison professor of geology and mineralogy in his own univer-
sity. At the latter date he resigned to take up the general
directorship of the Geological Survey of the United Kingdom,
which he held till 1901. He has been an extremely productive
investigator, and his achievements have brought him many dis-
tinctions from learned societies at home and abroad, including
the honor of knighthood, the presidency of the British Associ-
ation, and the secretaryship of the Royal Society, an office
which he still holds.
Among his more important writings may be mentioned "The
Scenery of Scotland, Viewed in Connection With its Physical
Geography" (1865), "Text-book of Geology" (1882), "Ancient
Volcanoes of Britain" (1897), "Types of Scenery and Their
Influence on Literature" (1898), and (t Landscape in History"
(1905).
The following paper on "Geographical Evolution," published
among his "Geological Sketches at Home and Abroad" (1882),
might be entitled with less ambiguity "Geological Evolution,"
since it is in this sense rather than in the broader modern
signification that the word "geographical" is employed through-
out the essay.
The interest in literature indicated by the names of several
of Geikie 's books has not been without influence on his writings,
for there are few scientific authors to-day who can command
so fascinating a style. His power of exposition is shown by
the skill with which, in the present paper, he maps out in com-
paratively few pages, the views of his school as to the geo-
logical history of the earth.
338
GEOGRAPHICAL EVOLUTION'
IN the quaint preface to his Navigations and Voyages
of the English Nation, Hakluyt calls geography and chro-
nology " the sunne and moone, the right eye and the left
of all history." The position thus claimed for geography
three hundred years ago by the great English chronicler was
not accorded by his successors, and has hardly been admitted
even now. The functions of the geographer and the travel-
ler, popularly assumed to be identical, have been supposed
to consist in descriptions of foreign countries, their climate,
productions, and inhabitants, bristling on the one hand with
dry statistics, and relieved on the other by as copious an
introduction as may be of stirring adventure and per-
sonal anecdote. There has indeed been much to justify this
popular assumption. It was not until the key-note of its
future progress was struck by Karl Ritter within the present
century that geography advanced beyond the domain of
travellers' tales and desultory observation into that of
orderly, methodical, scientific progress. This branch of
inquiry, however, is now no longer the pursuit of mere
numerical statistics, nor the chronicle of marvellous and
often questionable adventures by flood and fell. It seeks
to present a luminous picture of the earth's surface, its
various forms of configuration, its continents, islands, and
oceans, its mountains, valleys, and plains, its rivers and
lakes, its climates, plants and animals. It thus endeavours
to produce a picture which shall not be one of mere topo-
graphical detail. It ever looks for a connection between
scattered facts., tries to ascertain the relations which subsist
between the different parts of the globe, their reactions
on each other and the function of each in the general econ-
1 A Lecture delivered at the Evening Meeting of the Royal Geographical
Society, 24th March, 1879.
339
340 GEIKIE
omy of the whole. Modern geography studies the dis-
tribution of vegetable and animal life over the earth's sur-
face, with the action and reaction between it and the sur-
rounding inorganic world. It traces how man, alike uncon-
sciously and knowingly, has changed the face of nature,
and how, on the other hand, the conditions of his geo-
graphical environment have moulded his own progress.
With these broad aims geography comes frankly for as-
sistance to many different branches of science. It does not,
however, claim in any measure to occupy their domain. It
brings to the consideration of their problems a central hu-
man interest in which these sciences are sometimes apt to
be deficient; for it demands first of all to know how the
problems to be solved bear upon the position and history
of man and of this marvellously-ordered world wherein he
finds himself undisputed lord. Geography freely borrows
from meteorology, physics, chemistry, geology, zoology, and
botany; but the debt is not all on one side. Save for the
impetus derived from geographical research, many of these
sciences would not be in their present advanced condition.
They gain in vast augmentation of facts, and may cheer-
fully lend their aid in correlating these for geographical
requirements.
In no respect does modern geography stand out more
prominently than in the increased precision and fulness of
its work. It has fitted out exploratory expeditions, and in
so doing has been careful to see them provided with the
instruments and apparatus necessary to enable them to con-
tribute accurate and definite results. It has guided and fos-
tered research, and has been eager to show a generous ap-
preciation of the labours of those by whom our knowledge of
the earth has been extended. Human courage and endurance
are not less enthusiastically applauded than they once were;
but they must be united to no common powers of obser-
vation before they will now raise a traveller to the highest
rank. When we read a volume of recent travel, while
warmly appreciating the spirit of adventure, fertility of
resource, presence of mind, and other moral qualities of its
author, we instinctively ask ourselves, as we close its pages,
what is the sum of its additions to our knowledge of the
GEOGRAPHICAL EVOLUTION 341
earth? From the geographical point of view—and it is to
this point alone that these remarks apply — we must rank
an explorer according to his success in widening our knowl-
edge and enlarging our views regarding the aspects of
nature.
The demands of modern geography are thus becoming
every year more exacting. It requires more training in its
explorers abroad, more knowledge on the part of its readers
at home. The days are drawing to a close when one can
gain undying geographical renown by struggling against man
and beast, fever and hunger and drought, across some
savage and previously unknown region, even though little can
be shown as the outcome of the journey. All honour to
the pioneers by whom this first exploratory work has been
so nobly done ! They will be succeeded by a race that will
find its laurels more difficult to win — a race from which
more will be expected, and which will need to make
up in the variety, amount, and value of its detail, what it
lacks in the freshness of first glimpses into new lands.
With no other science has geography become more in-
timately connected than with geology, and the connec-
tion is assuredly destined to become yet deeper and closer.
These two branches of human knowledge are, to use Hak-
luyt's phrase, " the sunne and moone, the right eye and
the left," of all fruitful inquiry into the character and his-
tory of the earth's surface. As it is impossible to under-
stand the genius and temperament of a people, its laws
and institutions, its manners and customs, its buildings and
its industries, unless we trace back the history of that people,
and mark the rise and effect of each varied influence by
which its progress has been moulded in past generations;
so it is clear that our knowledge of the aspect of a con-
tinent, its mountains and valleys, rivers and plains, and all
its surface-features, cannot be other than singularly feeble
and imperfect, unless we realise what has been the origin
of these features. The land has had a history, not less than
the human races that inhabit it.
One can hardly consider attentively the future progress
of geography without being convinced that in the wide
development yet in store for this branch of human inquiry,
342 GEIKIE
one of its main lines of advance must be in the direction
of what may be termed geographical evolution. The geog-
rapher will no longer be content to take continents and
islands, mountain chains and river valleys, table-lands and
plains, as initial or aboriginal outlines of the earth's surface.
He will insist on knowing what the geologist can tell him
regarding the growth of these outlines. He will try to
trace out the gradual evolution of a continent, and may
even construct maps to show its successive stages of de-
velopment. At the same time, he will seek information
regarding the history of the plants and animals of the
region, and may find much to reward his inquiry as to
the early migrations of the fauna and flora, including
those even of man himself. Thus his pictures of the living
world of to-day, as they become more detailed and accurate,
will include more and more distinctly a background of by-
gone geographical conditions, out of which, by continuous
sequence, the present conditions will be shown to have
arisen.
I propose this evening to sketch in mere outline the
aspects of one side of this evolutional geography. I wish
to examine, in the first place, the evidence whereby we
establish the fundamental fact that the present surface of any
country or continent is not that which it has always borne,
and the data by which we may trace backward the origin
of the land; and, in the second place, to consider, by way of
illustration, some of the more salient features in the gradual
growth of the framework of Europe.
The first of these two divisions of the subject deals with
general principles, and may be conveniently grouped into
two parts: ist, The Materials of the Land. 2d, The Build-
ing of the Land.
I. General Principles of Continental Evolution
i. — The Materials of the Land
Without attempting to enter into detailed treatment of
this branch of the subject, we may, for the immediate pur-
GEOGRAPHICAL EVOLUTION 343
pose in view, content ourselves with the broad, useful clas-
sification of the materials of the land into two great series
— Fragmental and Crystalline.
§ 1. Fragmental. — A very cursory examination of rocks
in almost any part of the world suffices to show that by
far the larger portion of them consists of compacted frag-
mentary materials. Shales, sandstones, and conglomerates,
in infinite variety of texture and colour, are piled above
each other to form the foundation of plains and the structure
of mountains. Each of these rocks is composed of distinct
particles, worn by air, rain, frost, springs, rivers, glaciers,
or the sea, from previously existing rocks. They are thus
derivative formations, and their source, as well as their
mode of origin, can be determined. Their component grains
are for the most part rounded, and bear evidence of having
been rolled about in water. Thus we easily and rapidly
reach a first and fundamental conclusion — that the substance
of the main part of the solid land has been originally laid
down and assorted under water.
The mere extent of the area covered by these water-
formed rocks would of itself suggest that they must have
been deposited in the sea. We cannot imagine rivers or
lakes of magnitude sufficient to have spread over the sites
of the present continents. The waters of the ocean, how-
ever, may easily be conceived to have rolled at different
times over all that is now dry land. The fragmental rocks
contain, indeed, within themselves proof that they were
mainly of marine, and not of lacustrine or fluviatile origin.
They have preserved in abundance the remains of foram-
inifera, corals, crinoids, mulluscs, annelides, crustaceans,
fishes, and other organisms of undoubtedly marine habitat,
which must have lived and died in the places where their
traces remain still visible.
But not only do these organisms occur scattered through
sedimentary rocks ; they actually themselves form thick
masses of mineral matter. The Carboniferous or Mountain
Limestone of Central England and Ireland, for example,
reaches a thickness of from 2000 to 3000 feet, and covers
thousands of square miles of surface. Yet it is almost en-
tirely composed of congregated stems and joints and plates
344 GEIKIE
of crinoids, with foraminifera, corals, bryozoans, brachio-
pods, lamellibranchs, gasteropods, fish-teeth, and other un-
equivocally marine organisms. It must have been for ages
the bottom of a clear sea, over which generation after
generation lived and died, until their accumulated remains
had gathered into a deep and compact sheet of rock. From
the internal evidence of the stratified formations we thus
confidently announce a second conclusion — that a great
portion of the solid land consists of materials which have
been laid down on the floor of the sea.
From these familiar and obvious deductions we may
proceed further to inquire under what conditions these
marine formations, spreading so widely over the land, were
formed. According to a popular belief, shared in perhaps
by not a few geologists, land and sea have been continually
changing places. It is supposed that while, on the one hand,
there is no part of a continent over which sea-waves may
not have rolled, so, on the other hand, there is no lonely
abyss of the ocean where a wide continent may not have
bloomed. That this notion rests upon a mistaken interpreta-
tion of the facts may be shown from an examination —
(i) of the rocks of the land, and (2) of the bottom of the
present ocean.
(1) Among the thickest masses of sedimentary rock —
those of the ancient palaeozoic systems — no features recur
more continually than alterations of different sediments, and
surfaces of rock covered with well-preserved ripple-marks,
trails and burrows of annelides, polygonal and irregular
desiccation marks, like the cracks at the bottom of a sun-
dried muddy pool. These phenomena unequivocally point
to shallow and even littoral waters. They occur from
bottom to top of deposits which reach a thickness of several
thousand feet. They can be interpreted only in one way,
viz., that their deposition began in shallow water ; that dur-
ing their formation the area of deposit gradually subsided
for thousands of feet; yet that the rate of accumulation of
sediment kept pace on the whole with this depression; and
hence, that the original shallow-water character of the de-
posits remained, even after the original sea-bottom had been
buried under a vast mass of sedimentary matter. Now, if
GEOGRAPHICAL EVOLUTION 345
this explanation be true, even for the enormously thick
and comparatively uniform systems of older geological
periods, the relatively thin and much more varied stratified
groups of later date can offer no difficulty. In short, the
more attentively the stratified rocks of the crust of the
earth are studied, the more striking becomes the absence
of any deposits among them which can legitimately be con-
sidered those of a deep sea. They have all been deposited
in comparatively shallow water.
The same conclusion may be arrived at from a consider-
ation of the circumstances under which the deposition must
have taken place. It is evident that the sedimentary rocks
of all ages have been derived from degradation of land.
The gravel, sand, and mud, of which they consist, existed
previously as part of mountains, hills, or plains. These
materials carried down to the sea would arrange themselves
there as they do still, the coarser portions nearest the shore,
the finer silt and mud farthest from it. From the earliest
geological times the great area of deposit has been, as it
still is, the marginal belt of sea-floor skirting the land. It
is there that nature has always strewn " the dust of conti-
nents to be. ,, The decay of old rocks has been unceasingly
in progress on the land, and the building up of new rocks
has been unintermittently going on underneath the adjoining
sea. The two phenomena are the complementary sides of
one process, which belongs to the terrestrial and shallow
oceanic parts of the earth's surface and not to the wide and
deep ocean basins.
(2) Recent explorations of the bottom of the deep sea
all over the world have brought additional light to this
question. No part of the results obtained by the Challenger
Expedition has a profounder interest for geologists and
geographers than the proof which they furnish that the
floor of the ocean basins has no real analogy among the
sedimentary formations that form most of the frame-
work of the land. We now know by actual dredging and
inspection that the ordinary sediment washed off the land
sinks to the sea-bottom before it reaches the deeper abysses,
and that, as a rule, only the finer particles are carried more
than a few score of miles from the shore. Instead of such
346 GEIKIE
sandy and pebbly material as we find so largely among the
sedimentary rocks of the land, wide tracts of the sea-
bottom at great depths are covered with various kinds of
organic ooze, composed sometimes of minute calcareous fo-
raminifera, sometimes of siliceous radiolaria or diatoms.
Over other areas vast sheets of clay extend, derived ap-
parently from the decomposition of volcanic detritus, of
which large quantities are floated away from volcanic
islands, and much of which may be produced by submarine
volcanoes. On the tracts farthest removed from any land
the sediment seems to settle scarcely so rapidly as the
dust that gathers over the floor of a deserted hall. Mr.
Murray of the Challenger staff, has described how from
these remote depths large numbers of shark's teeth and ear-
bones of whales were dredged up. We cannot suppose the
number of sharks and whales to be much greater in these
regions than in others where their relics were found much
less plentifully. The explanation of the abundance of their
remains was supplied by their varied condition of decay
and preservation. Some were comparatively fresh, others
had greatly decayed, and were incrusted with or even com-
pletely buried in a deposit of earthy manganese. Yet the
same cast of the dredge brought up these different stages
of decay from the same surface of the sea-floor. While
generation after generation of sea-creatures drops its bones
to the bottom, now here, now there, so exceedingly feeble
is the rate of deposit of sediment that they lie uncovered,
mayhap for centuries, so that the remains which sink to-day
may lie side by side with the mouldered and incrusted bones
that found their way to the bottom hundreds of years ago.
Another striking indication of the very slow rate at which
sedimentation takes place in these abysses has also been
brought to notice by Mr. Murray. In the clay from the
bottom he found numerous minute spherical granules of
native iron, which, as he suggests, are almost certainly of
meteoric origin — fragments of those falling stars which,
coming to us from planetary space, burst into fragments
when they rush into the denser layers of our atmosphere.
In tracts where the growth of silt upon the sea-floor is
excessively tardy, the fine particles, scattered by the dissipa-
GEOGRAPHICAL EVOLUTION 347
tion of these meteorites, may remain in appreciable quan-
tity. In this case, again, it is not needful to suppose that
meteorites have disappeared over these ocean depths more
numerously than over other parts of the earth's surface.
The iron granules have no doubt been as plentifully showered
down elsewhere, though they cannot be so readily detected in
accumulating sediment. I know no recent observation in
physical geography more calculated to impress deeply the
imagination than the testimony of this presumably meteoric
iron from the most distant abysses of the ocean. To be
told that mud gathers on the floor of these abysses at an
extremely slow rate conveys but a vague notion of the tardi-
ness of the process. But to learn that it gathers so slowly,
that the very star-dust which falls from outer space forms
an appreciable part of it, brings home to us, as hardly any-
thing else could do, the idea of undisturbed and excessively
slow accumulation.
From all this evidence we may legitimately conclude
that the present land of the globe, though formed in great
measure of marine formations, has never lain under the
deep sea; but that its site must always have been near
land. Even its thick marine limestones are the deposits
of comparatively shallow water. Whether or not any trace
of aboriginal land may now be discoverable, the characters of
the most unequivocally marine formations bear emphatic
testimony to this proximity of a terrestrial surface. The
present continental ridges have probably always existed in
some form, and as a corollary we may infer that the pres-
ent deep ocean basins likewise date from the remotest
geological antiquity.
§ 2. Crystalline. — While the greater part of the frame-
work of the land has been slowly built up of sedimentary
materials, it is abundantly varied by the occurrence of
crystalline masses, many of which have been injected in
a molten condition into rents underground, or have been
poured out in lava-streams at the surface.
Without entering at all into geological detail, it will be
enough for the present purpose to recognise the characters
and origin of two great types of crystalline material which
have been called respectively the Eruptive and Metamorphic.
348 GEIKIE
(a) Eruptive. — As the name denotes, eruptive or igneous
rocks have been ejected from the heated interior of the
earth. In a modern volcano lava ascends the central fun-
nel, and issuing from the lip of the crater or from lateral
fissures pours down the slopes of the cone in sheets of
melted rock. The upper surface of the lava column within
the volcano is kept in constant ebullition by the rise of
steam through its mass. Every now and then a vast body
of steam rushes out with a terrific explosion, scattering the
melted lava into impalpable dust, and filling the air with
ashes and stones, which descend in showers upon the sur-
rounding country. At the surface, therefore, igneous rocks
appear, partly as masses of congealed lava, and partly as
more or less consolidated sheets of dust and stones. But
beneath the surface there must be a downward prolongation
of the lava column, which no doubt sends out veins into
rents of the subterranean rocks. We can suppose that the
general aspect of the lava which consolidates at some depth
will differ from that which solidifies above ground.
As a result of the revolutions which the crust of the
earth has undergone, the roots of many ancient volcanoes
have been laid bare. We have been, as it were, admitted
into the secrets of these subterranean laboratories of nature,
and have learned much regarding the mechanism of vol-
canic action which we could never have discovered from
any modern volcano. Thus, while on the one hand we meet
with beds of lava and consolidated volcanic ashes which
were undoubtedly erupted at the surface of the ground in
ancient periods, and were subsequently buried deep beneath
sedimentary accumulations now removed, on the other hand
we find masses of igneous rock which certainly never came
near the surface, but must have been arrested in their
ascent from below while still at a great depth, and have
been laid bare to the light after the removal of the pile of
rock under which they originally lay.
By noting these and other characters, geologists have
learnt that, besides the regions of still active volcanoes, there
are few large areas of the earth's surface where proofs of
former volcanic action or of the protrusion of igneous rocks
may not be found. The crust of the earth, crumpled and
GEOGRAPHICAL EVOLUTION 349
fissured, has been, so to speak, perforated and cemented
together by molten matter driven up from below.
(b) Metamorphic— The sedimentary rocks of the land
have undergone many changes since their formation, some
of which are still far from being satisfactorily accounted
for. One of these changes is expressed by the term
Metamorphism, and the rocks which have undergone this
process are called Metatnorphic. It seems to have taken
place under widely varied conditions, being sometimes con-
fined to small local tracts, at other times extending across
a large portion of the continent. It consists in the rear-
rangement of the component materials of rocks, and nota-
bly in their recrystallisation along particular lines or laminae.
It is usually associated with evidence of great pressure; the
rocks in which it occurs having been corrugated and crum-
pled, not only in vast folds, which extend across whole
mountains, but even in such minute puckerings as can only
be observed with the microscope. It shows itself more par-
ticularly among the older geological formations, or those
which have been once deeply buried under more recent
masses of rock, and have been exposed as the result of the
removal of these overlying accumulations. The original
characters of the sandstones, shales, grits, conglomerates,
and limestones, of which, no doubt, these metamorphic
masses once consisted, have been more or less effaced, and
have given place to that peculiar crystalline laminated
or foliated structure so distinctively a result of meta-
morphism.
An attentive examination of a metamorphic region shows
that here and there the alteration and recrystallisation have
proceeded so far that the rocks graduate into granites and
other so-called igneous rocks. A series of specimens may
be collected showing unaltered or at least quite recognisable
sedimentary rocks at the one end, and thoroughly crystalline
igneous rocks at the other. Thus the remarkable fact is
brought home to the mind that ordinary sandstones, shales,
and other sedimentary materials may in the course of ages
be converted by underground changes into crystalline
granite. The framework of the land, besides being knit
together by masses of igneous rock intruded from below,
350 GEIKIE
has been strengthened by the welding and crystallisation of
its lowest rocks. It is these rocks which rise along the
central crests of mountain chains, where, after the lapse of
ages, they have been uncovered and laid bare, to be bleached
and shattered by frost and storm.
ii. — The Architecture of the Land.
Let us now proceed to consider how these materials, sedi-
mentary and crystalline, have been put together, so as to
constitute the solid land of the globe.
It requires but a cursory examination to observe that the
sedimentary masses have not been huddled together at ran-
dom; that, on the contrary, they have been laid down in
sheets one over the other. An arrangement of this kind at
once betokens a chronological sequence. The rocks cannot
all have been formed simultaneously. Those at the bottom
must have been laid down before those at the top. A truism
of this kind seems hardly to require formal statement. Yet
it lies at the very foundation of any attempt to trace the
geological history of a country. Did the rocks everywhere
lie undisturbed one above another as they were originally
laid down, their clear order of succession would carry with
it its own evident interpretation. But such have been the
changes that have arisen, partly from the operation of forces
from below, partly from that of forces acting on the surface,
that the true order of a series of rocks is not always so easily
determined. By starting, however, from where the succes-
sion is normal and unbroken, the geologist can advance with
confidence into regions where it has been completely inter-
rupted; where the rocks have been shattered, crumpled, and
even inverted.
The clue which guides us through these labyrinths is a
very simple one. It is afforded by the remains of once living
plants and animals which have been preserved in the rocky
framework of the land. Each well-marked series of sedimen-
tary accumulations contains its own characteristic plants, cor-
als, crustaceans, shells, fishes, or other organic remains. By
these it can be identified and traced from country to country
across a whole continent. When, therefore, the true order
of superposition of the rocks has been ascertained by ob-
GEOGRAPHICAL EVOLUTION 351
serving how they lie upon each other, the succession of their
fossils is at the same time fixed. In this way the sedi-
mentary part of the earth's crust has been classified into
different formations, each characterised by its distinct
assemblage of organic remains. In the most recent forma-
tions, most of these remains are identical with still living
species of plants and animals; but as we descend in the
series and come into progressively older deposits the pro-
portion of existing species diminishes until at last all the
species of fossils are found to be extinct. Still lower and
older rocks reveal types and assemblages of organisms
which depart farther and farther from the existing order.
By noting the fossil contents of a formation, therefore,
even in a district where the rocks have been so disturbed
that their sequence is otherwise untraceable, the geologist
can confidently assign their relative position to each of the
fractured masses. He knows, for instance, using for our
present purpose the letters of the alphabet to denote the
sequence of the formations, that a mass of limestone con-
taining fossils typical of the formation B must be younger
than another mass of rock containing the fossils of A. A
series of strata full of the fossils of H resting immediately
on others charged with those of C, must evidently be sepa-
rated from these by a great gap, elsewhere filled in by the
intervening formations D, E, F, G. Nay, should the rocks
in the upper part of a mountain be replete with the fossils
proper to D, while those in the lower slopes showed only
the fossils of E, F, and G, it could be demonstrated that
the materials of the mountain had actually been turned
upside down, for, as proved by its organic remains, the
oldest and therefore lowest formation had come to lie at
the top, and the youngest, and therefore highest, at the
bottom.
Of absolute chronology in such questions science can as
yet give no measure. How many millions of years each
formation may have required for its production, and how
far back in time may be the era of any given group of
fossils, are problems to which no answer, other than a mere
guess, can be returned. But this is a matter of far less
moment than the relative chronology, which can usually be
352 GEIKIE
accurately fixed for each country, and on which all attempts
to trace back the history of the land must be based.
While, then, it is true that most of the materials of the
solid land have been laid down at successive periods under
the sea, and that the relative dates of their deposition can be
determined, it is no less certain that the formation of these
materials has not proceeded uninterruptedly, and that they
have not finally been raised into land by a single movement.
The mere fact that they are of marine origin shows, of
course, that the land owes its origin to some kind of terres-
trial disturbance. But when the sedimentary formations are
examined in detail, they present a most wonderful chronicle
of long-continued, oft-repeated, and exceedingly complex
movements of the crust of the globe. They show that the
history of every country has been long and eventful; that,
in short, hardly any portion of the land has reached its
present condition, save after a protracted series of geo-
logical revolutions.
One of the most obvious and not the least striking features
in the architecture of the land is the frequency with which
the rocks, though originally horizontal, or approximately so,
have been tilted up at various angles, or even placed on end.
At first it might be supposed that these disturbed positions
have been assumed at random, according to the capricious
operations of subterranean forces. They seem to follow no
order, and to defy any attempt to reduce them to system.
Yet a closer scrutiny serves to establish a real connection
among them. They are found, for the most part, to belong
to great, though fractured, curves, into which the crust of
the earth has been folded. In low countries far removed
from any great mountain range, the rocks often present
scarcely a trace of disturbance, or if they have been
affected, it is chiefly by having been thrown into gentle
undulations. As we approach the higher grounds, how-
ever, they manifest increasing signs of commotion. Their
undulations become more frequent and steeper, until, en-
tering within the mountain region, we find the rocks curved,
crumpled, fractured, inverted, tossed over each other into
yawning gulf and towering crest, like billows arrested at
the height of a furious storm.
GEOGRAPHICAL EVOLUTION 353
Yet even in the midst of such apparent chaos it is not
impossible to trace the fundamental law and order by which
it is underlaid. The prime fact to be noted is the universal
plication and crumpling of rocks which were at first nearly
horizontal. From the gentle undulations of the strata
beneath the plains to their violent contortion and inversion
among the mountains, there is that insensible gradation
which connects the whole of these disturbances as parts of
one common process. They cannot be accounted for by
any mere local movements, though such movements no
doubt took place abundantly. The existence of a mountain
chain is not to be explained by a special upheaval or series
of upheavals caused by an expansive force acting from
below. Manifestly the elevation is only one phase of a vast
terrestrial movement which has extended over whole con-
tinents, and has affected plains as well as high grounds.
The only cause which, so far as our present knowledge
goes, could have produced such widespread changes is a
general contraction of the earth's mass. There can be no
doubt that at one time our planet existed in a gaseous,
then in a liquid condition. Since these early periods it has
continued to lose heat, and consequently to contract and to
grow more and more solid, until, as the physicists insist, it
has now become practically as rigid as a globe of glass or
of steel. But in the course of the contraction, after the
solid external crust was formed, the inner hot nucleus has
lost heat more rapidly than the crust, and has tended to
shrink inward from it. As a consequence of this internal
movement, the outer solid shell has sunk down upon the
retreating nucleus. In so doing, it has of course had to
accommodate itself to a diminished area, and this it could
only accomplish by undergoing plication and fracture.
Though the analogy is not a very exact one, we may liken
our globe to a shrivelled apple. The skin of the apple does
not contract equally. As the internal moisture passes off,
and the bulk of the fruit is reduced, the once smooth ex-
terior becomes here and there corrugated and dimpled.
Without entering into this difficult problem in physical
geology, it may suffice if we carry with us the idea that
our globe must once have had a greater diameter than it
hc vol. xxx — L
354 GEIKIE
now possesses, and that the crumpling of its outer layers,
whether due to mere contraction or, as has been suggested,
to the escape also of subterranean vapours, affords evidence
of this diminution. A little reflection suffices to show us
that, even without any knowledge of the actual history of
the' contraction, we might anticipate that the effects would
neither be continuous nor everywhere uniform. The solid
crust would not, we may be sure, subside as fast as the
mass inside. It would, for a time at least, cohere and sup-
port itself, until at last, gravitation proving too much for
its strength, it would sink down. And the areas and
amount of descent would be greatly regulated by the vary-
ing thickness and structure of the crust. Subsidence would
not take place everywhere; for, as a consequence of the
narrower space into which the crust sank, some regions
would necessarily be pushed up. These conditions appear
to have been fulfilled in the past history of the earth. There
is evidence that the terrestrial disturbance has been renewed
again and again, after long pauses, and that, while the
ocean basins have on the whole been the great areas of
depression, the continents have been the lines of uprise or
relief, where the rocks were crumpled and pushed out of
the way. Paradoxical, therefore, as the statement may
appear, it is nevertheless strictly true, that the solid land,
considered with reference to the earth's surface as a whole,
is the consequence of subsidence rather than of upheaval.
Grasping, then, this conception of the real character of
the movements to which the earth owes its present surface
configuration, we are furnished with fresh light for explor-
ing the ancient history and growth of the solid land. The
great continental ridges seem to lie nearly on the site of
the earliest lines of relief from the strain of contraction.
They were forced up between the subsiding oceanic basins
at a very early period of geological history. In each suc-
ceeding epoch of movement they were naturally used over
again, and received an additional push upward. Hence
we see the meaning of the evidence supplied by the sedi-
mentary rocks as to shallow seas and proximity of land.
These rocks could not have been otherwise produced. They
were derived from the waste of the land, and were depos-
GEOGRAPHICAL EVOLUTION 355
ited near the land. For it must be borne in mind that
every mass of land as soon as it appeared above water was
at once attacked by the ceaseless erosion of moving water
and atmospheric influences, and immediately began to fur-
nish materials for the construction of future lands to be
afterwards raised out of the sea.
Each great period of contraction elevated anew the much-
worn land, and at the same time brought the consolidated
marine sediments above water as parts of a new terrestrial
surface. Again a long interval would ensue, marked per-
haps by a slow subsidence both of the land and sea-bottom.
Meanwhile the surface of the land was channelled and
lowered, and its detritus was spread over the sea-floor, until
another era of disturbance raised it once more with a por-
tion of the surrounding ocean-bed. These successive up-
ward and downward movements explain why the sedi-
mentary formations do not occur as a continuous series, but
often lie each upon the upturned and worn edges of its
predecessors.
Returning now to the chronological sequence indicated
by the organic remains preserved among the sedimentary
rocks, we see how it may be possible to determine the
relative order of the successive upheavals of a continent.
If, for example, a group of rocks, which as before may be
called A, were found to have been upturned and covered
over by undisturbed beds C, the disturbance could be
affirmed to have occurred at some part of the epoch repre-
sented elsewhere by the missing series B. If, again, the
group C were observed to have been subsequently tilted, and
to pass under gently-inclined or horizontal strata E, a
second period of disturbance would be proved to have
occurred between the time of C and E.
I have referred to the unceasing destruction of its sur-
face which the land undergoes from the time when it
emerges out of the sea. As a rule, our conceptions of the
rate of this degradation are exceedingly vague. Yet they
may easily be made more definite by a consideration of
present changes on the surface of the land. Every river
carries yearly to the sea an immense amount of sand and
mud. But this amount is capable of measurement. It
356 GEIKIE
represents, of course, the extent to which the general level
of the surface of the river's drainage basin is annually
lowered. According to such measurements and computa-
tions as have been already made, it appears that some-
where about ^ff of a foot is every year removed from
the surface of its drainage basin by a large river. This
seems a small fraction, yet by the power of mere addition
it soon mounts up to a large total. Taking the mean level
of Europe to be 600 feet, its surface, if everywhere worn
away at what seems to be the present mean normal rate,
would be entirely reduced to the sea-level in little more than
three and a half millions of years.
But of course the waste is not uniform over the whole
surface. It is greatest on the slopes and valleys, least on
the more level grounds. A few years ago, in making some
estimates of the ratios between the rates of waste on these
areas, I assumed that the tracts of more rapid erosion
occupy only one-ninth of the whole surface affected, and
that in these the rate of destruction is nine times greater
than on the more level spaces. Taking these proportions,
and granting that ^V? of a foot is the actual ascertained
amount of loss from the whole surface, we learn by a
simple arithmetical process that xV of an inch is carried
away from the plains and tablelands in seventy-five years,
while the same amount is worn out of the valleys in eight
and a half years. One foot must be removed from the
former in 10,800 years, and from the latter in 1200 years.
Hence, at the present rate of erosion, a valley 1000 feet
deep may be excavated in 1,200,000 years — by no means a
very long period in the conception of most geologists.
I do not offer these figures as more than tentative results.
They are based, however, not on mere guesses, but on data
which, though they may be corrected by subsequent inquiry,
are the best at present available, and are probably not far
from the truth. They are of value in enabling us more
vividly to realise how the prodigious waste of the land,
proved by the existence of such enormous masses of sedi-
mentary rock, went quietly on age after age, until results
were achieved which seem at first scarcely possible to so
slow and gentle an agency.
GEOGRAPHICAL EVOLUTION 357
It is during this quiet process of decay and removal that
all the distinctive minor features of the land are wrought
out. When first elevated from the sea, the land doubtless
presents on the whole a comparatively featureless surface.
It may be likened to a block of marble raised out of the
quarry — rough and rude in outline, massive in solidity and
strength, but giving no indication of the grace into which
it will grow under the hand of the sculptor. What art
effects upon the marble block, nature accomplishes upon
the surface of the land. Her tools are many and varied —
air, frost, rain, springs, torrents, rivers, avalanches,
glaciers, and the sea — each producing its own characteristic
traces in the sculpture. With these implements, out of the
huge bulk of the land she cuts the valleys and ravines,
scoops the lake-basins, hews with bold hand the colossal
outlines of the mountains, carves out peak and crag, crest
and cliff, chisels the courses of the torrents, splinters the
sides of the precipices, spreads out the alluvium of the
rivers, and piles up the moraines of the glaciers. Patiently
and unceasingly has this great earth-sculptor sat at her
task since the land first rose above the sea, washing down
into the ocean the debris of her labour, to form the ma-
terials for the framework of future countries; and there
will she remain at work so long as mountains stand, and
rain falls, and rivers flow.
II. The Growth of the European Continent.
Passing now from the general principles with which we
have hitherto been dealing, we may seek an illustration of
their application to the actual history of a large mass of
land. For this purpose let me ask your attention to some
of the more salient features in the gradual growth of
Europe. This continent has not the simplicity of structure
elsewhere recognisable; but without entering into detail
or following a continuous sequence of events, our present
purpose will be served by a few broad outlines of the con-
dition of the European area at successive geological periods.
It is the fate of continents, no less than of the human
communities that inhabit them, to have their first origin
358 GEIKIE
shrouded in obscurity. When the curtain of darkness
begins to rise from our primeval Europe, it reveals to us
a scene marvellously unlike that of the existing continent.
The land then lay chiefly to the north and north-west,
probably extending as far as the edge of the great sub-
marine plateau by which the European ridge is prolonged
under the Atlantic for 230 miles to the west of Ireland.
Worn fragments of that land exist in Finland, Scandinavia,
and the north-west of Scotland, and there are traces of
what seem to have been some detached islands in Central
Europe, notably in Bohemia and Bavaria. Its original
height and extent can of course never be known; but some
idea of them may be formed by considering the bulk of
solid rock which was formed out of the waste of that land.
I find that if we take merely one portion of the detritus
washed from its surface and laid down in the sea — viz.
that which is comprised in what is termed the Silurian
system — and if we assume that it spreads over 60,000 square
miles of Britain with an average thickness of 16,000 feet,
or 3 miles, which is probably under the truth, then we
obtain the enormous mass of 180,000 cubic miles. The
magnitude of this pile of material may be better realised if we
reflect that it would form a mountain ridge three times as
long as the Alps, or from the North Cape to Marseilles
(1800), with a breadth of more than 33 miles, and an
average height of 16,000 feet — that is, higher than the sum-
mit of Mont Blanc. All this vast pile of sedimentary rock
was worn from the slopes and shores of the primeval
northern land. Yet it represents but a small fraction of
the material so removed, for the sea of that ancient time
spread over nearly the whole of Europe eastwards into
Asia, and everywhere received a tribute of sand and mud
from the adjoining shores.
There is perhaps no mass of rock so striking in its
general aspect as that of which this northern embryo of
Europe consisted. It lacks the variety of composition,
structure, colour, and form, which distinguishes rocks of
more modern growth; but in dignity of massive strength it
stands altogether unrivalled. From the headlands of the
Hebrides to the far fjords of Arctic Norway it rises up
GEOGRAPHICAL EVOLUTION 359
grim and defiant of the elements. Its veins of quartz,
felspar, and hornblende project from every boss and crag
like the twisted and knotted sinews of a magnificent torso.
Well does the old gneiss of the north deserve to have been
made the foundation-stone of a continent.
What was the character of the vegetation that clothed
this earliest prototype of Europe is a question to which at
present no definite answer is possible. We know, however,
that the shallow sea which spread from the Atlantic south-
ward and eastward over most of Europe was tenanted by
an abundant and characteristic series of invertebrate ani-
mals — trilobites, graptolites, cystideans, brachiopods, and
cephalopods, strangely unlike, on the whole, to anything
living in our waters now, but which then migrated freely
along the shores of the Arctic land between what are now
America and Europe.
The floor of this shallow sea continued to sink, until
over Britain, at least, it had gone down several miles. Yet
the water remained shallow because the amount of sedi-
ment constantly poured into it from the north-west filled it
up about as fast as the bottom subsided. This slow subter-
ranean movement was varied by uprisings here and there,
and notably by the outburst at successive periods of a great
group of active submarine volcanoes over Wales, the Lake
district, and the south of Ireland; but at the close of the
Silurian period a vast series of disturbances took place, as
the consequence of which the first rough outlines of the
European continent were blocked out. The floor of the
sea was raised into long ridges of land, among which were
some on the site of the Alps, the Spanish peninsula, and
the hills of the west and north of Britain. The thick mass
of marine sediment was crumpled up, and here and there
even converted into hard crystalline rock. Large enclosed
basins, gradually cut off from the sea, like the modern
Caspian and Sea of Aral, extended from beyond the west
of Ireland across to Scandinavia and even into the west of
Russia. These lakes abounded in bone-covered fishes of
strange and now long-extinct types, while the land around
was clothed with a club-moss and reed-like vegetation —
Psilophyton, Sigillaria, Catamite, etc. — the oldest terrestrial
360 GEIKIE
flora of which any abundant records have yet been found
in Europe. The sea, dotted with numerous islands, appears
to have covered most of the heart of the continent.
A curious fact deserves to be noticed here. During the
convulsions by which the sediments of the Silurian sea-floor
were crumpled up, crystallised, and elevated into land, the
area of Russia seems to have remained nearly unaffected.
Not only so, but the same immunity from violent disturb-
ance has prevailed over that vast territory during all sub-
sequent geological periods. The Ural Mountains on the
east have again and again served as a line of relief, and
have been from time to time ridged up anew. The German
domains on the west have likewise suffered extreme con-
vulsion. But the wide intervening plateau of Russia has
apparently always maintained its flatness either as sea-
bottom or as terrestrial plains. As I have already remarked,
there has been a remarkable persistence alike in exposure
to and immunity from terrestrial disturbance. Areas that
lay along lines of weakness have suffered repeatedly in
successive geological revolutions, while tracts outside of
these regions of convulsion have simply moved gently up
or down without material plication or fracture.
By the time of the coal growths, the aspect of the
European area had still further changed. It then consisted
of a series of low ridges or islands in the midst of a shallow
sea or of wide salt-water lagoons. A group of islands
occupied the site of some of the existing high grounds of
Britain. A long, irregular ridge ran across what is now
France from Brittany to the Mediterranean. The Spanish
peninsula stood as a detached island. The future Alps
rose as a long, low ridge, to the north of the eastern edge
of which lay another insular space, where now we find
the high grounds of Bavaria and Bohemia. The shallow
waters that wound among these scattered patches of land
were gradually silted up. Many of them became marshes,
crowded with a most luxuriant cryptogamic vegetation,
specially of lycopods and ferns, while the dry grounds
waved green with coniferous trees. By a slow intermittent
subsidence, islet after islet sank beneath the verdant
swamps. Each fresh depression submerged the rank
GEOGRAPHICAL EVOLUTION 361
jungles and buried them under sand and mud, where they
were eventually compressed into coal. To this united
co-operation of dense vegetable growth, accumulation of
sediment, and slow subterranean movement, Europe owes
her coal-fields.
All this time the chief area of high ground in Europe
appears still to have lain to the north and north-west. The
old gnarled gneiss of that region, though constantly worn
down and furnishing materials towards each new formation,
yet rose up as land. It no doubt received successive eleva-
tions during the periods of disturbance, which more or less
compensated for the constant loss from its surface.
The next scene we may contemplate brings before us a
series of salt lakes, covering the centre of the continent
from the north of Ireland to the heart of Poland. These
basins were formed by the gradual cutting off of portions
of the sea which had spread over the region. Their waters
were red and bitter, and singularly unfavourable to life.
On the low intervening ridges a coniferous and cycadaceous
vegetation grew, sometimes in quantity sufficient to supply
materials for the formation of coal-seams. The largest of
these salt lakes stretched from the edge of the old plateau
of Central France along the base of the Alpine ridge to the
high grounds of Bohemia, and included the basin of the
Rhine from Bale down to the ridge beyond Mayence, which
has been subsequently cut through by the river into the
picturesque gorge between Bingen and the Siebengebirge.
This lake was filled up with red sand and mud, limestone,
and beds of rock salt. Where the eastern Alps now rise
the opener waters were the scene of a long-continued
growth of dolomite, out of which in later ages the famous
dolomite mountains of the Tyrol were carved.
These salt lakes of the Triassic period seem to have
been everywhere quietly effaced by a widespread depres-
sion, which allowed the water of the main ocean once more
to overspread the greater part of Europe. This slow sub-
sidence went on so long as to admit of the accumulation of
masses of limestone, shale, and sandstone, several thousand
feet in thickness, and probably to bring most of the insular
tracts of Central Europe under water. To this period,
362 GEIKIE
termed by geologists the Jurassic, we can trace back the
origin of a large part of the rock now forming the surface
of the continent, from the low plains of Central England
up to the crests of the northern Alps, while in the Mediter-
ranean basin, rocks of the same age cover a large area of
the plateau of Spain, and form the central mass of the
chain of the Apennines. It is interesting to know that the
north-west of Britain continued still to rise as land in spite
of all the geographical changes which had taken place to
the south and east. We can trace even yet the shores of
the Jurassic sea along the skirts of the mountains of Skye
and Ross-shire.
The next long era, termed the Cretaceous, was likewise
more remarkable for slow accumulation of rock under the
sea than for the formation of new land. During that time
the Atlantic sent its waters across the whole of Europe and
into Asia. But they were probably nowhere more than a
few hundred feet deep over the site of our continent, even
at their deepest part. Upon their bottom there gathered a
vast mass of calcareous mud, composed in great part of
foraminifera, corals, echinoderms, and molluscs. Our En-
glish chalk which ranges across the north of France, Bel-
gium, Denmark, and the North of Germany, represents a
portion of the deposits of that sea-floor, probably accu-
mulated in a northern, somewhat isolated basin, while the
massive hippurite limestone of Southern Europe represents
the deposits of the opener ocean. Some of the island
spaces which had remained for a vast period above water,
and had by their degradation supplied materials for the
sediment of successive geological formations, now went
down beneath the Cretaceous sea. The ancient high-
grounds of Bohemia, the Alps, the Pyrenees, and the
Spanish tableland were either entirely submerged, or at
least had their area very considerably reduced. The sub-
mergence likewise affected the north-west of Britain; the
western highlands of Scotland lay more than iooo feet
below their present level.
When we turn to the succeeding geological period, that
of the Eocene, the proofs of widespread submergence are
still more striking. A large part of the Old World seems
GEOGRAPHICAL EVOLUTION 363
to have sunk down ; for we find that one wide sea extended
across the whole of Central Europe and Asia. It was at
the close of this period of extreme depression that those
subterranean movements began to which the present con-
figuration of Europe is mainly due. The Pyrenees, Alps,
Apennines, Carpathians, the Caucasus, and the heights of
Asia Minor mark, as it were, the crests of the vast earth-
waves into which the solid framework of Europe was then
thrown. So enormous was the contortion that, as may be
seen along the northern Alps, the rocks for thousands of
feet were completely inverted, this inversion being accom-
panied by the most colossal folding and twisting. The mas-
sive sedimentary formations were crumpled up, and doubled
over each other, as we might fold a pile of cloth. In the
midst of these commotions the west of Europe remained
undisturbed. It is strange to reflect that the soft clays and
sands under London are as old as some of the hardened
rocks which have been upheaved into such picturesque
peaks along the northern flanks of the Alps.
After the completion of these vast terrestrial disturb-
ances, the outlines of Europe began distinctly to shape
themselves into their present form. The Alps rose as a
great mountain range, flanked on the north by a vast lake
which covered all the present lowlands of Switzerland, and
stretched northward across a part of the Jura Mountains,
and eastward into Germany. The size of this fresh-water
basin may be inferred from the fact that one portion only
of the sand and gravel that accumulated in it even now
measures 6000 feet in thickness. The surrounding land
was densely clothed with a vegetation indicative of a much
warmer climate than Europe now can boast. Palms of
American types, as well as date palms, huge Californian
pines (Sequoia), laurels, cypresses, and evergreen oaks,
with many other evergreen trees, gave a distinctive charac-
ter to the vegetation. Among the trees too were planes,
poplars, maples, willows, oaks, and other ancestors of our
living woods and forests; numerous ferns grew in the
underwood, while clematis and vine wound themselves
among the branches. The waters were haunted by huge
pachyderms, such as the dinotherium and hippopotamus;
364 GEIKIE
while the rhinoceros and mastodon roamed through the
woodlands.
A marked feature of this period in Europe was the
abundance and activity of its volcanoes. In Hungary,
Rhineland, and Central France, numerous vents opened
and poured out their streams of lava and showers of ashes.
From the south of Antrim, also, through the west coast
of Scotland, the Faroe Islands, and Iceland, even far
into Arctic Greenland, a vast series of fissure-eruptions
poured forth successive floods of basalt, fragments of
which now form the extensive volcanic plateaux of these
regions.
The mild climate indicated by the vegetation in the
deposits of the Swiss lake prevailed even into Polar lati-
tudes, for the remains of numerous evergreen shrubs, oaks,
maples, walnuts, hazels, and many other trees have been
found in the far north of Greenland, and even within 8° 15'
of the pole. The sea still occupied much of the lowlands
of Europe. Thus it ran as a strait between the Bay of
Biscay and the Mediterranean, cutting off the Pyrenees
and Spain from the rest of the continent. It swept round
the north of France, covering the rich fields of Touraine
and the wide flats of the Netherlands. It rolled far up
the plains of the Danube, and stretched thence eastward
across the south of Russia into Asia.
By this time some of the species of shells which still
people the European seas had appeared. So long have
they been natives of our area that they have witnessed the
rise of a great part of the continent. Some of the most
stupendous changes which they have seen have taken place
in the basin of the Mediterranean, where, at a comparatively
recent geological period, parts of the sea-floor were up-
heaved to a height of 3000 feet. It was then that the
breadth of the Italian peninsula was increased by the belt
of lower hills that flanks the range of the Apennines. Then,
too, Vesuvius and Etna began their eruptions. Among
these later geographical events also we must place the
gradual isolation of the Sea of Aral, the Caspian, and the
Black Sea from the rest of the ocean, which is believed to
have once spread from the Arctic regions down the west
GEOGRAPHICAL EVOLUTION 365
of Asia, along the base of the Ural Mountains into the
south-east of Europe.
The last scene in this long history is one of the most
unexpected of aK. Europe, having nearly its present height
and outlines, is found swathed deep in snow and ice.
Scandinavia and Finland are one vast sheet of ice, that
creeps down from the watershed into the Atlantic on the
one side, and into the basin of the Baltic on the other.
All the high grounds of Britain are similarly buried. The
bed of the North Sea as well as of the Baltic is in great
measure choked with ice. The Alps, the Pyrenees, the
Carpathians, and the Caucasus send down vast glaciers into
the plains at their base. Northern plants find their way
south even to the Pyrenees, while the reindeer, musk-ox,
lemming, and their Arctic companions, roam far and wide
over France.
As a result of the prolonged passage of solid masses of
ice over them, the rocks on the surface of the continent,
when once more laid bare to the sun, present a worn, flow-
ing outline. They have been hollowed into basins, ground
smooth, and polished. Long mounds and wide sheets of
clay, gravel, and sand have been left over the low grounds,
and the hollows between them are filled with innumerable
tarns and lakes. Crowds of boulders have been perched on
the sides of the hills and dropped over the plains. With the
advent of a milder temperature the Arctic vegetation gradu-
ally disappeared from the plains. Driven up step by step
before the advancing flora from more genial climates, it
retired into the mountains, and there to this day continues
to maintain itself. The present Alpine flora of the Pyrenees,
the Alps, Britain, and Scandinavia, is thus a living record
of the ice-age. The reindeer and his friends have long
since been forced to return to their northern homes.
After this long succession of physical revolutions, man
appears as a denizen of the Europe thus prepared for him.
The earliest records of his presence reveal him as a fisher
and hunter, with rude flint-pointed spear and harpoon. And
doubtless for many a dim century such was his condition.
He made no more impress on external nature than one of
the beasts which he chased. But in course of time, as
366 GEIKIE
civilisation grew, he asserted his claim to be one of the
geographical forces of the globe. Not content with gather-
ing the fruits and capturing the animals which he found
needful for his wants, he gradually entered into a contest
with nature to subdue the earth and to possess it. Nowhere
has this warfare been fought out so vigorously as on the
surface of Europe. On the one hand, wide dark regions of
ancient forest have given place to smiling cornfields. Peat
and moor have made way for pasture and tillage. On the
other hand, by the clearance of woodlands the rainfall has
been so diminished that drought and barrenness have spread
where verdure and luxuriance once prevailed. Rivers have
been straitened and made to keep their channels, the sea
has been barred back from its former shores. For many
generations the surface of the continent has been covered
with roads, villages and towns, bridges, aqueducts and
canals, to which this century has added a multitudinous
network of railways, with their embankments and tunnels.
In short, wherever man has lived, the ground beneath him
bears witness to his presence. It is slowly covered with a
stratum either wholly formed by him or due in great
measure to his operations. The soil under old cities has
been increased to a depth of many feet by the rubbish of
his buildings; the level of the streets of modern Rome
stands high above that of the pavements of the Caesars, and
that again above the roadways of the early Republic. Over
cultivated fields his potsherds are turned up in abundance
by the plough. The loam has risen within the walls of his
graveyards as generation after generation has mouldered
into dust.
It must be owned that man, in much of his struggle with
the world around him, has fought blindly for his own ulti-
mate interests. His contest, successful for the moment,
has too often led to sure and sad disaster. Stripping forests
from hill and mountain, he has gained his immediate object
in the possession of their abundant stores of timber; but he
has laid open the slopes to be parched by drought, or swept
bare by rain. Countries once rich in beauty, and plenteous
in all what was needful for his support, are now burnt and
barren, or almost denuded of their soil. Gradually he has
GEOGRAPHICAL EVOLUTION 367
been taught, by his own bitter experience, that while his aim
still is to subdue the earth, he can attain it, not by setting
nature and her laws at defiance, but by enlisting them in
his service. He has learnt at last to be the minister and
interpreter of nature, and he finds in her a ready and un-
repining slave.
In fine, looking back across the long cycles of change
through which the land has been shaped into its present
form, let us realise that these geographical revolutions are
not events wholly of the dim past, but that they are still
in progress. So slow and measured has been their march,
that even from the earliest times of human history they
seem hardly to have advanced at all. But none the less
are they surely and steadily transpiring around us. In the
fall of rain and the flow of rivers, in the bubble of springs
and the silence of frost, in the quiet creep of glaciers and
the tumultuous rush of ocean waves, in the tremor of the
earthquake and the outburst of the volcano, we may recog-
nise the same play of terrestrial forces by which the frame-
work of the continents has been step by step evolved. In
this light the familiar phenomena of our daily experience
acquire an historical interest and dignity. Through them
we are enabled to bring the remote past vividly before us,
and to look forward hopefully to that great future in which,
in the physical not less than in the moral world, man is to
be a fellow-worker with God.
J 1 " 7-197 21748 5793
Date Due
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HAK I J m
Bri
gham Young Univen
;ity
