0551
0551f
The Methods of Glass Blowing
THE METHODS OF GLASS BLOWING
AND OF
WORKING SILICA
BY THE SAME AUTHOR _With 25 Illustrations. Crown 8vo, 2s._
=A Practical Introduction to Chemistry.= Intended to
give a _practical_ acquaintance with the Elementary Facts
and Principles of Chemistry.
LONGMANS, GREEN, AND CO.
LONDON, NEW YORK, BOMBAY, CALCUTTA, AND MADRAS.
The Methods of Glass Blowing
AND OF
Working Silica in the Oxy-Gas Flame
_FOR THE USE OF CHEMICAL AND
PHYSICAL STUDENTS_
BY
W. A. SHENSTONE, F.R.S.
FORMERLY LECTURER ON CHEMISTRY IN CLIFTON COLLEGE
_NINTH IMPRESSION_
LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON
FOURTH AVENUE & 30TH STREET, NEW YORK
BOMBAY, CALCUTTA, AND MADRAS
1916
PREFACE
This book consists of a reprint of the third edition of my Methods of
Glass-blowing, together with a new chapter in which I have described the
comparatively new art of working vitreous silica.
The individual operations of glass-blowing are much less difficult than
is usually supposed, and considerable success in the performance of most
of them may be attained by any one who is endowed with average powers of
manipulation and who is moderately persistent. Constructing finished
apparatus is often more difficult, as it may involve the performance of
several operations under disadvantageous conditions, and may demand a
little ingenuity on the part of the operator. But I think the
suggestions in Chapter IV. will make this comparatively easy also to
those who have mastered the operations described in Chapter III.
The working of vitreous silica, though more tedious and expensive than
glass-blowing, is not really more difficult, and as it seems certain
that this new material will soon play a useful part in chemical and
physical research, I believe the addition now made to the earlier book
will add considerably to its value.
As glass is much less expensive to work with than silica, the beginner
will find it best to spend a few days working with the common gas
blow-pipe and glass before he attempts to manipulate the new and more
refractory material. Therefore, in writing the new chapter, I have
assumed that the reader is already more or less familiar with the rest
of the book, and have given only such instructions and advice as will be
required by one who is already able to carry out simple work at the
blow-pipe.
W. A. SHENSTONE.
CLIFTON COLLEGE,
_Dec. 1901_.
CONTENTS
CHAPTER I.
GLASS-BLOWER'S APPARATUS.
PAGE
Introductory--The Working-place--The Blow-pipe--The
Bellows--Automatic Blower--Blow-pipe Flames, 1-11
CHAPTER II.
VARIETIES OF GLASS AND THEIR MANAGEMENT.
Characters of good Glass--Cleaning and Preparing a
Tube--Presenting Glass to the Flame--Methods of working with
Lead and Soft Soda Glass respectively--Management of Soda
Glass--Annealing--The Use of Combustion Tube, 12-25
CHAPTER III.
CUTTING AND BENDING GLASS--FORMING GLASS APPARATUS BEFORE
THE BLOW-PIPE--MAKING AND GRINDING STOPPERS TO APPARATUS,
ETC.
Cutting Glass Tubes--Bending Glass Tubes--Rounding and
Bordering the Ends of Tubes--Sealing--Choking, or
Contracting the Bore of a Glass Tube--Widening
Tubes--Piercing Tubes--Uniting Pieces of Glass to Each
Other, Known as Welding, or Soldering--Blowing a Bulb or
Globe of Glass--Making and Grinding Stoppers, 26-54
CHAPTER IV.
MAKING THISTLE FUNNELS, U-TUBES, ETC.--COMBINING THE PARTS
OF COMPLICATED APPARATUS--MERCURY, AND OTHER AIR-TIGHT
JOINTS--VACUUM TAPS--SAFETY TAPS--AIR-TRAPS.
Electrodes--U-Tubes--Spiral Tubes--Thistle Funnels--Closing
Tubes containing Chemicals--Construction of Apparatus
Consisting of Several Parts--Modes of Combining the Parts of
Heavy Apparatus--Mercury Joints--Vacuum Taps--Lubricating
Taps--Air-Traps, 55-69
CHAPTER V.
GRADUATING AND CALIBRATING GLASS APPARATUS.
To Graduate Tubes, etc.--To Divide a Given Line into Equal
Parts--To Calibrate Apparatus--To Calibrate Tubes for
Measuring Gases--To Calibrate the Tube of a Thermometer, 70-81
CHAPTER VI.
GLASS TUBING.
Diagrams of Glass Tubes, showing the chief sizes in which
they are made, 82-83
CHAPTER VII.
VITREOUS SILICA
Introductory--Properties of Vitreous Silica--Preparing
non-splintering Silica from Brazil Pebble--Apparatus--The
Method of Making Silica Tubes--Precautions--Making Larger
Tubes and other Apparatus of Silica--Quartz Fibres, 84-95
INDEX, 97
CHAPTER I.
_GLASS-BLOWER'S APPARATUS._
=Introductory.=--I shall endeavour to give such an account of the
operations required in constructing glass apparatus as will be useful to
chemical and other students; and as this book probably will come into
the hands of beginners who are not in a position to secure any further
assistance, I shall include descriptions even of the simple operations
which are usually learned during the first few hours of practical work
in a chemical or physical laboratory. I shall not give any particular
account of the manufacture of such apparatus as thermometers, taps,
etc., because, being in large demand, they can be bought so cheaply that
time is not profitably spent in making them. But it will be found that
what is included will enable any one, who will devote sufficient time to
acquiring the necessary manipulative dexterity, to prepare such
apparatus as test-tubes, distillation flasks, apparatus for washing
gases, ozone generating tubes, etc., when they are required, as they
often are, without delay or for special purposes. The amateur probably
will not succeed in turning out apparatus so finished in appearance as
that of the professional glass-blower until after long practice, but
after a little daily practice for the space of a few weeks, any one who
is fairly skilful in ordinary manipulation, and who perseveres in the
face of failure at first, will find himself able to make almost all the
apparatus he needs for lecture or other experiments, with a considerable
saving in laboratory expenses, and, which very often is more important,
without the delay that occurs when one depends upon the professional
glass-worker. In the case of those who, like myself, work in the
provinces, this latter advantage is a very weighty one.
After the description of the instruments used in glass-blowing, which
immediately follows, the following arrangement of the subject has been
adopted. In the first place, an account of the two chief kinds of glass
is given, and of the peculiarities in the behaviour of each of them
before the blow-pipe, which is followed by a tolerably minute
description of the method of performing each of the fundamental
operations employed in fashioning glass apparatus. These are not very
numerous, and they should be thoroughly mastered in succession,
preferably upon tubes of both soda and lead glass. Then follows, in
Chapter IV., an account of the application of these operations to
setting up complete apparatus, full explanations of the construction of
two or three typical pieces of apparatus being given as examples, and
also descriptions of the modes of making various pieces of apparatus
which in each case present one or more special difficulties in their
construction; together with an account, which, I think, will be found
valuable, of some apparatus that has been introduced, chiefly during
recent years, for experimenting upon gases under reduced pressure,
_e.g._ vacuum taps and joints. Finally, in Chapter V., there is a short
account of the methods of graduating and calibrating glass apparatus for
use in quantitative experiments.
=The Working-place.=--The blow-pipe must be placed in a position
perfectly free from draughts. It should not face a window, nor be in
too strong a light, if that can be avoided, for a strong light will
render the non-luminous flames, which are used in glass-blowing, almost
invisible, and seriously inconvenience the operator, who cannot apply
the various parts of the flames to his glass with the degree of
certainty that is necessary; neither can he perceive the condition of
the glass so correctly in a strong light, for though in many operations
the glass-worker is guided by feeling rather than by seeing, yet sight
plays a very important part in his proceedings.
My own blow-pipe is placed near a window glazed with opaque glass, which
looks southwards, but is faced by buildings at a short distance. In dull
weather the light obtained is good; but on most days I find it
advantageous to shade the lower half of the window with a green baize
screen. Some glass-blowers prefer gaslight to daylight.
The form of the table used is unimportant, provided that it is of a
convenient height, and allows free play to the foot which works the
blower underneath it. The blower should be _fixed_ in a convenient
position, or it will get out of control at critical moments. The table,
or that part of it which surrounds the blow-pipe, should be covered with
sheet-iron to protect it from the action of the fragments of hot glass
that will fall upon it. The tubes that supply air and gas to the
blow-pipe should come from beneath the table, and may pass through holes
cut for the purpose.
Many glass-blowers prefer to work at a rather high table, and sit on a
rather high stool, so that they are well above their work. No doubt this
gives extra command over the work in hand, which is often valuable. On
the other hand, it is somewhat fatiguing. For a long spell of labour at
work which is not of a novel character nor specially difficult, I am
disposed to recommend sitting on a chair or low stool, at a table of
such height as will enable the elbows to rest easily upon it whilst the
glass is held in the flame. The precise heights that are desirable for
the table and stool, and the exact position of the blow-pipe, will
depend upon the height and length of arm of the individual workman, and
it must therefore be left to each person to select that which suits him
best. A moveable rest made of wood, for supporting the remote end of a
long piece of glass tube a few inches above the table, whilst the other
end is being heated in the flame, will be found convenient.
=The Blow-pipe.=--Formerly a lamp, in which sweet oil or tallow was
burnt, was employed for glass-working, and such lamps are still
occasionally used. Thus, lamps burning oil or tallow were used on board
the _Challenger_ for hermetically sealing up flasks of water collected
at various depths to preserve them for subsequent examination. I shall
not, however, give an account of such a lamp, for the gas apparatus is
so much more convenient for most purposes that it has now practically
superseded the oil lamps. Fig. 1 shows a gas blow-pipe of exceedingly
simple construction, which can be easily made, and with which good work
can be done.
[Illustration: FIG. 1.]
The tube _A_ is of brass, and has a side tube _B_ brazed to it, ten to
twelve centimetres from the end _E_, according to the dimensions of the
tube. A tube of glass, _EC_, is fitted into _A_ by a cork at _D_. _B_
is connected to a supply of gas by a flexible tube, _C_ is similarly
connected to the blower. By means of _CE_ a stream of air can be forced
into gas burning at the mouth of the blow-pipe _G_, and various flames,
with the characters described in a later section, can be produced with
this instrument. For producing the pointed flame (Fig. 3, p. 9) the
opening _E_ of the air-tube should be contracted to the size of a large
knitting needle. For producing a flame of large size, rich in air (Fig.
4, p. 9), the internal diameter of _E_ may be nearly half as great as
that of _A_ without disadvantage.
This blow-pipe may be fixed in position by the spike _F_, which will fit
into holes in a block of wood or a large cork. Several of these holes in
various positions should be made in the block, so that the position of
the blow-pipe may be varied easily. Two taps must be provided in
convenient positions near the edge of the table to enable the workman to
regulate the supplies of air and gas. These taps should be fixed to the
table and be connected with the gas and air supplies respectively on one
side, and with the blow-pipe on the other, by flexible tubes. If
blow-pipes of this kind be used, at least two of them should be
provided; one of small dimensions for working on small tubes and joints,
the other of larger size for operations on larger tubes. It will be
convenient to have both of them ready for use at all times, as it is
sometimes necessary to employ large and small flames on the same piece
of work in rapid succession. By having several air-tubes of different
sizes fitted to each blow-pipe, a greater variety of work may be done.
For the larger blow-pipe, the internal diameter of _A_ may be fifteen to
seventeen millimetres.
For the smaller instrument, eleven millimetres for the diameter of _A_
would be a useful size.
When a slightly greater outlay can be afforded it will be most
convenient to purchase the blow-pipe. They can be obtained of compact
form, supported on stands with universal joints giving great freedom of
movement, and with taps for regulating the supplies of gas and air, at
comparatively small cost.
As figures of various blow-pipes can be seen in the price-lists of most
dealers in apparatus, they are not given here. Their introduction would
be of but little service, for the construction of that which is adopted
can be readily ascertained by taking it to pieces. The simplest
blow-pipe usually used for glass-working is that of Herapath. This has
two taps to regulate the air and gas supplies respectively, and will
give a considerable variety of flames, which will be discussed
subsequently.
An excellent blow-pipe, made on the same principle as that shown in Fig.
1, but more substantially and with interchangeable jets, can be obtained
from Messrs. Muller of Holborn for a moderate outlay.
Another very good blow-pipe is the Automaton blow-pipe of Mr. Fletcher
of Warrington. In this, one tap regulates the supply both of air and
gas, which is a great gain when difficult work is in hand. Automaton
blow-pipes are made of two sizes. I have found that the larger size,
with a powerful bellows, heats large pieces of lead glass very
satisfactorily. On the other hand, the fine-pointed oxidising flame of
the Herapath blow-pipe is, perhaps, the most suitable for working joints
of lead glass. Therefore a good equipment would be a small Herapath
blow-pipe and a large-sized Automaton. If only one blow-pipe is
purchased it should be either a medium-sized Herapath, or the smaller
Automaton, as those are most useful for general work.
Mr. Fletcher also makes an ingenious combination of two blow-pipes in
which the gas and air supplies are regulated by a single lever-handle.
This is very convenient, and gives flames that answer well with tubes
made of soft soda glass, and it is very useful for general work. For use
with lead glass the supply of air is rather too small, and does not
enable one to get such good results. This can be easily amended,
however. By slightly increasing the size of the air-tube of the smaller
blow-pipe, and having increased the supply of air to the larger
blow-pipe also, by reducing the external diameter of the end of the
innermost tube, I now get medium-sized brush flames and pointed flames
with this blow-pipe, that are equal to any I have used for heating lead
glass.
For small laboratories the inexpensive No. 5 Bunsen burner of Mr.
Fletcher, which is convertible into a blow-pipe, will be very useful.
Jets of several sizes to fit the air-tubes of blow-pipes may be obtained
with them, and will serve for regulating the supply of air to the flame.
=The Bellows.=--The usual blowing apparatus is some form of foot-blower.
These may be obtained fitted to small tables with sheet-iron tops. But a
much less expensive apparatus is the large foot-blower made by Mr.
Fletcher of Warrington, which can be used at an ordinary table or
laboratory bench. Good foot-blowers can also be obtained from makers of
furnace bellows.
No part of the glass-blower's equipment exceeds the bellows in
importance. The best blower procurable should therefore be adopted. A
bellows which, when used with a large blow-pipe, will not enable you to
heat large pieces of lead glass tube to redness without blackening the
glass when the directions for heating lead glass on pages 17-21 are
followed, should on no account be received. I am told that at some
places, where the water-supply is at very high pressure, it is utilised
for working blow-pipes by means of the apparatus described below, and
that some glass-workers find it advantageous to use such automatic
blowers. But after a little practice, the effort of working the blower
with the foot whilst manipulating the glass is not a source of serious
inconvenience. Indeed, as it gives a certain degree of control over the
flame without the use of the hands, the foot-blower is preferable. It is
worth while to describe an automatic blower, however.
=Automatic Blower= (Fig. 2).--A strong glass tube _A_ is welded into a
somewhat larger tube _B_ so that its end is about 2 mm. from the
contraction at _G_. _B_ has a side tube _C_ joined to it. The narrow end
of _B_ is fixed by an india-rubber cork to a strong bottle _D_ of two or
three litres capacity. The india-rubber cork also carries an exit tube
_E_, and _D_ is pierced near its bottom by a small hole at _F_.
[Illustration: FIG. 2]
In using the apparatus _A_ is connected with the water-supply, and water
passing through _G_, carries air with it into _D_. The water escapes
from _D_ by the opening at _F_, and the air is allowed to pass out by
the tube _E_, its passage being regulated by a tap. Fresh supplies of
air enter _B_ by _C_.
=Blow-pipe Flames=--_The Pointed Flame._--If the gas tap of a Herapath
blow-pipe be adjusted so that comparatively little gas can pass, and if
the foot-blower be then worked cautiously, a long tongue of flame ending
in a fine point will be produced (Fig. 3). This flame will subsequently
be described as the _pointed flame_. It should be quite free from
luminosity, and as the amount of air necessary for securing a pointed
flame is large, in proportion to the gas, there is excess of oxygen
towards the end _C_. By adjusting the proportions of air and gas,
pointed flames of various dimensions can be obtained with the same
blow-pipe. The part of a pointed flame to be used in glass-working is
the tip, or in some cases the space slightly beyond the tip.
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
_The Brush Flame._--If a large supply of gas be turned on and a
considerable blast of air sent into the flame, a non-luminous flame of
great size will be obtained (Fig. 4). In form it somewhat resembles a
large camel's hair pencil, and may conveniently be described as a
_brush flame_. The chief advantage of a large-sized blow-pipe is, that
with it a large brush flame may be produced, which is often invaluable.
By gradually diminishing the supply of gas and air smaller brush flames
may be produced.
The jet used to supply air to the Herapath blow-pipe is usually too
fine, and consequently does not permit the passage of sufficient air to
produce a brush flame that contains excess of oxygen, even with the aid
of a very powerful blower. My own Herapath blow-pipe only gives a
satisfactory oxidising brush flame when the jet is removed altogether
from the end of the air-tube. For producing pointed flames the finer jet
of the air-tube must be used, but when a highly oxidising flame of large
size is required it must be removed. The internal diameter of the
central air-tube should be nearly half as great as that of the outer or
gas-supply tube. Fletcher's Automaton with the large air jet gives a
very liberal supply of air, and produces an excellent oxidising brush
flame. In the case of the larger-sized Automaton a consequence of this
is, however, that when fitted with the large jet it will not give so
good a pointed flame as the Herapath, which, in its turn, gives an
inferior oxidising brush. By fitting finer jets to the air-tube of
Fletcher's apparatus pointed flames can be secured when necessary.
_The Smoky Flame._--By turning on a very free supply of gas, and only
enough air to give an outward direction to the burning gas, a smoky
flame, chiefly useful for annealing and for some simple operations on
lead glass, is produced.
The Gimmingham blow-pipe and Fletcher's combination blow-pipe, in
addition to the above flames, are also adapted to produce a non-luminous
flame, resembling that of the Bunsen gas-burner, which is very
convenient for the preliminary heating of the glass, and also for
gradually cooling finished apparatus. It is not necessary to describe
the method of using these last-mentioned blow-pipes. With the more
complicated of them directions for its use are supplied.
Mr. Madan has suggested the use of oxygen in place of air for producing
the oxidising flame required for working lead glass, and to produce a
flame of high temperature for softening tubes of hard, or combustion,
glass. For the latter purpose the employment of oxygen may be adopted
with great advantage. For working lead glass, however, it is quite
unnecessary if the directions already given are followed.
The student's subsequent success will so largely depend upon his
acquaintance with the resources of his blow-pipe, and on the facility
with which he can take advantage of them, that no pains should be spared
in the effort to become expert in its management as soon as possible. A
few experiments should now be made, therefore, upon the adjustment of
the flame, until the student is able to produce and modify any form of
flame with promptness and certainty.
[Illustration: FIG. 5.]
The remaining apparatus used in glass-working consists of triangular and
other files, charcoal pastils for cutting glass, pieces of sound
charcoal of various diameters with conical ends; it is convenient to
have one end somewhat less pointed than the other (Fig. 5). Corks of
various sizes; the smallest, which are most frequently needed, should be
carefully cut with sharpened cork borers from larger corks. Besides
these there should be provided some freshly distilled turpentine in
which camphor has been dissolved,[1] fine and coarse emery powder, and
some sheets of cotton-wadding, an india-rubber blowing-bottle, glass
tubes, a little white enamel, and a pair of iron tongs.
[1] Half an ounce of camphor to about six ounces of turpentine will do
very well.
CHAPTER II.
_VARIETIES OF GLASS AND THEIR MANAGEMENT._
All the varieties of glass that are ordinarily met with contain silica
(SiO_{2}) associated with metallic oxides. In a true glass there are at
least two metallic oxides. The unmixed silicates are not suitable for
the purposes of glass. They are not so capable of developing the viscous
condition when heated as mixtures--some of them are easily attacked by
water, and many of those which are insoluble are comparatively
infusible. There is generally excess of silica in glass, that is, more
than is necessary to form normal silicates of the metals present. The
best proportions of the various constituents have been ascertained by
glass-makers, after long experience; but the relation of these
proportions to each other, from a chemical point of view, is not easy to
make out.
The varieties of glass from which tubes for chemical glass-blowing are
made may be placed under three heads, and are known as[2]--
Soft soda glass. Also known as French glass.
Lead glass. Also known as English glass.
Hard glass.
[2] For details of the composition of the various glasses, some work on
glass-making may be consulted.
In purchasing glass tubes, it is well to lay in a considerable stock of
tubes made of each of the two first varieties, and, if possible, to
obtain them from the manufacturer, for it frequently happens that pieces
of glass from the same batch may be much more readily welded together
than pieces of slightly different composition. Yet it is not well to lay
in too large a stock, as sometimes it is found that glass deteriorates
by prolonged keeping.
As it is frequently necessary to make additions, alterations, or repairs
to purchased apparatus, it is best to provide supplies both of soft soda
glass and lead glass, for though purchased glass apparatus is frequently
made of lead glass, yet sometimes it is formed from the soda glass, and
as it is a matter of some difficulty to effect a permanent union between
soda glass and lead glass, it is desirable to be provided with tubes of
both kinds.
Many amateurs find that soda glass is in some respects easier to work
with than lead glass. But, on the other hand, it is somewhat more apt to
crack during cooling, which causes much loss of time and disappointment.
Also, perhaps in consequence of its lower conductivity for heat, it very
often breaks under sudden changes of temperature during work. If,
however, a supply of good soda glass is obtained, and the directions
given in this book in regard to annealing it are thoroughly carried out,
these objections to the use of soda glass will, to a great extent, be
removed. I find, however, that when every precaution has been taken,
apparatus made of soda glass will bear variations of temperature less
well than that made of lead glass. Therefore, although the comparatively
inexpensive soda glass may be employed for most purposes without
distrust, yet I should advise those who propose to confine themselves to
one kind of glass, to take the small extra trouble required in learning
to work lead glass.
In order to secure glass of good quality, a few pieces should be
obtained as a sample, and examined by the directions given below. When
the larger supply arrives, a number of pieces, taken at random, should
be examined before the blow-pipe, to compare their behaviour with that
of the sample pieces, and each piece should be separately examined in
all other respects as described subsequently.
Hard glass is used for apparatus that is required to withstand great
heat. It is difficult to soften, especially in large pieces. It should
only be employed, therefore, when the low melting points of soda or lead
glass would render them unsuitable for the purpose to which the finished
apparatus is to be put. What is sold as Jena combustion tube should be
preferred when this is the case.
=Characters of good Glass.=--Glass tubes for glass-blowing should be as
free as possible from knots, air-bubbles, and stripes. They should be in
straight pieces of uniform thickness, and cylindrical bore. It is not
possible to obtain glass tubes of absolutely the same diameter from one
end to the other in large quantities, but the variations should not be
considerable.
When a sharp transverse scratch is made with a good file on a piece of
tube, and the scratch is touched with a rather fine point of red-hot
glass (this should be lead glass for a lead glass tube, and soda glass
for a tube of soda glass), the crack which is started should pass round
the glass, so that it may be broken into two pieces with regular ends.
If the crack proceeds very irregularly, and especially if it tends to
extend along the tube, the glass has been badly annealed, and should not
be employed for glass-blowing purposes. It is important that the point
of hot glass used shall be very small, however. Even good glass will
frequently give an irregular fracture if touched with a large mass of
molten glass.
Finally, glass tube which is thin and of small diameter should not
crack when suddenly brought into a flame. But larger and thicker tubes
will not often withstand this treatment. They should not crack, however,
when they are brought into a flame gradually, after having been held in
the warm air in front of it for a minute or so.
Good glass does not readily devitrify when held in the blow-pipe flame.
As devitrified glass very often may be restored to its vitreous
condition by fusion, devitrification most frequently shows itself round
the edges of the heated parts, and may be recognised by the production
of a certain degree of roughness there. It is believed to be due to the
separation of certain silicates in the crystallised form. Hard glass,
which contains much calcium, is more apt to devitrify than the more
fusible varieties.[3]
[3] The presence of silicates of calcium and aluminum are considered to
promote a tendency to devitrification in glass; and glasses of complex
composition are more apt to devitrify than the simpler varieties. See
_Glass-making_, by Powell, Chance, and Harris, Chap. IV.
Glass tubes are made of various sizes. When purchasing a supply, it is
necessary to be somewhat precise in indicating to the vendor the sizes
required. I have therefore placed at the end of the book, in an
appendix, a table of numbered diagrams. In ordering tubes it will
usually only be necessary to give the numbers of the sizes wished for,
and to specify the quantity of each size required. In ordering glass
tubes by weight, it must be remembered that a great many lengths of the
smaller sizes, but very few lengths of the larger sizes, go to the
pound. Larger-sized tubes than those on the diagram are also made. In
ordering them the external diameter and thickness of glass preferred
should be stated.
=Cleaning and Preparing a Tube.=--It is frequently much easier to clean
the tube from which a piece of apparatus is to be made than to clean
the finished apparatus. A simple method of cleaning a tube is to draw a
piece of wet rag which has been tied to a string through the tube once
or twice, or, with small tubes, to push a bit of wet paper or cotton
wool through them. If the dirt cannot be removed in this way, the
interior of the tube should be moistened with a little sulphuric acid in
which some bichromate of potassium has been dissolved. In any case, it
must finally be repeatedly rinsed with distilled water, and dried by
cautiously warming it, and sucking or blowing air through it. In order
to avoid heating delicate apparatus which has become damp and needs
drying, the water may be washed out with a few drops of spirit, which is
readily removed at a low temperature.
Before using a glass tube for an operation in which it will be necessary
to blow into it, one end of it must be contracted, unless it is already
of such a size that it can be held between the lips with perfect ease;
in any case, its edges must be rounded. For descriptions of these
operations, see page 35. The other end must be closed. This may be done
by means of a cork.
=Presenting Glass to the Flame.=--Glass tubes must never be brought
suddenly into the flame in which they are to be heated. All glass is
very likely to crack if so treated. It should in all cases be held for a
little while in front of the flame, rotated constantly in the hot air
and moved about, in order that it may be warmed over a considerable
area. When it has become pretty hot by this treatment, it may be
gradually brought nearer to the flame, and, finally, into contact with
it, still with constant rotation and movement, so as to warm a
considerable part of the tube. When the glass has been brought fairly
into contact with the flame, it will be safe to apply the heat at the
required part only. Care must be taken in these preliminary operations
to avoid heating the more fusible glasses sufficiently to soften them.
=Methods of working with Lead and soft Soda Glass respectively.=--When
lead glass is heated in the brush flame of the ordinary Herapath
blow-pipe, or within the point of the pointed flame, it becomes
blackened on its surface, in consequence of a portion of the lead
becoming reduced to the metallic state by the reducing gases in the
flame. The same thing will happen in bending a lead glass tube if it is
made too hot in a luminous flame. A practical acquaintance with this
phenomenon may be acquired by the following experiment:--
Take a piece of lead glass tube, bring it gradually from the point of a
pointed flame to a position well within the flame, and observe what
happens. When the glass reaches the point _A_ (Fig. 3), or thereabouts,
a dark red spot will develop on the glass, the area of the spot will
increase as the glass is brought further in the direction _A_ to _B_. If
the glass be then removed from the flame and examined, it will be found
that a dark metallic stain covers the area of the dark red spot
previously observed. Repeat the experiment, but at the first appearance
of the dark spot slowly move the glass in the direction _A_ to _C_. The
spot will disappear, and, if the operation be properly performed, in its
place there will be a characteristically greenish-yellow luminous spot
of highly heated glass. In this proceeding the reduced lead of the dark
spot has been re-oxidised on passing into the hot gases, rich in oxygen,
which abound at the point of the flame. If one end of the tube has been
previously closed by a piece of cork, and if air be forced into the tube
with the mouth from the open end before the luminous spot has become
cool, the glass will expand. If the experiment be repeated several
times, with pointed flames of various sizes, the operator will quickly
learn how to apply the pointed flame to lead glass so that it may be
heated without becoming stained with reduced lead.
If the spot of reduced metal produced in the first experiment be next
brought into the oxidising flame, it also may gradually be removed. On
occasion, therefore, apparatus which has become stained with lead during
its production, may be rendered presentable by suitable treatment in the
oxidising flame. The process of re-oxidising a considerable surface in
this way after it has cooled down is apt to be very tedious, however,
and, especially in the case of thin tubes or bulbs, often is not
practicable. In working with lead glass, therefore, any reduction that
occurs should be removed by transferring the glass to the oxidising
flame at once.
Small tubes, and small areas on larger tubes of English glass, may be
softened without reduction by means of the pointed oxidising flame; but
it is not easy to heat any considerable area of glass sufficiently with
a pointed flame. And though it is possible, with care, to employ the hot
space immediately in front of the visible end of an ordinary brush
flame, which is rich in air, yet, in practice, it will not be found
convenient to heat large masses of lead glass nor tubes of large size,
to a sufficiently high temperature to get the glass into good condition
for blowing, by presenting them to the common brush flame.
It may seem that as glass which has become stained with reduced lead can
be subsequently re-oxidised by heating it with the tip of the pointed
flame, the difficulty might be overcome by heating it for working in the
brush flame, and subsequently oxidising the reduced lead. It is,
however, difficult, as previously stated, to re-oxidise a large surface
of glass which has been seriously reduced by the action of the reducing
gases of the flame, after it has cooled. Moreover, there is this very
serious objection, that if, as may be necessary, the action of the
reducing flame be prolonged, the extensive reduction that takes place
diminishes the tendency of the glass to acquire the proper degree of
viscosity for working it, the glass becomes difficult to expand by
blowing, seriously roughened on its surface, and often assumes a very
brittle or rotten condition.
When it is only required to bend or draw out tubes of lead glass, they
may be softened sufficiently by a smoky flame, which, probably owing to
its having a comparatively low temperature, does not so readily reduce
the lead as flames of higher temperature. But for making joints,
collecting masses of glass for making bulbs, and in all cases where it
is required that the glass shall be thoroughly softened, the smoky flame
does not give good results.
In the glass-works, where large quantities of ornamental and other glass
goods are made of lead or flint glass, the pots in which the glass is
melted are so constructed that the gases of the furnace do not come into
contact with the glass;[4] and as the intensely-heated sides of the
melting-pot maintain a very high temperature within it by radiation, the
workman has a very convenient source of heat to his hand,--he has, in
fact, only to introduce the object, or that part of it which is to be
softened, into the mouth of the melting-pot, and it is quickly heated
sufficiently for his purpose, not only without contact of reducing
gases, but in air. He can therefore easily work upon very large masses
of glass. In a special case, such a source of heat might be devised by
the amateur. Usually, however, the difficulty may be overcome without
special apparatus. It is, in fact, only necessary to carry out the
instructions given below to obtain a considerable brush flame rich in
air, in which the lead glass can be worked, not only without
discoloration, but with the greatest facility.
[4] See _Principles of Glass-making_, p. 31.
_To Produce an Oxidising Brush Flame._--The blower used must be
powerful, the air-tube of the blow-pipe must be about half as great in
diameter as the outer tube which supplies the gas. The operator must
work his bellows so as to supply a strong and _steady_ blast of air, and
the supply of gas must be regulated so that the brush flame produced is
free from every sign of incomplete combustion,[5] which may be known by
its outer zone being only faintly visible in daylight, and quite free
from luminous streaks (see Fig. 4, p. 9). When a suitable flame has been
produced, try it by rotating a piece of lead glass at or near the end of
the inner blue part of the flame (_A_ Fig. 4); the appearance of the
glass will quickly indicate reduction. When this occurs move the glass
forward to the end of the outer zone _B_, but keep it sufficiently
within the flame to maintain it at a high temperature. If all is right
the metallic reduction will quickly disappear, the glass will become
perfectly transparent once more, and will present the appearance
previously observed in the experiments with the pointed flame, or, if
very hot, assume a brownish-red appearance. If this does not occur, the
supply of air must be increased or the supply of gas diminished until
the proper effects are secured.
[5] Nevertheless the supply of air must not be so excessive as to reduce
the temperature of the flame sufficiently to prevent the thorough
softening of the glass, which will occur if the bellows is worked with
too much zeal.
In working upon lead glass with the highly oxidising brush flame, it is
a good plan to heat it in the reducing part of the flame _A_ for
thoroughly softening the glass, and to remove it to the oxidising flame
_B_ to burn away the reduced metal. In prolonged operations, in order
that reduction may never go too far, hold the glass alternately in the
hot reducing flame and in the oxidising flame. The inferiority of the
outer oxidising flame to those portions nearer the inner blue zone for
softening the glass, may perhaps be accounted for by the presence of a
larger proportion of unconsumed air in the former, which being heated at
the expense of the hot gases produced by combustion, thereby lowers the
temperature of the flame. At or near _A_ (Fig. 4) where the combustion
is nearly complete, but no excess of air exists, the temperature will
naturally be highest.
If a very large tube be rotated in the oxidising flame at _B_ (Fig. 4)
it may happen that the flame is not large enough to surround the tube,
and that as it is rotated those parts of it which are most remote from
the flame will cool down too considerably to allow all parts of the tube
to be simultaneously brought into the desired condition. This difficulty
may be overcome by placing two blow-pipes exactly opposite to each
other, at such a distance that there is an interval of about an inch
between the extremities of their flames, and rotating the tube between
the two flames. It may be necessary to provide two blowers for the
blow-pipes if they are large.
Again, if a very narrow zone of a tube of moderate size is to be heated,
two pointed flames may be similarly arranged with advantage.
Occasionally more than two flames are made to converge upon one tube in
this manner.
Another method of preventing one side of a tube from cooling down whilst
the other is presented to the flame, is to place a brick at a short
distance from the extremity of the flame. The brick checks the loss of
heat considerably. A block of beech wood may be used for the same
purpose, the wood ignites and thereby itself becomes a source of heat,
and is even more effective than a brick.
Fuller details of the management of lead glass under various
circumstances will be found in the subsequent descriptions of operations
before the blow-pipe.
Before proceeding to work with soda glass, the student should not only
verify by experiments what has been already said, but he should
familiarise himself with the action of the blow-pipe flame on lead glass
by trying the glass in every part of the flame, varying the proportions
of gas and air in every way, repeating, and repeating, his experiments
until he can obtain any desired effect with certainty and promptitude.
He should practice some of the simpler operations given in Chapter III.
in order to impress what he has learned well on his mind.
=Management of Soda Glass.=--In working with soda glass the following
points must be constantly kept in mind. That as it is much more apt than
lead glass to crack when suddenly heated, great caution must be
exercised in bringing it into the flame; and that in making large joints
or in making two joints near each other, all parts of the tube adjacent
to that which, for the moment, is being heated, must be kept hot, as it
is very apt to crack when adjacent parts are unequally heated. This may
be effected by stopping work at short intervals and warming the cooler
parts of the tube, or by the use of the brick or block of wood to check
radiation, or even by placing a supplementary blow-pipe or Bunsen burner
in such a position that its flame plays upon the more distant parts of
the work, not coming sufficiently into contact to soften the glass,
however, but near enough to keep it well heated. Lastly, to prevent the
finished work from falling to pieces after or during cooling, the
directions given under the head of annealing must be carefully carried
out.
In very much of his work the glass-blower is guided more by the _feel_
of the glass than by what he sees. The power of feeling glass can only
be acquired by practice, and after a certain amount of preliminary
failure. As a rule I have observed that beginners are apt to raise their
glass to a higher temperature than is necessary, and that they employ
larger flames than are wanted. If glass be made too soft it may fall so
completely out of shape as to become unworkable except in very skilful
hands. The following rules, therefore, should be strictly adhered to.
Always employ in the first instance the smallest flame that is likely to
do the work required. In operations involving _blowing out_ viscous
glass, attempt to blow the glass at low temperatures before higher ones
are tried. After a little experience the adoption of the right-sized
flame for a given purpose, and the perception of the best condition of
glass for blowing it, become almost automatic.
I may add that glass which is to be bent needs to be much less heated
than glass which is to be blown.
=Annealing.=--If apparatus, the glass of which is very thin and of
uniform substance, be heated, on removal from the source of heat it will
cool equally throughout, and therefore may often be heated and cooled
without any special precautions. If the glass be thick, and especially
if it be of unequal thickness in various parts, the thinner portions
will cool more quickly than those which are more massive; this will
result in the production of tension between the thicker and thinner
parts in consequence of inequality in the rates of contraction, and
fractures will occur either spontaneously or upon any sudden shock.
Thus, if a hot tube be touched with cold or wet iron, or slightly
scratched with a cold file, the inequality of the rate of cooling is
great, and it breaks at once. It is therefore necessary to secure that
hot glass shall cool as regularly as possible. And this is particularly
important in the case of articles made of soda glass. Some glass-blowers
content themselves with permitting the glass to cool gradually in a
smoky flame till it is covered with carbon, and then leave it to cool
upon the table. But under this treatment many joints made of soda glass
which are not quite uniform in substance, but otherwise serviceable,
will break down. In glass-works the annealing is done in ovens so
arranged that the glass enters at the hottest end of the oven where it
is uniformly heated to a temperature not much below that at which it
becomes viscous, and slowly passed through the cooler parts of the
chamber so that it emerges cold at the other end. This method of
annealing is not practicable in a small laboratory. But fortunately very
good results can be obtained by the following simple device, viz.:--
By wrapping the hot apparatus that is to be annealed closely in cotton
wool, and leaving it there till quite cold. The glass should be wrapped
up immediately after it is blown into its final shape, as soon as it is
no longer soft enough to give way under slight pressure. And it should
be heated as uniformly as possible, not only at the joint, but also
about the parts adjacent to the joint, at the moment of surrounding it
with the cotton. Lead glass appears to cool more regularly than soda
glass, and these precautions may be more safely neglected with apparatus
made of lead glass; but not always. At the date of writing I have had
several well-blown joints of thick-walled capillary tube to No. 16 (see
diagram, p. 82), break during cooling, in consequence of circumstances
making it dangerous to heat the neighbourhood of the joint so much as
was necessary.
The black carbonaceous coat formed on hot glass when it is placed in
cotton wool may be removed by wiping with methylated spirit, or, if it
be very closely adherent, by gently rubbing with fine emery, moistened
with the spirit.
Cotton wool is rather dangerously inflammable; it should therefore be
kept out of reach of the blow-pipe flame, and care should be taken that
the glass is not placed in contact with it at a sufficiently high
temperature to cause its ignition.
Another method of annealing is to cover the hot glass with hot sand, and
allow it to cool therein.
As in the case of lead glass, so with soda glass. A thorough
acquaintance with the effect of the various parts of the flame upon it
should be gained before further work is entered upon, for which purpose
an hour or more spent in observing its behaviour in the flame will be
fully repaid by increased success subsequently.
=The Use of Combustion Tube.=--It is often necessary to construct
apparatus of what is known as hard glass or combustion tube. It is
almost as easy to work combustion tube as to deal with lead and soda
glass if the oxy-hydrogen flame be employed.
It is not necessary to set up a special apparatus for this purpose; many
of the ordinary blow-pipes can be used with oxygen instead of with air.
It is only necessary to connect the air-tube of the blow-pipe with a
bottle of compressed oxygen instead of with the bellows. The connecting
tube should not be too wide nor too long, in order to avoid the
accumulation in it, by accident, of large quantities of explosive
mixtures.
Two precautions are necessary in manipulating hard glass in the
oxy-hydrogen flame. The glass must _not_ be overheated. At first one is
very apt to go wrong in this direction. The supply of oxygen must _not_
be too great; a small hissing flame is not what is wanted. If either of
these precautions are neglected most glass will devitrify badly. With a
little care and experience, devitrification can be absolutely avoided.
Ordinary combustion tube can be used, but I find that the glass tube
(Verbrennungsröhr) made by Schott & Co. of Jena, which can be obtained
through any firm of dealers in apparatus, is far better than the
ordinary tube.
By following these instructions, any one who has learned how to work
with lead or soda glass will find it easy to manipulate hard glass.
CHAPTER III.
_CUTTING AND BENDING GLASS--FORMING GLASS APPARATUS BEFORE THE
BLOW-PIPE--MAKING AND GRINDING STOPPERS TO APPARATUS, ETC._
In the later pages of this Chapter it will be assumed that the
operations first described have been mastered. The beginner should
therefore practise each operation until he finds himself able to perform
it with some degree of certainty. Generally speaking, however, after the
failure of two or three attempts to perform any operation, it is best to
give up for a few hours, and proceed to the work next described,
returning to that upon which you have failed subsequently. If,
unfortunately, it should happen that the work next in order involves the
performance of the operation in which the failure has occurred, it is
best to pass on to some later work which does not demand this particular
accomplishment, or to rest a while, and re-attack the difficulty when
refreshed.
=Cutting Glass Tubes.=--The simplest method of cutting a glass tube is
to make a sharp scratch with a file across the glass at the point where
it is desired to cut it, and on pulling apart the two ends, it will
break clean off. It is important that the file be sharp. In pulling
apart the ends the scratch should be held upwards, and the pull should
have a downward direction, which will tend to open out the scratch. In
the case of a large tube, a scratch will not ensure its breaking clean
across. The tube must be filed to some depth, half-way, or even all
round it. A good way of breaking a tube is to place the file in the
table after scratching the glass, to hold the glass tube above its edge
with one hand on each side of the scratch, and to strike the under side
of the tube a sharp blow upon the edge of the file, directly beneath the
scratch. In this way very even fractures of large and moderately thin
tubes may be made. It answers particularly well for removing short ends
of tube, not long enough to hold; the tube is held firmly upon the file,
and a sharp blow given to the short end with a piece of large tube or a
key.
A file whose faces have been ground till they are nearly smooth, so as
to leave very finely-serrated edges, will be found useful for cutting
glass tubes. Such a file should be used almost as a knife is used for
cutting a pencil in halves.
The simple methods just described are too violent to be applied to
delicate apparatus, too tedious when employed upon the largest tubes,
and very difficult to apply when the tube to be cut is very thin, or too
short to permit the operator to get a good grip of it on either side of
the file mark. In such cases, one or other of the following methods will
be useful:--
1. Make a scratch with a file, and touch it with the end of a _very
small_ piece of glass drawn out and heated at the tip to its melting
point. It is important that the heated point of glass be very small,
or the fracture is likely to be uneven, or to spread in several
directions. Also, it is best to use hot soda glass for starting cracks
in tubes of soda glass, and lead glass for doing so in lead glass
tubes. If the crack does not pass quite round the tube, you may pull
it asunder, as previously described, or you may bring the heated piece
of glass with which the crack was started to one end of the crack, and
slowly move it (nearly touching the glass) in the required direction;
the crack will extend, following the movements of the hot glass.
Instead of hot glass, pastils of charcoal are sometimes employed for
this purpose. They continue to burn when once lighted, and there is
no need to re-heat them from time to time. They should be brought as
close to the glass as is possible without touching it, and, when no
longer needed, should be extinguished by placing the lighted end under
sand, or some other incombustible powder, for they must not be wetted.
2. A method much practised by the makers of sheet glass, and suitable
for large objects, is to wrap a thread of hot glass round the tube, at
once removing it, and touching any point of the glass which the thread
covered with water or a cold iron, when a crack will be started and will
pass round the glass where it was heated by the thread.
3. Tubes which are large and slightly conical may have a ring of red-hot
iron passed over them till it comes into contact with the glass, then,
the iron being removed, and a point on the heated glass being at once
touched with cold iron as before, it will break as desired. Or a string,
moistened with turpentine, may be loosely twisted round the tube, and
the turpentine ignited, afterwards the application of sudden cold to any
point on the zone of hot glass will usually start a crack, which, if
necessary, may be continued in the usual manner. The last three methods
are chiefly useful in dealing with the largest and thickest tubes, and
with bottles.
A fairly stout copper wire, bent into the form of a bow so that it can
be applied when hot to a considerable surface of a glass tube, will be
found superior to the point of hot glass or metal usually employed, for
leading cracks in glass tubes. With such a wire a tube can be cut so
that the cross section of the end is at any desired angle to the axis of
the tube, with considerable precision. I am indebted for this suggestion
to Mr. Vernon Boys and Dr. Ebert.
=Bending Glass Tubes.=--The blow-pipe flame is not a suitable source of
heat for bending tubes, except in certain cases which will be mentioned
in a subsequent paragraph. For small tubes, and those of moderate size,
a fish-tail burner, such as is used for purposes of illumination, will
answer best. Use a flame from one to two inches in breadth--from _A_ to
_A_ (Fig. 6), according to the size of the tube which is to be bent. If
the length of tube that is heated be less than this, the bend will
probably buckle on its concave side.
[Illustration: FIG. 6.]
The tube to be heated should be held in the position shown in Fig. 6,
supported by the hands on each side. It should be constantly rotated in
the flame, that it may be equally heated on all sides. In the figure the
hands are represented above the tube, with their backs upwards. A tube
can be held equally well from below, the backs of the hands being then
directed downwards, and this, I think, is the more frequent habit. It is
difficult to say which position of the hands is to be preferred. I
lately observed how a tube was held by three skilful amateurs and by a
professional glass-blower. All the former held the tube with the hands
below it. The latter, however, held it from above, as in Fig. 6. He,
however, was working with a rather heavy piece of tube, and I am
inclined myself to recommend that position in such cases. During a long
spell of work, the wrist may be rested from time to time by changing the
position of the hands.
When the tube has softened, remove it from the flame, and gently bend
it to the desired angle. The side of the tube last exposed to the flame
will be slightly hotter, and therefore softer, than that which is
opposite to it. This hotter side should form the concave side of the
bent tube.
[Illustration: FIG. 7.]
The exact condition in which the glass is most suitable for bending can
only be learned by making a few trials. If it is too soft in consequence
of being overheated, the sides will collapse. If, in the endeavour to
heat the side _A_ of Fig. 7 a little more than _B_, _B_ is
insufficiently heated, the tube will be likely to break on the convex
side _B_. If the bent tube be likely to become flattened, and this
cannot always be prevented in bending very thin tubes, the fault may be
avoided by blowing gently into one end of the tube whilst bending it,
for which purpose the other end should be closed beforehand. A tube
already flattened may, to some extent, be blown into shape after
closing one end and re-heating the bent portion, but it is not easy to
give it a really good shape.
When making a bend like that in Fig. 7, to secure that the arms of the
tube _C_ and _D_, and the curve at _B_, shall be in one plane, the tube
should be held in a position perpendicular to the body, and brought into
the position shown in the figure during bending, by which means it will
be found easy to secure a good result. Lead glass tubes must be removed
from the flame before they become hot enough to undergo reduction. If
they should become blackened, however, the stain may be removed by
re-heating in the oxidising flame (see p. 18).
When a very sharp bend is to be made, it is sometimes best to heat a
narrow zone of the glass rather highly in the blow-pipe flame, and to
blow the bend into shape at the moment of bending it, as previously
described, one end having been closed for that purpose beforehand. Lead
glass should be heated for this purpose in the oxidising flame (pp. 17
to 22).
The processes of bending large tubes, making U-tubes and spiral tubes,
are more difficult operations, and will be explained in Chap. IV.
=Rounding and Bordering the Ends of Tubes.=--After cutting a piece of
glass tube in two pieces, the sharp edges left at its ends should be
rounded by holding them in a flame for a few moments till the glass
begins to melt. The oxidising point of a pointed flame may be used for
both kinds of glass. The flame will be coloured yellow by soda glass at
the moment of melting. This indication of the condition of soda glass
should be noted, for it serves as a criterion of the condition of the
glass. The ends of soda glass tubes may also be rounded in the flame of
a common Bunsen's burner.
When the end of a tube is to be closed with a cork or stopper, its
mouth should be expanded a little, or =bordered=. To do this, heat the
end of the tube by rotating it in the flame till it softens, then remove
it from the flame, at once introduce the charcoal cone (Fig. 5, p. 11),
and rotate it with gentle pressure against the softened glass till the
desired effect is produced. In doing this it is very important that the
end of the tube shall be uniformly heated, in order that the enlargement
shall be of regular form. If the tube cannot be sufficiently expanded at
one operation, it should be re-heated and the process repeated.
Borders, such as are seen on test-tubes, are made by pressing the
softened edge of the tube against a small iron rod. The end of the rod
should project over the softened edge of the tube at a slight angle, and
be pressed against it, passing the rod round the tube, or rotating the
tube under the rod.
=Sealing=, that is closing the ends of tubes, or other openings, in
glass apparatus.
In performing this and all the other operations of glass blowing, the
following points must be constantly kept in mind:--
(_a._) That it is rarely safe to blow glass whilst it is still in the
flame, except in certain special cases that will be mentioned
subsequently. Therefore always remove apparatus from the flame before
blowing.
(_b._) That when heating glass tubes, unless it is specially desired to
heat one portion only, the tube must be constantly rotated in the flame
to ensure that it shall be uniformly heated, and to prevent the tube or
mass of glass from assuming an irregular form.
(_c._) Always blow gently at first, and slowly increase the force
applied till you feel or see the glass giving way. It is a good plan to
force the air forward in successive short blasts rather than in one
continued stream.
(_d._) When it is necessary to force air into tubes of fine bore, such
as thermometer tubes, the mouth must not be used, for moisture is
thereby introduced into the tube, which it is very difficult to remove
again in many cases. All tubes of very small bore should be blown with
the aid of an india-rubber blowing-bottle, such as are used for
spray-producers, Galton's whistles, etc. The tube to be blown must be
securely fixed to the neck of the bottle, which is then held in one
hand, and air is forced from it into the tube as it is required. These
bottles are frequently of service to the glass-blower--_e.g._, when
tubes of very fine bore have to be united, it is necessary to maintain
an internal pressure slightly exceeding that of the air throughout the
operation, in order to prevent the viscous glass from running together
and closing the tube. An india-rubber blowing-ball is very convenient
for this purpose.
To seal the end of a glass tube (Fig. 8), adjust the flame so that it
will heat a zone of glass about as broad as the diameter of the tube to
be sealed (see _A_, Fig. 8). Hold the tube on each side of the point
where it is to be sealed in the manner described in the description of
bending glass tubes (p. 28). Bring the tube gradually into the flame,
and heat it with constant rotation, till the glass softens (for lead
glass the oxidising flame must be used, as has been already
explained).[6] When the glass begins to thicken, gently pull asunder the
two ends, taking care not to pull out the softened glass too much, but
to allow the sides to fall together, as shown at _A_. When this has
occurred, heat the glass at the narrow part till it melts, and pull
asunder the two ends. The closed end should present the appearance
shown at _D_. If the glass be drawn out too quickly its thickness will
be unduly reduced, and it will present the appearance shown at _B_. In
that case apply a pointed flame at _b_, and repeat the previous
operation so as to contract the tube as at _c_, taking care not to allow
the glass to become much increased nor decreased in thickness.
[6] Remember that when the lead glass is heated to the proper
temperature it will present an appearance which may be described as a
greenish phosphorescence. At higher temperatures it assumes an
orange-red appearance. If it loses its transparency and assumes a dull
appearance, it must be moved further into the oxidising parts of the
flame.
If a considerable mass of glass be left at _d_, it may be removed by
heating it to redness, touching it with the pointed end of a cold glass
tube, to which it will adhere, and by which it may be pulled away.
[Illustration: FIG. 8.]
When the end of the tube presents the appearance shown in the diagram
_D_, and the mass of glass at _d_ is small, the small lump that remains
must be removed by heating it till it softens, and _gently_ blowing with
the mouth, so as to round the end and distribute the glass more
regularly, as shown in _E_. The whole end, from the dotted line _e_,
must then be heated with constant rotation in the flame. If this final
heating of the end _e_ be done skilfully, the glass will probably
collapse and flatten, as at _F_. The end must then be gently blown into
the form shown at _G_.
If a flat end to the tube be desired, the tube may be left in the
condition shown by _F_, or a thin rounded end may be flattened by
pressure on a plate of iron.
If a concave end be wished for, it is only necessary to gently suck air
from the tube before the flattened end has become solid.
In each case, _immediately_ after the tube is completed, it must be
closely wrapped in cotton wool and left to cool. With good lead glass
this last process, though advantageous, is not absolutely necessary; and
as glass cools slowly when enveloped in cotton wool, this precaution may
frequently be neglected in the case of apparatus made from lead glass.
[Illustration: FIG. 9.]
In order to draw out tubes for sealing, close to one end, and thus to
avoid waste of material, it is a good plan to heat simultaneously the
end of the glass tube _A_ which is to be sealed, and one end of a piece
of waste tube _E_ of about the same diameter, and when they are fused to
bring them together as at _DD_ (Fig. 9). _E_ will then serve as a handle
in the subsequent operations on _A_. Such a rough joint as that at _D_
must not be allowed to cool too much during the work in hand, or _E_ and
_A_ may separate at an inconvenient moment. Or the glass at the end of
the tube may be pressed together to close the tube, and the mass of
glass may be seized with a pair of tongs and drawn away.
=Choking, or Contracting the Bore of a Glass Tube.=--If it be not
desired to maintain the uniformity of external dimensions of the tube
whilst decreasing the diameter of the bore, the tube may be heated and
drawn out as described in the description of sealing tubes on pp. 32-35.
This may be done as shown at _A_ or _B_ in Fig. 8, according to the use
to which the contracted tube is to be put.
[Illustration: FIG. 10.]
Greater strength and elegance will be secured by preserving the external
diameter of the tube unchanged throughout, as shown in Fig. 10. For this
purpose heat the tube with the pointed flame, if it be small, or in the
brush flame if it be of large size, constantly rotating it till the
glass softens and the sides show an inclination to fall together, when
this occurs, push the two ends gently towards _A_. If the tube should
become too much thickened at _A_, the fault may be corrected by removing
it from the flame and gently pulling the two ends apart till it is of
the proper size. If the bore at the contracted part of the tube should
become too much reduced, it may be enlarged by closing one end of the
tube with a small cork, and blowing gently into the open end after
sufficiently heating the contracted part. The tube should be rotated
during blowing or the enlargement produced may be irregular.
When the external diameter of the tube is to be increased as well as its
bore diminished, press together the ends of a tube heated at the part to
be contracted, as already described, and regulate the size of the bore
by blowing into the tube if at any time it threatens to become too much
contracted.
=Widening Tubes.=--Tubes may be moderately expanded at their extremities
by means of the charcoal cone (see Bordering, p. 31). They may be
slightly expanded at any other part by closing one end and gently
blowing into the open end of the tube, after softening the glass at the
part to be widened before the blow-pipe. But the best method of
obtaining a wide tube with narrow extremities (Fig. 11) is to join
pieces of narrow tube _AA_ to the ends of a piece of wider tube _B_ of
the desired dimensions. The method of performing this operation is
described under welding, on pp. 39-47.
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
=Piercing Tubes.=--The glass-blower very frequently requires to make a
large or small opening in some part of a tube or other piece of
apparatus. This is known as piercing. Suppose it is desired to make a
small hole at the point _a_ in _A_ (Fig. 12). When the tube has been
brought to the flame with the usual precautions, allow the end of the
pointed flame to touch it at _a_ till an area corresponding to the
desired size of the opening is thoroughly softened. Then expand the
softened glass by blowing to the form shown at _B_. Re-heat _a_, blow a
small globe as at _C_, and carefully break the thin glass, then smooth
the rough edges by rotating them in the flame till they form a mouth
like that of _D_. Instead of leaving the bulb to be broken at the third
stage _C_, it is a good plan to blow more strongly, so that the bulb
becomes very thin and bursts, the removal of the thin glass is then
accompanied by less risk of producing a crack in the thicker parts of
the glass. Openings may be made in a similar manner in the sides of
tubes or in globes, in fact, in almost any position on glass apparatus.
If another tube is to be attached at the opening, it is a good plan to
proceed to this operation before the tube has cooled down.
[Illustration: FIG. 13.]
The openings obtained by the method above described are too large when
platinum wires are to be sealed into them. Suppose that it is necessary
to pierce the tube _A_ of Fig. 13 in order to insert a platinum wire at
_a_; direct the smallest pointed flame that will heat a spot of glass to
redness on the point _a_. When the glass is viscous, touch it with the
end of a platinum wire _w_, to which the glass will adhere; withdraw the
wire and the viscous glass will be drawn out into a small tube, as shown
at _B_; by breaking the end of this tube a small opening will be made.
Introduce a platinum wire into the opening, and again allow the flame to
play on the glass at that point; it will melt and close round the wire.
Before the hot glass has time to cool, blow gently into the mouth of the
tube to produce a slightly curved surface, then heat the neighbouring
parts of the tube till the glass is about to soften, and let it cool in
cotton wool. Unless this is done, I find that glass tubes into which
platinum wires have been sealed are very apt to break during or after
cooling.
To ensure that the tube shall be perfectly air-tight, a small piece of
white enamel should be attached to the glass at _a_ before sealing in
the wire.
=Uniting Pieces of Glass to Each Other, known as Welding, or
Soldering.=--The larger and more complicated pieces of glass apparatus
are usually made in separate sections, and completed by joining together
the several parts. This is therefore a very important operation, and
should be thoroughly mastered before proceeding to further work.
In order to produce secure joints, the use of tubes made of different
kinds of glass must be avoided. Soda glass may be joined securely to
soda glass, especially if the tubes belong to the same batch, and lead
glass to lead glass. But, though by special care a joint between lead
glass and soda glass, if well made, will often hold together, yet it is
never certain that it will do so.
_To join two Tubes of Equal Diameters._--Close one end of one of the
tubes with a small cork. Heat the open end of the closed tube, and
either end of the other tube in a small flame until they are almost
melted, taking care that only the ends of the tubes are heated, and not
to let the glass be thickened; bring the two ends together with
sufficient pressure to make them adhere, but not sufficient to compress
the glass to a thickened ring. Before the joint has time to cool too
much, adjust your blow-pipe for a pointed flame, if you are not already
working with that kind of flame, and allow the point of the flame to
play on any spot on the joint till it is heated to redness; rotate the
tube a little so as to heat the glass adjacent to that which is already
red-hot, and repeat this till the whole circumference of the rough joint
has been heated.[7] Repeat the operation last described, but, when each
spot is red-hot, blow gently into the open end of the tube so as to
slightly expand the viscous glass. Finally, rotate the whole joint in
the flame till the glass is softened, and blow gently as before into the
open end of the tube, still rotating it, in order that the joint may be
as symmetrical as possible. If in the last operation the diameter of the
joint becomes greater than that of the rest of the tube, it may be
cautiously re-heated and reduced by pulling it out, or this may be
secured by gently pulling apart the two ends, whilst the operator blows
it into its final shape.
[7] Some glass-blowers at once work on the glass as next described,
without this preliminary treatment. I find that some glass, usually soda
glass, will not always bear the necessary movements without breaking
unless first heated all round.
[Illustration: FIG. 14.]
When small tubes, or tubes of fine bore, are to be joined, in order to
prevent the fused glass from running together and closing the tube, it
is a good plan to border and enlarge the ends that are to be united, as
at _A_ (Fig. 14). Some glass-blowers prefer to border all tubes before
uniting them.
When a narrow tube is to be joined to one that is only slightly wider,
expand the end of the narrow tube till it corresponds in size to the
larger tube. If the tube be too narrow to be enlarged by inserting a
charcoal cone, seal one end and pierce it as directed (on p. 37).
For joining small thin-walled tubes Mr. Crookes recommends the use of a
small Bunsen flame.
In welding pieces of lead glass tube, take care that the heated glass is
perfectly free from reduced lead at the moment when the two ends of
viscous glass are brought into contact.
[Illustration: FIG. 15.]
_To join Tubes of Unequal Sizes End to End_ (Fig. 15).--Draw out the
larger tube and cut off the drawn-out end at the part where its diameter
is equal to that of the smaller tube, then seal the smaller tube to the
contracted end of the larger according to the directions given for
joining tubes of equal size. When a good joint has been made, the tube
presents the appearance of _A_, Fig. 15, the union being at about _bb_.
Next heat the whole tube between the dotted lines _aa_, and blow it into
the shape of _B_ in which the dotted line _dd_ should correspond to the
actual line of junction of the two tubes.
In making all joints it is important to leave no thick masses of glass
about them. If the glass be fairly thin and uniformly distributed, it is
less likely to break during or after annealing under any circumstances,
and especially if it has to bear alternations of temperature.
_Joining a Tube to the Side of another Tube_ (Fig. 16).--One of the
tubes must be pierced as at _A_ in Fig. 16 (for the method, see p. 37),
and its two ends closed with small pieces of cork. The edges of the
opening, and one end of the other tube, must then be heated till they
melt, and united by pressing them together. The joint may then be
finished as before.
[Illustration: FIG. 16.]
A properly blown joint will not present the appearance of _B_ (Fig. 16),
but rather that of _C_. This is secured by directing the pointed flame
upon the glass at _aa_ (_B_) spot by spot, and blowing out each spot
when it is sufficiently softened. If the tubes are large, the whole
joint should subsequently be heated and blown, but in the case of small
tubes this is of less importance. Finally it is to be wrapped whilst hot
in cotton wool for the annealing process.
If a second tube has to be joined near to the first one, say at _b_, it
is well to proceed with it before the joint first made cools down, and
the joint first made, especially if soda glass be used, must be held in
the flame from time to time during the process of making the second
joint to keep it hot; if this be not done the first joint is very likely
to break. A joint previously made may, however, be re-heated, if well
made and well annealed.
A three-way tube, like that in Fig. 17, is made by bending _A_ (Fig. 16)
to an angle, and joining _B_ to an opening blown on the convex side of
the angle; or, _A_ of Fig. 16 may be bent as desired after attaching _B_
in the ordinary way.
[Illustration: FIG. 17.]
Tubes may also be joined to openings made in the sides of globes or
flasks; great care must be taken, however, especially if the walls of
the globe be thin, to secure that the tube is well attached to the mouth
of the opening when the melted ends are first brought into contact, for,
with thin glass, any hole that may be left will probably increase whilst
the joint is being blown into shape, owing to cohesion causing the glass
to gather in a thickened ring round an enlargement of the original
opening.[8]
[8] If such an opening be observed, it may usually be closed by touching
its edges with a fused point of glass at the end of a drawn out tube.
In order to unite a tube of soda glass to a tube of lead glass, the end
of the soda glass tube must be carefully covered with a layer of soft
arsenic glass.[9] This must be done so perfectly that when the ends to
be united are brought together the lead and soda glass are separated by
the enamel at every point.
[9] This can be obtained from Messrs. Powells, Whitefriars Glassworks.
_To Seal a Tube inside a Larger Tube or Bulb._--Suppose that an air-trap
(3 of Fig. 18) is to be constructed from a small bulb (_A_) blown on a
glass tube (1).
[Illustration: FIG. 18]
Either cut off the tube close to the bulb at _B_, or better, remove the
end by melting the glass and pulling it away from _B_, and then pierce
_A_ at _B_, No. 2, by heating the glass there and blowing out a small
bulb as described under Piercing.
Prepare a tube (4) drawn out at _E_ with a bulb blown at _D_. Insert _E_
into the opening _B_, press _D_ well against the mouth _B_ and slowly
rotate before the blow-pipe till _D_ adheres to _B_. Then heat and blow
the joint spot by spot as in other cases, taking care that the glass is
blown out on each side of the joint; lastly, heat the whole joint
between _aa_, and blow it into its final shape.
These joints are very apt to break after a few minutes or hours if the
glass of _D_ be much thicker than that of the bulb _A_. They should be
wrapped in cotton wool for annealing as soon as possible, as the rate at
which the tube _E_ cools is likely to be less rapid than that of the
parts of the apparatus which are more freely exposed to the air;
therefore all such internal joints require very careful annealing, and
they should always be made as thin as is consistent with the use to
which they are to be put.
Tubes may also be sealed into the ends or sides of larger tubes by
piercing them at the point at which the inserted tube is to be
introduced, and proceeding as in the case of the air-trap just
described.
Ozone generators of the form shown on next page (Fig. 19), afford an
interesting example of the insertion of smaller tubes into larger.
On account of the small space that may be left between the inner and
outer tubes of an ozone generator, and of the length of the inner tube,
its construction needs great care. I find the following mode of
procedure gives good results. Select the pieces of tube for this
instrument as free from curvature as possible. For the inner tube, a
tube 12 mm., or rather more, in external diameter, and of rather thin
glass, is drawn out, as for closing, until only a very narrow tube
remains at _C_, the end of _C_ is closed the area round _C_ is
carefully blown into shape, so that by melting off _C_ the tube _A_ will
be left with a well-rounded end. A small bulb of glass is next blown on
_A_ at _B_. This bulb must be of slightly greater diameter than the
contracted end _E_ of the larger tube (II.), so that _B_ will just fail
to pass through _E_. The length from _B_ to _C_ must not be made greater
than from _E_ to _G_ on the outside tube. The end at _C_ is then to be
cut off so as to leave a pin-hole in the end of _A_.
[Illustration: FIG. 19.]
The outer tube (II.), whose diameter may be 5 or 6 mm. greater than that
of _A_, is prepared by sealing a side tube on it at _F_, after
previously contracting the end _E_. For this purpose the end _E_ should
be closed and rounded, and then re-heated and blown out till the bulb
bursts. To ensure that the diameter of the opening is less than that of
the tube, care must be taken not to re-heat too large an area of the end
before blowing it out. It is very important that the cross section at
_E_ shall be in a plane at right angles to the axis of the tube.
Wrap a strip of writing paper, one inch in breadth, closely round the
end of _A_ at _C_ till the tube and paper will only just pass easily
into the mouth _D_ of the outer tube, push the inner tube _A_, with the
paper upon it, into _D_, and when the paper is entirely within _D_,
withdraw _A_, and cautiously push the paper a little further into the
outer tube. Insert _A_ into _DE_ through _E_, so that the bulb _B_ is
embraced by _E_. Close _D_ with a cork. Ascertain that the paper does
not fit sufficiently tightly between the two tubes to prevent the free
passage of air, by blowing into the mouth _K_ of _A_. Air should escape
freely from _E_ when this is done. Gradually bring the line of contact
of _B_ and _E_ and the surrounding parts of the tube before a pointed
flame, after previously warming them by holding near a larger flame, and
rotate them before the flame so that the glass may soften and adhere.
Then heat the joint spot by spot as usual. In blowing this joint, take
care that the glass on each side of the actual joint is slightly
expanded. It should present the form shown by the dotted lines in III.
(these are purposely exaggerated, however). Finally, heat the whole
joint between the lines _JI_ till it softens, and simultaneously blow
and draw it into its final shape as seen at III.
The side tube _F_ should not be too near the end _E_. If, however, it is
necessary to have them close together, the joint _F_ must be very
carefully annealed when it is made; it must also be very cautiously
warmed up before the construction of the joint at _H_ is begun, and must
be kept warm by letting the flame play over it from time to time during
the process of making the latter joint.
A good joint may be recognised by its freedom from lumps of glass, its
regularity of curve, and by a sensibly circular line at _H_, where the
two tubes are united.
When the joint after annealing has become quite cold, the pin-hole at
_C_ on the inner tube may be closed, after removing the paper support,
by warming the outer tube, and then directing a fine pointed flame
through _D_ on to _C_. And the end _D_ of the outer tube may be closed
in the ordinary manner, or a narrow tube may be sealed to it. As the end
of glass at _D_ will be too short to be held by the fingers when hot,
another piece of tube of similar diameter must be attached to it to
serve as a handle (see p. 35, Fig. 9).
=Blowing a Bulb or Globe of Glass.=--For this purpose it is very
important that the glass tube employed shall be of uniform substance.
The size and thickness of the tube to be employed depends partly on the
dimensions of the bulb desired, and partly on the size of neck that is
required for the bulb. It is easier to blow large bulbs on large-sized
tubes than on those of smaller size. When it is necessary to make a
large globe on a small tube, it can be done, however, if great care be
taken to avoid overheating that part of the small tube which is nearest
to the mass of viscous glass from which the bulb is to be formed. For
the purpose of blowing a very large bulb on a small tube, it is best to
unite a wide tube to that which is to serve as the neck, as it will save
some time in collecting the necessary mass of glass from which to form
the globe.
[Illustration: FIG. 20.]
_To blow a Bulb at the End of a Tube._--Select a good piece of tube, say
1·5 cm. in diameter, and about 30 cm. long; draw out one end to a light
tail (_a_, Fig. 20) about 3 inches in length. Then heat up a _short_
length of the tube at _b_, with a small brush flame, by rotating the
glass in the flame, and gently press it together when soft to thicken
it; blow into it if necessary to preserve the regularity of its figure.
Repeat this process on the portion of tube nearest to that which has
been first thickened, and so on, till as much glass has been heated and
thickened as you judge will serve to make a bulb of the size desired.
You should have a mass of glass somewhat resembling that shown at _B_
(Fig. 20), but probably consisting of the results of more successive
operations than are suggested in that diagram. Apply the flame as before
to the narrower parts _cc_ of _B_, gently compress and blow until all
the small bulbs first made are brought together into a mass still
somewhat resembling the enlarged end of _B_, but more nearly
cylindrical, with the glass as regularly distributed as possible, and of
such length from _d_ to the contracted part that the whole of it may
easily be heated simultaneously with the large brush flame of your
blow-pipe. Take great care in the foregoing operations not to allow the
sides of the mass of glass to fall in and run together, and, on the
other hand, do not reduce the thickness of the glass needlessly by
blowing it more than is necessary to give the glass as regular a form as
possible. When you are satisfied with the mass of glass you have
collected, melt off the tail _a_, and remove the pointed end of glass
that remains, as directed on page 33. Turn on as large a brush flame as
is necessary to envelop the whole mass of glass that you have collected,
and heat it with constant rotation, so that it may gradually run
together to the form seen at _C_ (Fig. 20), taking care that it does not
get overheated near _d_, or the tube which is to form the neck will
soften and give way.
The position in which the mass of heated glass is to be held will depend
upon circumstances; if the mass of glass be not too great, it is best to
keep it in a nearly horizontal position. If the mass of glass be very
large, it may be necessary to incline the end _B_ downwards; but as that
is apt to result in an excess of glass accumulating towards _d_, avoid
doing so if possible by rotating the glass steadily and rapidly. If at
any time the glass shows indications of collapsing, it must be removed
from the flame and gently blown into shape, during which operation it
may be rotated in the perpendicular position; indeed, to promote a
regular distribution of the glass by allowing it plenty of time to
collect, it is well from time to time to remove the heated mass of glass
from the flame, and slightly expand it by blowing. Finally, when a
regular mass of glass, such as is shown at _C_ (Fig. 20) has been
obtained, remove it from the flame, and blow it to its final dimensions.
A succession of gentle puffs _quickly_ succeeding each other should be
employed, in order that the progress of the bulb may be more easily
watched and arrested at the right moment. During the process of blowing,
the hot glass must be steadily rotated.
To collect the glass for blowing a bulb of lead glass, employ the flame
described on pp. 17-22 for heating lead glass.
If the tube be held horizontally whilst the globe is blown, its form
will most nearly approach that of a true globe. If it be held in the
perpendicular position, with the mass of glass depending from it, the
form of the bulb will usually be somewhat elongated. If it be held
perpendicularly, with the mass of glass upwards, the resulting bulb will
be flattened.
When a bulb is not of a sufficiently regular form, it may sometimes be
re-made by re-collecting the glass, and re-blowing it. The greatest care
is needed at the earlier stages of re-heating to prevent the glass from
collapsing into a formless and unworkable mass. This is to be prevented
in all such cases by gently blowing it into shape from time to time
whilst gathering the glass.
[Illustration: FIG. 21.]
_To blow a Bulb between two Points_ (Fig 21).--Select a piece of
suitable tube, seal or cork one end, gather together a mass of glass at
the desired part, as directed for blowing a bulb at the end of a tube;
when a mass of glass has been collected of sufficient thickness, blow it
into shape from the open end of the tube by a rapid succession of short
blasts of air, till the expanding glass attains the desired dimensions.
The tube must be held horizontally, and must be rotated steadily during
the process. By slightly pressing together the glass while blowing, the
bulb will be flattened; by slightly drawing apart the two ends of the
tube, it will be elongated.
A pear-shaped bulb may be obtained by gently re-heating an elongated
bulb, say from _a_ to _a_, and drawing it out. It is easiest to perform
this operation on a bulb which is rather thick in the glass.
If the tubes _bb_ are to be small, and a globe of considerable size is
wanted, contract a tube as shown in Fig. 22, taking care that the narrow
portions of the tube are about the same axis as the wider portions, for
if this be not the case, the mouths of the bulb will not be
symmetrically placed; seal at _C_, cut off the wider tube at _B_, and
make the bulb, as previously described, from the glass between _AA_.
If, as probably will be the case, the contracted portions of the tube be
not very regular, they may be cut off, one at a time, near the bulb, and
replaced by pieces of tube of the size desired.
[Illustration: FIG. 22.]
When a bulb has to be blown upon a very fine tube, for example upon
thermometer tubing, the mouth should not be employed, for the moisture
introduced by the breath is extremely difficult to remove afterwards. A
small india-rubber bottle or reservoir, such as those which are used in
spray-producers, Galton's whistles, etc., securely attached to the open
end of the tube, should be used. With the help of these bottles bulbs
can be blown at the closed ends of fine tubes with ease, though some
care is necessary to produce them of good shape, as it is difficult to
rotate the hot glass properly when working in this way.
=Making and Grinding Stoppers.=--Apparatus which is to contain chemicals
that are likely to be affected by the free admission of air, needs to
have stoppers fitted to it. Making a good stopper is a much less tedious
process than is commonly supposed.
Suppose that the tube I. of Fig. 23 is to be stoppered at _A_, it must
be slightly enlarged by softening the end and opening it with a pointed
cone of charcoal; or a conical mouth for the stopper may be made by
slightly contracting the tube near one end, as at _B_, cutting off the
cylindrical end of the tube at the dotted line _C_, and then very
slightly expanding the end at _C_ with a charcoal cone after its edges
have been softened by heat. In either case the conical mouth should be
as long and regular as possible.
[Illustration: FIG. 23.]
For the stopper take a piece of rather thick tube, of such size that it
will pass easily, but not too easily, into _A_ or _B_. Expand this tube
at _D_, as shown in II., by softening the glass and gently compressing
it. The configuration of the enlarged tube as shown at _D_ may be
obtained by heating and compressing two or more zones of the tube that
are adjacent, one zone being less expanded than the other, so as to give
the sides of the imperfect stopper as nearly as possible the form shown
at _D_, which, however, is much less regular than may easily be
obtained. Seal off the head of the tube at _H_, and heat the glass till
it runs together into a nearly solid mass; compress this with a pair of
iron tongs to the flattened head _E_. In making _D_, aim at giving it a
form which will as nearly as possible correspond to that of the tube
into which it is to be ground, and make it slightly too large, so that
only the lower part at _D_ can be introduced into the mouth of _A_ or
_B_. Before it is ground, the stopper must be heated nearly to its
softening-point and annealed.
Moisten _D_ with a solution of camphor in recently distilled
turpentine, and dust the wet surface with finely-ground emery, then
gently grind it into its place till it fits properly. In this operation
the tail _G_, which should fit loosely into the tube _A_, will be of
assistance by preventing _D_ from unduly pressing in any direction on
_A_ in consequence of irregular movements. The stopper should be
completely rotated in grinding it. It must not be worked backwards and
forwards, or a well-fitting stopper will not be produced. Renew the
emery and camphorated turpentine frequently during the earlier part of
the grinding; when the stopper almost fits, avoid using fresh emery, but
continue to remove the stopper frequently at all stages of the
operation. That added at the earlier stages will be reduced to a state
of very fine division, and will therefore leave the stopper and mouth of
_A_ with smoother surfaces than fresh emery.[10]
[10] Mr. Gimmingham recommends giving stoppers a final polish with
rotten-stone (_Proceedings of the Royal Society_, p. 396, 1876).
NOTE.--The addition of camphor to the turpentine used for grinding glass
is very important. Notwithstanding its brittle nature, glass will work
under a file moistened with this solution almost as well as the metals.
Small quantities should be made at a time, and the solution should be
kept in a well-closed vessel, for after long exposure to the air it is
not equally valuable.
If the stopper is to fit a tube contracted like _B_, it must be
constructed from a piece of tube that will pass through the contraction
at _B_. The tail _GF_ will not do such good service as it does in the
case of a tube which has been opened out to receive its stopper, but it
will help to guide the stopper, and should be retained.
When the stopper has been ground into its place, melt off the tail at
_F_. The flame must be applied very cautiously, as glass which has been
ground is particularly apt to crack on heating. To avoid all risk of
this, the tail may simply be cut off, and its edges filed smooth with a
file moistened freely with camphorated turpentine.
The stoppers of bottles are not made exactly in the manner described
above, though, on occasion, a new stopper may be made for a bottle by
following those directions. Ill-fitting stoppers, which are very common,
can be very easily re-ground with emery and camphorated turpentine.
CHAPTER IV.
_MAKING THISTLE FUNNELS, U-TUBES, ETC.--COMBINING THE PARTS OF
COMPLICATED APPARATUS--MERCURY, AND OTHER AIR-TIGHT JOINTS--VACUUM
TAPS--SAFETY TAPS--AIR-TRAPS._
In Chapter III. the simpler operations used in making the separate parts
of which apparatus is composed have been described. In this Chapter
finished apparatus will be described, and the combination of the
separate parts into the more or less complicated arrangements used in
experiments will be so far explained as to enable the student to set up
such apparatus as he is likely to require. I have thought it would be
useful that I should add a short account of various contrivances that
have come much into use of late years for experimenting under reduced
pressure, such as safety taps, air-traps, vacuum joints, etc.
[Illustration: FIG. 24.]
=Electrodes.=--On page 38 (Fig. 13) is shown a simple form of electrode
sealed into a glass tube, which for many purposes answers very well. But
frequently, in order that there may be less risk of leakage between the
glass and the metal, the latter is covered for a considerable part of
its length with solid glass, which at one extremity is united to the
apparatus. In Fig. 24 _W_ is the metal core of the electrode, and _G_
the glass covering around it. The wire is fused into the glass, and the
glass is then united to the apparatus; a little white enamel should be
applied at one end and combined with the glass by fusion.
=U-Tubes.=--A U-tube is but a particular case of a bent glass tube. It
is scarcely possible when bending very large tubes in the manner
described on p. 29 to produce regular curves of sufficient strength.
To make a U-tube, or to bend a large tube, close one end of the tube
selected with a cork, soften and compress the glass in the flame at the
part where it is to be bent till a sufficient mass of glass for the bend
is collected, then remove the mass of glass from the flame, let it cool
a little, and simultaneously draw out the thickened glass, bend it to
the proper form, and blow the bend into shape from the open end of the
tube. Small irregularities may be partly corrected afterwards.
To make a good U-tube of large size, and of uniform diameter from end to
end, requires much practice, but to make a tolerably presentable piece
of apparatus in which the two limbs are bent round till they are
parallel, without any considerable constriction at the bend, can be
accomplished without much difficulty.[11]
[11] Large tubes may also be bent by rotating a sufficient length of the
tube in a large flame till it softens, and bending in the same manner as
in the case of smaller tubes, and after filling them with sand, closing
one end completely, and the other so that the sand cannot escape, though
heated air can do so.
=Spiral Tubes.=--These may be made by twisting a tube gradually softened
by heat round a metal cylinder. Spiral tubes made of small thin tubes
possess considerable elasticity, and have been used by Mr. Crookes for
making air-tight connections between separate pieces of apparatus when a
rigid connection would have been unnecessary and inconvenient. By the
use of such spiral tubes it is possible to combine comparatively free
movement with all the advantages attached to hermetically-sealed joints.
To make a flexible spiral tube, mount a copper cylinder on a screw, so
that the cylinder will travel in the direction of its axis when it is
rotated. Fix a fine glass tube to the cylinder, and direct a flame
towards the cylinder so as to heat and soften the glass, which will then
bend to the form of the cylinder. Gradually rotate the cylinder before
the source of heat, so that fresh portions of tube are successively
brought into position, softened, and bent. Useful spirals may also be
made by hand without a cylinder. As each length of tube is bent, a fresh
length may be united to it until the spiral is completed. The fine tubes
employed are prepared by heating and drawing out larger tubes.
[Illustration: FIG. 25.]
=Thistle Funnels= (Fig. 25).--Seal a moderately thick piece of small
glass tube at _A_, then heat a wide zone of it a little below _A_ by
rotating it horizontally in the blow-pipe flame till the glass softens,
and expand the glass to a bulb, as shown at _B_ of 1; during the
operation of blowing this bulb, the end _A_ must be directed to the
ground.
Soften the end _A_ and a small portion of _B_ as before, and, holding
the tube horizontally from the mouth, blow out the end _C_ as at 2. Heat
the end of _C_ gradually, till the glass softens and collapses to the
dotted line _dd_, and at once blow a steady stream of air into the open
end of the tube, rotating it steadily, till it is about to burst;
finally clean off the thin glass from round the edges of the funnel,
which should have the form shown at 3, and round them. An inspection of
a purchased thistle funnel will generally show that the head _B_ has
been formed from a larger tube sealed to _E_ at _f_.
[Illustration: FIG. 26.]
=Closing Tubes containing Chemicals= for experiments at high
temperatures.--Tubes of the hard glass used for organic analyses answer
best for this purpose; the operation of drawing out the end of such a
tube is practically identical with what has been described under the
head of choking, p. 35. A well-sealed tube presents the appearance of
that shown by Fig. 26.
In order to secure a thick end to the point of the tube _a_, about an
inch or so of the tube near the contracted part should be warmed a
little, if it is not already warm, at the moment of finally sealing it;
the contraction of the air in the tube, in consequence of the cooling of
the warm tube, will then ensure the glass at _a_ running together to a
solid end when it is melted in the flame.
If it will be necessary to collect a gas produced during a chemical
action from such a tube, make the contracted end several inches long,
and bend it into the form of a delivery tube. It will then be possible
to break the tip of this under a cylinder in a trough of liquid.
=In order to explain the construction of apparatus consisting of several
parts=, it will be sufficient to take as examples, two very well-known
instruments, and to describe their construction in detail. From what is
learned in studying these, the student will gather the information that
is wanted.
[Illustration: FIG. 27.]
1. _To make Hofman's Apparatus for the electrolysis of water_ (Fig. 27).
Take two tubes about 35 cm. in length, and 14 mm. in diameter for _AA_,
join taps _TT_ to the end _B_ of each of them, draw out the other end,
as shown at _D_, after sheets of platinum foil with wires attached to
them[12] have been introduced into the tubes, and moved by shaking to
_BB_. Then allow the platinum wires to pass through the opening _D_ left
for the purpose, and seal the glass at _D_ round the platinum as at _E_.
Pierce the tubes at _JJ_, and join them by a short piece of tube _K_,
about 14 mm. in diameter, to which the tube _T_, carrying the reservoir
_R_, has been previously united. _R_ may be made by blowing a bulb from
a larger piece of tube attached to the end of _T_. The mouth _M_ of the
reservoir being formed from the other end of the wide tube afterwards.
One of the taps can be used for blowing through at the later stages.
Each joint, especially those at _JJ_, must be annealed after it is
blown. Some operators might prefer to join _AA_ by the tube _K_ in the
first instance, then to introduce the electrodes at _E_ and _D_. In some
respects this plan would be rather easier than the other, but, on the
whole, it is better to make the joints at _JJ_ last in order, as they
are more apt to be broken than the others during the subsequent
manipulations.
[12] Red-hot platinum welds very well. The wire may be joined to the
sheet of foil by placing the latter on a small piece of fire-brick,
holding the wire in contact with it at the place where they are to be
united, directing a blow-pipe flame upon them till they are at an
intense heat, and smartly striking the wire with a hammer. The blow
should be several times repeated after re-heating the metal.
2. I have before me the vacuum tube shown by Fig. 28, in which the
dotted lines relate to details of manipulation only.
[Illustration: FIG. 28.]
It is usually possible to detect the parts of which a piece of apparatus
has been built up, for even the best-made joints exhibit evidence of
their existence. Thus, although I did not make the tube that is before
me, and cannot therefore pretend to say precisely in what order its
parts were made and put together, the evidence which it exhibits of
joints at the dotted lines _A_, _B_, _C_, _D_, _E_, _F_, enables me to
give a general idea of the processes employed in its construction, and
to explain how a similar tube might be constructed. I should advise
proceeding as follows:--
Join a piece of tube somewhat larger than _M_ to its end _A_, draw out
the other end of the larger tube, and blow a bulb _L_ as directed on p.
47. Then seal the electrode _R_ into the bulb _L_ (p. 55).
Blow a similar but larger bulb _N_ from a large piece of tube sealed
between two tubes of similar size to _M_, as described at p. 50. Cut off
one of the tubes at _B_, and join the bulb _N_ to _M_ at _B_. Form the
bulb _Q_ in the same manner as in the case of _L_, seal into it the
electrode _R_, and add the tube marked by the dotted lines at _F_.
Seal a narrow tube _P_ to the end of a larger tube, and blow out the
tube at the joint till the glass is thin and regular. Take a tube _O_,
of similar size to _M_, slightly longer than _P_, contract its mouth
slightly to meet the wide end of _P_ at _D_, and after loosely
supporting _P_ inside _O_ with a cork, or otherwise, close the end _N_
of _O_ by sealing or corking it, and join _P_ to _O_ at _D_. Cut off _O_
just above _D_ at _E_, and join it to the bulb _Q_, closing either _O_
or _F_ for the purpose. Cut off the end of _O_ at _C_ parallel to the
end of _P_, and connect _O_ to _N_, using _F_ for blowing the joint at
_C_. _F_ may be used subsequently for introducing any gas into the tube,
and, when a vacuum has been established, may be sealed before the
blow-pipe.
[Illustration: FIG. 29.]
=Modes of combining the Parts of Heavy Apparatus.=--It is often
necessary to connect pieces of apparatus which are too heavy to be
freely handled before the blow-pipe, and which, therefore, cannot be
welded together as described on p. 39, by some more effective method
than the ordinary one of connecting by india-rubber tubing. For example,
apparatus which is to be exhausted by a Sprengel air-pump must be
attached to the pump by a joint as perfectly air-tight as can be
obtained. This, indeed, often may be done by welding the apparatus to be
exhausted to the air-pump before the blow-pipe. But such a method is
open to the obvious objection that it is very troublesome to connect and
disconnect the parts as often as may be necessary, and that there is
some risk of accidental breakages. Nevertheless it may be done on
occasion, especially if there be no objection to the use of the
flexible spiral tubes already alluded to. When the use of a spiral
connecting-tube is not admissible the difficulty is considerably
increased. For example, the author has lately required to attach an
ozone generator, of the form shown by Fig. 19, which previously had been
cemented into a heavy copper jacket, to a pressure-gauge rigidly fixed
to a support, and of considerable size. The employment of a flexible
spiral connection was prohibited by the fact that it was necessary that
the volume of the connecting-tube should be but a small fraction of that
of the ozone generator, a condition which compelled the use of a tube of
almost capillary bore, and of inconsiderable length. At the same time
the frailness of such a connection made it necessary to fix the
generator and pressure-gauge rigidly to their supports, in order to
avoid the possibility of breakage by slight accidental movements of
either of them, and it was obviously necessary to fix the pieces of
apparatus in their final positions before joining them, lest the fine
tube which connected them should be fractured during adjustment. The
possibility of a strain being caused by the contraction that would occur
during the cooling down of the joint last made had to be provided for
also. The desired object was effected as follows. In Fig. 29 _A_
represents a section of the ozone generator at the point where the tube
to connect it to the gauge was fixed. _B_ represents the top of the
gauge, with the side tube _C_, which was to be connected with that from
_A_, viz. _D_. The ends of _C_ and _D_ were expanded as shown at _D_ (by
melting them and blowing them out), so that one of them, made rather
smaller than the other, could be overlapped by the larger one. _A_ and
_B_ being rigidly fixed in their final positions, with _C_ and _D_ in
contact, as shown in the figure, all openings in the apparatus were
closed, except one, to which was attached an india-rubber blowing-bottle
by means of a tube of india-rubber long enough to be held in the hand of
the operator, and to allow him to observe the operation of joining the
tubes at _D_. When everything was in readiness, a very small-pointed
flame from a moveable blow-pipe held in the hand was directed upon the
glass at _D_ till it melted and the two tubes united. To prevent the
fine tube when melted from running into a solid mass of glass, and so
becoming closed, a slight excess of pressure was maintained inside the
apparatus during the operation by forcing air into it with the
india-rubber blower from the moment at which _C_ and _D_ united. A point
of charcoal was kept in readiness to support the softened glass at _D_
in case it showed any tendency to fall out of shape.
The V-tube at _C_ served to prevent the subsequent fracture of the joint
in consequence of any strain caused by the contraction of the glass in
cooling.[13]
[13] For a method of joining soda glass to lead glass, see p. 81.
It is not difficult to connect several pieces of apparatus successively
in this manner, nor is this method only useful in such cases as that
just described. Pieces of apparatus of great length and weight may be
joined in a similar manner, irrespective of the size of the tubes to be
united.
The ends to be joined, prepared as before, so that one slightly overlaps
the other, must be held firmly in contact by clamps, and heated in
successive portions by a blow-pipe held in the hand of the operator,
each patch of glass being re-heated and gently blown, after a rough
joint has been made. Finally, a larger flame may be used to heat up the
whole joint for its final blowing. It is important to place the
apparatus so that the operator has free access to it on all sides. A
revolving table might be employed. An assistant to work the bellows is
necessary. Or, better still, air may be admitted to the blow-pipe from a
large gas-bag placed in some convenient position.
But in most cases one or other of the following air-tight joints can be
employed, and will be found to be very convenient:--
=Mercury Joints.=--The simplest form of mercury joint is shown at Fig.
30. _A_ and _B_ are the two tubes which are to be connected. A larger
tube or cup _F_ is attached to _A_ by the india-rubber tube _E_, and
placed on _A_ so that the end of _B_ may be brought into contact with
_A_ at _C_, and connected to it by a well-fitting piece of india-rubber
tube _C_. The cup _E_ is then brought into the position shown in Fig.
30, and mercury is introduced till the india-rubber tube at _C_ is
covered. As mercury and glass do not come into true contact, however,
such a joint, though said to give good results in practice, is not
theoretically air-tight, for air _might_ gradually find its way between
the liquid and the glass. By covering the mercury with a little
sulphuric acid or glycerine the risk of this occurring may be removed.
The same result may be attained by the use of glycerine in place of the
mercury in the cup _F_; but glycerine is less pleasant to work with than
mercury.[14]
[14] If the india-rubber tube _C_ be secured by wires, iron wire, not
copper wire, should be employed.
[Illustration: FIG. 30.]
When sulphuric acid is to be employed in such a joint, or when for any
other reason the use of an india-rubber tube is undesirable, the joint
may consist of a hollow stopper _B_ (Fig. 31), made of glass tube, and
ground to fit the neck of a thistle funnel _A_. _A_ and _B_ are joined
respectively to the pieces of apparatus to be connected, and connection
is made by placing _B_ in position in the neck of _A_; the joint is made
air-tight by introducing mercury with strong sulphuric acid above it
into the cup _A_. The joint may be rendered air-tight by introducing
sulphuric acid only into the cup. But this plan must not be adopted if
the interior of the apparatus is to be exhausted, as sulphuric acid is
easily forced between the ground glass surfaces by external pressure.
Mercury, however, will not pass between well-ground glass surfaces, and
is therefore to be employed for connecting apparatus which is to be
exhausted, and, if necessary, protected by a layer of strong sulphuric
acid to completely exclude air.
[Illustration: FIG. 31.]
Tubes placed horizontally may be joined by a glycerine or mercury joint
such as is shown in Fig. 32. The two tubes _A_ and _B_ are joined as
before by an india-rubber connection _C_, or one may be ground to fit
the other, and the joint is then enclosed within a larger jacketing-tube
_D_, with a mouth at _F_, which is filled with glycerine or mercury. _D_
is easily made by drawing out both ends of a piece of tube, leaving them
large enough to pass over the connection at _C_, however, and piercing
one side at _F_.
[Illustration: FIG. 32.]
=Vacuum Taps.=--It is not necessary to enter into a description of the
construction of ordinary glass taps, which can be purchased at very
reasonable prices. It may be remarked here, however, as a great many of
them are very imperfectly ground by the makers, that they may easily be
made air-tight by hand-grinding with camphorated turpentine and fine
emery, finishing with rotten-stone. A well-ground tap, which is well
lubricated, should be practically air-tight under greatly reduced
pressure for a short period; but when it is necessary to have a tap
which absolutely forbids the entrance of air into apparatus, one of the
following may be employed:--
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
(1.) _Mr. Cetti's Vacuum Tap_ (Fig. 34): This tap is cupped at _A_ and
sealed at _B_, and the cup _A_ is filled with mercury when the tap is in
use, so that if, for example, the end _C_ be attached to a flask, and
_D_ to an apparatus for exhausting the flask, it will be possible to
close the flask by turning off the tap _E_, and if no air be allowed
access through _D_, the vacuum produced in the flask at _C_ cannot be
affected by air leaking through the tap at _A_ or _B_.
A passage _F_ must be drilled from the bottom of the plug _E_ to meet
_G_, in order that when the plug is in position no residue of air shall
be confined within _B_, whence it might gradually leak into any
apparatus connected to it.
It is obvious, however, that this tap does not protect a flask sealed
to _C_ from the entrance of air through _D_, which, in fact, is the
direction in which air is most likely to effect an entrance. When using
one of these taps as part of an apparatus for supplying pure oxygen, I
have guarded against this by attaching a trap (Fig. 33) to the end _D_,
_C_ being joined to the delivery tube from the gas-holder. The structure
and mode of action of the trap are as follows:--
A narrow tube _G_ is joined to _D_ of Fig. 34, and terminates in the
wide tube _I_, which is connected above to _H_, and below to the
air-trap _J_. _J_ is connected at _K_, by a piece of flexible tube, to a
reservoir of mercury, from which mercury enters the air-trap, and
passing thence to _I_, can be employed for filling the V-trap _HLG_. The
air-trap _J_ is in the first instance filled with mercury, and then
serves to intercept any stray bubbles of air that the mercury may carry
with it. The particular form of the trap shown at _HLG_ was adopted
because with it the arm _LG_ is more readily emptied of mercury than
with any other form of trap made of small tube that I have tried. It has
been used in my apparatus in the following manner:--_H_ was connected
with a vessel to be filled with pure oxygen, the tap _E_ closed, and the
rise of mercury above _L_ prevented by a clamp on the flexible tube; the
vessel to be filled and the trap were then exhausted by a Sprengel pump,
and oxygen allowed to flow into the exhausted space by opening _E_, the
operation of exhausting the tubes and admitting oxygen being repeated as
often as necessary.
To prevent access of air to _E_ on disconnecting the vessel at _H_, the
mercury was allowed to flow into the trap till it reached to _MM_. _E_
was then closed, and _H_ exposed without danger of air reaching _E_, the
length of the arms of the trap being sufficient to provide against the
effects of any changes of temperature and pressure that could occur.
A delivery tube may be connected to _H_ and filled with mercury, by
closing _E_ and raising the mercury reservoir. All air being in that
way expelled from the delivery tube, and the supply of mercury cut off
by clamping the tube from the reservoir, oxygen can be delivered from
the tube by opening _E_, when it will send forward the mercury, and pass
into a tube placed to receive it without any risk of air being derived
from the delivery tube.
[Illustration: FIG. 35.]
(2.) _Gimmingham's Vacuum Tap_,[15] shown in Fig. 35, consists of three
parts. A tube _A_ is ground to fit the neck of _B_. _B_ is closed at its
lower end, and has a hole _d_ drilled through it; when _B_ is fitted to
_C_, _d_ can be made to coincide with the slit _e_. When _A_, _B_, _C_
are fitted together, if _d_ meet _e_, there is communication between any
vessels attached to _A_ and any other vessel attached to _C_, entrance
of external air being prevented by mercury being placed in the cups of
_C_ and _B_. The tap may be opened and closed at pleasure by rotating
_B_.
[15] From _Proceedings of Royal Society_, vol. XXV. p. 396.
If _A_ has to be removed, _C_ may be converted into a mercury joint _pro
tem._ by letting a little mercury from the upper cup fall into the tube
and cover _d_, the tap being closed. This mercury must be removed by a
fine pipette in order to use the tap again. It should be noted, however,
that though external air cannot enter by way of the ground glass joints,
there is no absolute protection against the passage of air between _A_
and _C_, or vessels joined to _A_ and _C_, even when the tap is closed.
The passage of air from _A_ to _C_ depends upon the grinding and
lubrication of the joint at _C_.
=Lubricating Taps.=--For general purposes resin cerate answers very
well. In special cases burnt india-rubber, or a mixture of burnt
india-rubber and vaseline will answer well, or vaseline may be used
alone. Sulphuric acid and glycerine are too fluid. When a lubricant is
wanted that will withstand the action of ether, the tap may be
lubricated by sprinkling phosphorus pentoxide upon it, and exposing it
to air till the oxide becomes gummy. The joint must then be protected
from the further action of the air if possible. For example, if a safety
tap be used the cup may be filled with mercury.
=Air-Traps.=--In Fig. 33, p. 66, an air-trap (_J_) is shown. An air-trap
is a device for preventing the mercury supplied to Sprengel pumps, etc.,
from carrying air into spaces that are exhausted, or are for any reason
to be kept free from air. Figs. 36 and 37 give examples of air-traps. In
the simpler of the two (Fig. 36) mercury flowing upwards from _C_ that
may carry bubbles of air with it passes through the bulb _A_, which is
_filled_ with mercury before use.[16] Any air which accompanies the
mercury will collect at _a_, the mercury will flow on through _b_. So
long as the level of the mercury in A is above _b_, the trap remains
effective.
[16] This may be done by clamping the tube which supplies mercury below
_C_, exhausting _A_, and then opening the clamped tube and allowing the
mercury to rise.
[Illustration: FIG. 36.]
[Illustration: FIG. 37.]
In the trap shown by Fig. 37, the tube _d_, which corresponds to _b_ in
Fig. 36, is protected at its end by the cup _E_. _E_ prevents the direct
passage of minute bubbles of air through _d_. This trap, like the other,
must be filled with mercury before it is used, and it will then remain
effective for some time.
CHAPTER V.
_GRADUATING AND CALIBRATING GLASS APPARATUS._
Although the subjects to which this concluding chapter is devoted do
not, properly speaking, consist of operations in glass-blowing, they are
so allied to the subject, and of such great importance, that I think a
brief account of them may advantageously be included.
=Graduating Tubes, etc.=--It was formerly the custom to graduate the
apparatus intended for use in quantitative work into parts of equal
capacity; for example, into cubic centimetres and fractions of cubic
centimetres. For the operations of volumetric analysis by liquids this
is still done. But for most purposes it is better to employ a scale of
equal divisions by length, usually of millimetres, and to determine the
relative values of the divisions afterwards, as described under
calibration. It rarely happens that the tube of which a burette or
eudiometer is made has equal divisions of its length of exactly equal
capacities throughout its entire length, and indeed, even for ordinary
volumetric work, no burette should be employed before its accuracy has
been verified. An excellent method for graduating glass tubes by
hand[17] has been described in Watts's _Dictionary of Chemistry_, and
elsewhere. Another excellent plan, which I have permission to describe,
has been employed by Professor W. Ramsay. It will be sufficient if I
explain its application to the operation of graduating a tube or strip
of glass in millimetre divisions.
[17] Originally suggested by Bunsen.
The apparatus required consists of a standard metre measure,[18] divided
into millimetres along each of its edges, with centimetre divisions
between them, a ruler adapted to the standard metre, as subsequently
explained, and a style with a fine point for marking waxed surfaces.
[18] Such measures can be obtained of steel for about _fifteen
shillings_ each. They are made by Mr. Chesterman of Sheffield. They can
be obtained also from other makers of philosophical instruments, at
prices depending upon their delicacy. Those of the greatest accuracy are
somewhat costly.
[Illustration: FIG. 38.]
Fig. 38 represents the standard measure, and the ruler.
At _AA_ are the millimetre divisions on the edges of the measure, the
longer transverse lines at _BB_ are placed at intervals of five
millimetres and of centimetres. The ruler is in the form of a
right-angled triangle; it is shown, by the dotted lines, in position on
the standard metre measure at _I_; and again, with its under surface
upwards, in the smaller figure at 2. It consists of a perfectly flat
sheet of metal, about ten centimetres in length from _C_ to _C_,
sufficiently thick to be rigid, and has a ledge, _DD_ in each figure,
which is pressed against the side of the measure when using it, to
ensure that the successive positions of the edge (_LL_) shall be
parallel to each other. At _GG_ are two small holes, into which fit
small screws with fine points. These must be in a line parallel to the
edge (_LL_), so that when the ruler is in position on the scale, the
points of the two screws, which project slightly, shall fall into
corresponding cuts on the divided scales (_AA_).
To graduate a strip of glass, or a glass tube (_HH_), the surface to be
marked must first be coated with wax, which should be mixed with a
little turpentine, and be applied to the surface of the glass,
previously made _warm_ and _dry_, by means of a fine brush, so as to
completely cover it with a thin, closely-adherent, and
evenly-distributed coat of wax, which must be allowed to cool.
Fix _HH_ firmly on a table, and fix the standard measure by the side of
_HH_. If the thickness of _HH_ be about equal to, but not greater than
that of the standard measure, this may be done by large drawing-pins.
If, however, a large tube or thick sheet of glass is to be graduated,
fix it in position by two strips of wood screwed to the table on each
side of it. One of these wooden strips, on which the measure may be
placed, may be about as broad as the standard measure, and of such
thickness that when the measure lies upon it beside the tube to be
graduated, the ruler, when moved along the measure, will move freely
above the tube, but will not be elevated more than is necessary to
secure free movement. The second strip of wood may be narrower, and of
the same thickness as the broader piece on which the standard measure
rests. In any case, let the standard measure and the object to be
graduated be very firmly secured in their places. Bring the ruler into
position at any desired part of the tube by placing the points of the
screws (_GG_) in corresponding divisions of the scales (_AA_). With the
style, which may be a needle mounted in a handle, make a scratch in the
wax along the edge of the ruler at _F_, move the ruler so that the
screws rest in the next divisions, and repeat the operation till the
required number of lines has been ruled. Longer marks may be made at
intervals of five and ten millimetres. Great care must be taken to hold
the needle perpendicularly, and to press it steadily against the edge
(_LL_) of the ruler in scratching the divisions.[19] The length of the
lines marking the millimetre divisions should not be too long; about 1
mm. is a good length. If they are longer than this, the _apparent_
distance between them is diminished, and it is less easy to read
fractions of millimetres. Before removing the scale to etch the glass,
carefully examine it to see that no mistakes have been made. If it is
found that any lines have been omitted, or that long lines have been
scratched in the place of short ones, remelt the wax by means of a
heated wire, and make new marks. Finally, mark the numbers on the scale
with a needle-point, or better, with a fine steel pen.
[19] To avoid variations of the position in which the needle is held
when marking the divisions, the edge (_LL_) should not be bevelled; and
an upright support may be placed upon the ruler, with a ring through
which the handle of the needle passes, thereby securing that the angle
formed by the needle and surface of the ruler is constant, and that
equal divisions are marked.
The marks on the wax should cut through it. When they are satisfactory,
they may be etched by one of the following processes:--
(1.) By moistening some cotton wool, tied to a stick, with solution of
hydrofluoric acid, and gently rubbing this over the scratched surface
for a minute or so; then washing away the acid with water, and cleaning
off the wax. This is the simplest method, but the marks made are
generally transparent, and therefore not very easy to read. The
simplicity of this method is a great recommendation, however.
(2.) Expose the tube to the fumes of hydrofluoric acid generated from a
mixture of powdered fluor-spar and strong sulphuric acid, in a leaden
trough. The marks produced in this way are usually opaque, and are
therefore very visible, and easily read.
After the above detailed account it will only be necessary to give an
outline of the other process of graduating tubes.
[Illustration: FIG. 39.]
The standard scale to be copied, _A_, which may in this case be another
graduated tube, or even a paper scale, and the object to be ruled, _B_,
are securely fixed, end to end, a little distance apart, in a groove
made in a board or in the top of a table. A stiff bar of wood, _C_, has
a point fixed at _D_, and a knife edge at _E_, _D_ is placed in any
division of _A_, _C_ is held firmly at _E_ and _D_, and a cut is made by
the knife through the wax on _B_, the point _D_ is then moved into the
next division, and the operation is repeated. To regulate the length and
position of the cuts, _B_ is usually held in position by two sheets of
brass projecting over the edges of the groove in which it lies; the
metal sheets have notches cut into them at the intervals at which longer
marks are to be made.
When the scale is completed, the equality of the divisions in various
parts of it may be, to some extent, verified as follows:--Adjust a
compass so that its points fall into two divisions 5, 10, or 20 mm.
apart. Then apply the points of the compass to various parts of the
scale. In every part the length of a given number of divisions should be
exactly the same. The individual divisions should also be carefully
inspected by the eye; they should be sensibly equal. If badly ruled,
long and short divisions will be found on the scale. Very often a long
and a short division will be adjacent, and will be the more easily
observed in consequence.
=To Divide a Given Line into Equal Parts.=--Occasionally it is necessary
to divide a line of given length into _x_ equal parts. For instance, to
divide the stem of a thermometer from the freezing-point to the
boiling-point into one hundred degrees.
The following outline will explain how a line may be so divided. Suppose
the line _AB_ (Fig. 40) is to be divided into nine equal parts. Adjust a
hinged rule so that the points _A_ and _B_ coincide with the inside
edges of the limbs, one of them, _A_, being at the ninth division
(_e.g._ the ninth inch) of _CE_. Then if lines parallel to _ED_ be drawn
from each division of the scale to meet _AB_, _AB_ will be divided into
nine equal parts.
[Illustration: FIG. 40.]
A very convenient and simple arrangement on this principle for dividing
a line into any number of equal parts with considerable accuracy, is
described by Miss S. Marks in the _Proceedings of the Physical Society_,
July 1885.[20] One limb of a hinged rule _D_ is made to slide upon a
plain rule fixed to it; the plain rule carries needles on its under
surface which hold the paper in position. The position of the divided
rule and line to be divided being adjusted, the hinged rule is gently
pushed forwards, as indicated by the arrow in Fig. 40, till division
eight coincides with the line _AB_. A mark is made at the point of
coincidence, and division seven on the scale is similarly brought to the
line _AB_, and so on. The inner edge of _EC_ should have the divisions
marked upon it, that their coincidence with _AB_ maybe more accurately
noted. The joint _E_ must be a very stiff one.
[20] Since this was printed I have observed that the above method is not
identical with that described by Miss Marks, but for ordinary purposes I
do not think it will be found to be inferior.
A line drawn of given length or a piece of paper may be divided into any
given number of equal parts, and will then serve as the scale _A_ of
Fig. 39, p. 74, the thermometer or other object to be graduated taking
the place of _B_.
Scales carefully divided according to any of the methods described will
be fairly accurate _if trustworthy instruments have been employed as
standards_.
It will be found possible when observing the volume of a gas over
mercury, or the height of a column of mercury in a tube, to measure
differences of one-sixth to one-eighth of a millimetre with a
considerable degree of accuracy. To obtain more delicate measurements a
vernier[21] must be employed.
[21] For the nature and use of the vernier, a treatise on Physics or
Physical Measurements may be consulted.
=To Calibrate Apparatus.=--The glass tubes of which graduated apparatus
is made are, as already stated, very rarely truly cylindrical
throughout their entire lengths. It follows that the capacities of equal
lengths of a tube will usually be unequal, and therefore it is necessary
to ascertain by experiment the true values of equal linear divisions of
a tube at various parts of it.
A burette may be calibrated by filling it with distilled water, drawing
off portions, say of 5 c.c. in succession, into a weighing bottle of
known weight, and weighing them.
Great care must be taken in reading the level of the liquid at each
observation. The best plan is to hold a piece of white paper behind the
burette, and to read from the lower edge of the black line that will be
seen. Each operation should be repeated two or three times, and the mean
of the results, which should differ but slightly, may be taken as the
value of the portion of the tube under examination.
If the weights of water delivered from equal divisions of the tube are
found to be equal, the burette is an accurate one, but if, as is more
likely, different values are obtained, a table of results should be
drawn up in the laboratory book showing the volume of liquid delivered
from each portion of the tube examined. And subsequently when the
burette is used, the volumes read from the scale on the burette must be
corrected. Suppose, for example, that a burette delivered the following
weights of water from each division of 5 c.c. respectively:--
C.C. Grams.
0 to 5 gave 4·90
5 " 10 " 4·91
10 " 15 " 4·92
15 " 20 " 4·93
20 " 25 " 4·94
25 " 30 " 4·95
30 " 35 " 4·96
35 " 40 " 4·97
40 " 45 " 4·98
45 " 50 " 4·99
and that in two experiments 20 c.c. and 45 c.c. respectively of a liquid
re-agent were employed. The true volumes calculated from the table would
be as 19·66 to 44·46.
If the temperature remained constant throughout the above series of
experiments, and if the temperature selected were 4° C., the weights of
water found, taken in grams, give the volumes in cubic centimetres, for
one gram of water at 4° C. has a volume of one cubic centimetre. If the
temperature at which the experiments were made was other than 4° C., and
if great accuracy be desired, a table of densities must be consulted,
with the help of which the volume of any weight of water at a known
temperature can be readily calculated.
Pipettes which are to be used as measuring instruments should also have
the relation one to another of the volumes of liquid which they deliver
determined, and also the proportions these bear to the values found for
the divisions of the burettes in conjunction with which they will be
employed.
=To Calibrate Tubes for Measuring Gases.=--Prepare a small glass tube
sealed at one end and ground at the other to a plate of glass. The tube
should hold about as much mercury as will fill 10 mm. divisions of the
graduated tube. Fill this tube with mercury, removing all bubbles of air
that adhere to the sides by closing the open end of the tube with the
thumb, and washing them away with a large air-bubble left for the
purpose. If any persistently remain, remove them by means of a fine
piece of bone or wood. Then completely fill the tube with mercury,
removing any bubbles that may be introduced in the operation, and remove
the excess of mercury by placing the ground-glass plate on the mouth of
the tube, and pressing it so as to force out all excess of mercury
between the two surfaces. Clean the outside of the tube, and place it on
a small stand (this may be a small wide-mouthed glass bottle), with
which it has been previously weighed when empty, and re-weigh. Repeat
this operation several times. From the mean of the results, which should
differ one from another but very slightly, the capacity of the tube can
be calculated.
The purest mercury obtainable should be used. Since the density of pure
mercury at 0° C. is 13·596, the weight of mercury required to fill the
tube at 0° C., taken in grams, when divided by 13·596, will give the
capacity of the tube at 0° C. in cubic centimetres. If the experiment be
not made at 0° C., and if a very exact determination of the capacity of
the tube be required, the density of mercury must be corrected for
expansion or contraction.
Having now a vessel of known capacity, it can be employed for
ascertaining the capacities of the divisions of a graduated tube in the
following manner:--The graduated tube is fixed perpendicularly, mouth
upwards, in a secure position. The small tube of known capacity is
filled with mercury as previously described, and its contents are
transferred to the divided tube. The number of divisions which the known
volume of mercury occupies is noted after all air-bubbles have been
removed. This process is repeated until the divided tube is filled. A
table of results is prepared, showing the number of divisions occupied
by each known volume of mercury introduced.
In subsequently using the tube the volumes of the gases measured in it
must be ascertained from the table of values thus prepared.
In observing the level of the mercury, unless a cathetometer is
available, a slip of mirror should be held behind the mercury close to
the tube, in such a position that the pupil which is visible on the
looking-glass is divided into two parts by the surface of the mercury.
A correction must be introduced for the error caused by the meniscus of
the mercury. As the closed end of the tube was downwards when each
measured volume of mercury was introduced, and as the surface of mercury
is convex, the volume of mercury in the tube when it is filled to any
division _l_ (Fig. 41) is represented by _A_ of 1. But in subsequently
measuring a gas over mercury in the same tube, when the mercury stands
at the same division _l_, the volume of the gas will be as represented
by _B_ of 2, which is evidently somewhat greater than _A_. This will be
seen still more clearly in 3, where _a_ represents the boundary of the
mercury, and _b_ the boundary of the air, when the tube is filled to the
mark _l_ with mercury or a gas over mercury respectively.
[Illustration: FIG. 41.]
It is plain that when the level of the mercury in measuring a gas is
read at _l_, the volume of the gas is greater than the volume of the
mercury recorded, by twice the difference between the volume _A_ of
mercury measured, and that which would fill the tube to the level _l_,
if its surface were plane.
The usual mode of finding the true volume of a gas collected over
mercury is as follows:--
Place the graduated tube mouth upwards, introduce some mercury, and,
after removing all bubbles, note the division at which it stands. Then
add a few drops of solution of mercuric chloride; the surface of the
mercury will become level, read and record its new position. Then, in
any measurement, having observed that the mercury stands at _n_
divisions of the tube, add twice the difference between the two
positions of the mercury to _n_, and ascertain the volume which
corresponds to this reading from the table of capacities.
=To Calibrate the Tube of a Thermometer.=--Detach a thread of mercury
from half an inch to one inch in length from the body of the mercury.
Move it from point to point throughout the length of the tube, and note
its length in each position. If in one part it occupies a length of tube
corresponding to eight degrees, and at another only seven degrees, then
at the former point the value of each division is only seven-eighths of
those at the latter position.
From the results obtained, a table of corrections for the thermometer
should be prepared.
It is sometimes necessary to join soda glass to lead glass. In this case
the edge of the lead glass tube may be bordered with white enamel before
making the joint. Enough enamel must be used to prevent the lead and
soda glasses from mingling at any point. The enamel is easily reduced,
and must be heated in the oxidising flame. Dr. Ebert recommends _Verre
d'urane_ for this purpose. It is supplied by Herr Götze of Leipzig
(Liebigstrasse).
CHAPTER VI.
_GLASS TUBING._
The diagrams given below show the sizes and thickness of the glass tubes
most frequently required. In ordering, the numbers of these diagrams may
be quoted, or the exact dimensions desired may be stated.
Glass tubes are usually sold by weight, and therefore the weight of tube
of each size that is wished for should be indicated, and also whether it
is to be of lead or soda glass.
[Illustration]
[Illustration]
CHAPTER VII.
_VITREOUS SILICA._
=Introductory.=--Vitreous Silica was made in fine threads by M. Gaudin
in 1839,[22] and small tubes of it were made in 1869 by M. A. Gautier,
but its remarkable qualities were not really recognised till 1889, when
Professor C. V. Boys rediscovered the process of making small pieces of
apparatus of this substance, and used the torsion of "quartz fibres" for
measuring small forces. More recently the author of this book has
devised a process for preventing the "splintering" of quartz which gave
so much trouble to the earlier workers, and jointly with Mr. H. G.
Lacell, has produced a variety of apparatus of much larger dimensions
than had been attempted =previously=. At the time of writing we can
produce by the processes described in the following pages tubes 1 to 1·5
cm. in diameter and about 750 cm. in length, globes or flasks capable of
containing about 50 cc., masses of vitreous silica weighing 100 grams or
more, and a variety of other apparatus.
[22] A brief summary of the history of this subject will be found in
_Nature_, Vol. 62, and in the Proceedings of the Royal Institution,
1901.
=Properties of Vitreous Silica.=--For the convenience of those who are
not familiar with the literature of this subject, I may commence this
chapter with a brief account of the properties and applications of
vitreous silica, as far as they are at present ascertained. Vitreous
silica is less hard than chalcedony, but harder than felspar. Tubes and
rods of it can be cut with a file or with a piece of sharpened and
hardened steel, and can afterwards be broken like similar articles of
glass. Its conducting power is low, and Mr. Boys has shown that fine
fibres of silica insulate remarkably well, even in an atmosphere
saturated with moisture. The insulating qualities of tubes or rods of
large cross sections have not yet been fully tested; one would expect
them to give good results provided that they are kept scrupulously
clean. A silica rod which had been much handled would probably insulate
no better than one of glass in a similar condition. The density of
vitreous silica is very near to that of ordinary amorphous silica. In
the case of a small rod not absolutely free from minute bubbles it was
found to be 2·21.
Vitreous silica is optically inactive, when homogeneous, and is highly
transparent to ultraviolet radiations.
The melting point of vitreous silica cannot be definitely stated. It is
plastic over a considerable range of temperature. Professor Callendar
has succeeded in measuring the rate of contraction of fine rods in
cooling from 1200° to 1500° C., so that its plasticity must be very
slight below the latter temperature. If a platinum wire embedded in a
thick silica tube be heated from without by an oxy-hydrogen flame the
metal may be melted at temperatures at which the silica tube will retain
its form for a moderate length of time, but silica softens to a marked
extent at temperatures a little above the melting point of platinum.
It has been observed by Boys, Callendar, and others that fine rods of
silica, and also the so-called "quartz fibres," are apt to become
brittle after they have been heated to redness. But I have not observed
this defect in the case of more massive objects, such as thick rods or
tubes; and as I have repeatedly observed that mere traces of basic
matter, such as may be conveyed by contact with the hand, seriously
injure the surface of silica, and have found that silica quickly becomes
rotten when it is heated to about 1000° in contact with an infusible
base such as lime, I am disposed to ascribe the above-mentioned
phenomenon to chemical rather than to purely physical causes.[23] It is
certain, however, that silica apparatus must never be too strongly
heated in contact with basic substances. Silica is easily attacked by
alkalis and by lime, less readily by copper oxide, and still less by
iron oxide.
[23] In a recent communication Professor Callendar tells me that the
devitrification commences at the outside and is hastened by particles of
foreign matter.
The rate of expansion of vitreous silica has been studied by H. le
Chatelier, and more recently by Callendar. The former found its mean
coefficient of expansion to be 0·0000007 between 0° and 10000°,[24] and
that it contracted when heated above 700°.
[24] The silica blocks used were prepared by fusion in an electric
furnace; it is therefore probable that they were not quite pure.
Professor Callendar used rods of silica prepared by the author from
"Brazil crystal"; these were drawn in the oxy-gas flame and had never
been heated in contact with solid foreign matter, so that they
consisted, presumably, of very pure silica. His results differ in some
respects from those obtained by Le Chatelier, for he finds the mean
coefficient of expansion to be only 0·00000059, _i.e._ about one
seventeenth as great as that of platinum. Callendar found the rods of
silica expanded very regularly up to 1000° but less regularly above that
temperature. Above 1200° they contracted when heated.
The behaviour of vitreous silica under sudden changes of temperature is
most remarkable. Large masses of it may be plunged suddenly when cold
into the oxy-gas flame, and tubes or rods at a white heat may be thrust
into cold water, or even into liquid air, with impunity. As a
consequence of this, it is in one respect much more easily worked in the
flame than any form of glass. Difficult joints can be thrust suddenly
into the flame, or removed from it, at any stage, and they may be heated
unequally in different parts with impunity. It is safe to say that
joints, etc., in silica never crack whilst one is making them nor during
the subsequent cooling. They may be set aside in an unfinished state and
taken up again without any precautions. Therefore it is possible for an
amateur to construct apparatus in silica which he would be quite unable
to produce from glass.
The behaviour of vitreous silica with solvents has not yet been fully
investigated, but Mr. H. G. Lacell has this subject in hand. If it
behaves like the other forms of anhydrous silica it will withstand the
action of all acids except hydrofluoric acid. It is, of course, very
readily acted upon by solutions of alkalis and alkaline salts.
As regards the use of silica in experiments with gases, it must be
remarked that vitreous silica, like platinum, is slightly permeable to
hydrogen when strongly heated. One consequence of this is that traces of
moisture are almost always to be found inside recently-made silica tubes
and bulbs, however carefully we may have dried the air forced into them
during the process of construction. Owing to the very low coefficient of
expansion of silica, it is not possible to seal platinum wires into
silica tubes. Nor can platinum be cemented into the silica by means of
arsenic enamel, nor by any of the softer glasses used for such purposes.
I have come near to success by using kaolin, but the results with this
material do not afford a real solution of the problem, though they may
perhaps point to a hopeful line of attack. Possibly platinum wires might
be soldered into the tubes (see _Laboratory Arts_, R. Threlfall), but
this also is uncertain.
The process of preparing silica tubes, etc., from Lumps of Brazil
Crystal may be described conveniently under the following headings. I
describe the various processes fully in these pages, as those who are
interested in the matter will probably wish to try every part of the
process in the first instance. But I may say that in practice I think
almost every one will find it advantageous to start with purchased
silica tubes, just as a glass-worker starts with a supply of purchased
glass tubes. The manufacturer can obtain his oxygen at a lower price
than the retail purchaser, and a workman who gives much time to such
work can turn out silica tube so much more quickly than an amateur, that
I think it will be found that both time and money can be saved by
purchasing the tube. At the same time the beginner will find it worth
while to learn and practise each stage of the process at first, as every
part of the work described may be useful in the production of finished
apparatus from silica tubes.
This being so, I am glad to be able to add that a leading firm of
dealers in apparatus[25] has commenced making silica goods on a
commercial scale, so that the new material is now available for all
those who need it or wish to examine its properties.
[25] Messrs. Baird and Tatlock.
=Preparing non-splintering Silica from Brazil Pebble.=--The best variety
of native Silica is Brazil Pebble, which may be obtained in chips or
larger masses. These should be thoroughly cleaned, heated in boiling
water, and dropped into cold water, the treatment being repeated till
the masses have cracked to such an extent that they may be broken easily
by blows from a clean steel pestle or hammer.
The fragments thus produced must be hand-picked, and those which are not
perfectly free from foreign matter should be rejected. The pure and
transparent pieces must then be heated to a yellow-red heat in a covered
platinum dish in a muffle or reverberatory furnace and quickly plunged
into a deep clean vessel containing clean distilled water; this process
being repeated, if necessary, till the product consists of semi-opaque
friable masses, very much like a white enamel in appearance. After these
have been washed with distilled water, well drained and dried, they may
be brought into the hottest part of an oxy-gas flame safely, or pressed
suddenly against masses of white hot silica without any preliminary
heating, such as is necessary in the case of natural quartz. Quartz
which has not been submitted to the above preparatory process, splinters
on contact with the flame to such an extent that very few would care to
face the trouble and expense of working with so refractory a material.
But after the above treatment, which really gives little trouble, all
the difficulties which hampered the pioneer workers in silica disappear
as if by magic.
=Apparatus.=--Very little special apparatus need be provided for working
with silica, but it is absolutely essential to protect the eyes with
very dark glasses. These should be so dark as to render it a little
difficult to work with them at first. If long spells of work are
undertaken, two pairs of spectacles should be provided, for the glasses
quickly become hot enough to cause great inconvenience and even injury
to the eyes.
Almost any of the available oxy-gas burners may be used, but they vary
considerably in efficiency, and it is economical to obtain a very
efficient burner. The 'blow-through' burners are least satisfactory, and
I have long since abandoned the use of them. Some of the safety
'mixed-gas jets' have an inconvenient trick of burning-back, with sharp
explosions, which are highly disconcerting, if the work be brought too
near the nozzle of the burner. I have found the patent burner of Mr.
Jackson (Brin's Oxygen Company, Manchester) most satisfactory, and it
offers the advantage that several jets can be combined in a group easily
and inexpensively for work on large apparatus. The large roaring flames
such as are used, I understand, for welding steel are very expensive,
and not very efficient for the work here described.
=The method of making Silica Tubes.=--Before commencing to make a tube a
supply of vitreous silica in rods about one or two millimetres in
diameter must be prepared. To make one of these, hold a fragment of the
non-splintering silica described above in the oxy-gas flame by means of
forceps tipped with platinum so as to melt one of its corners, press a
small fragment of the same material against the melted part till the two
adhere and heat it from below upwards,[26] till it becomes clear and
vitreous, add a third fragment in a similar manner, then a fourth, and
so on till an irregular rod has been formed. Finally re-heat this rod in
sections and draw it out whilst plastic into rods or coarse threads of
the desired dimensions. If one works carefully the forceps do not suffer
much. I have had one pair in almost constant use for several years; they
have been used in the training of five beginners and are still
practically uninjured.
[26] This is to avoid bubbles in the finished glass.
The beginner should work with a gauge and regulator on the bottle of
oxygen, and should watch the consumption of oxygen closely. A large
expenditure of oxygen does not by any means necessarily imply a
corresponding output of silica, even by one who has mastered the initial
difficulties.
When a supply of the small rods of vitreous silica has been provided,
bind a few of them round a rod of platinum (diameter say, 1 mm.) by
means of platinum wires at the two ends and heat the silica gradually,
beginning at one end after slightly withdrawing the platinum core from
that end, till a rough tube about four or five centimetres in length has
been formed. Close one end of this, expand it, by blowing, into a small
bulb, attach a silica rod to the remote end of the bulb, re-heat the
bulb and draw it out into a fine tube. Blow a fresh bulb on one end of
this and again draw it out, proceeding in this way till you have a tube
about six or eight centimetres in length. All larger tubes and vessels
are produced by developing this fine tube suitably.
=Precautions.=--The following points must be carefully kept in mind,
both during the making of the first tube and afterwards:--
(1) The hottest spot in the oxy-gas flame is at a point very near the
tip of the inner cone of the flame, and silica can be softened best at
this hot spot. The excellence of a burner does not depend on the size of
its flame, so much as on the temperature of its "hot spot," and the
success of the worker depends on his skill in bringing his work exactly
to this part of the flame. Comparatively large masses of silica may be
softened in a comparatively small jet if the hot spot is properly
utilised.
(2) Silica is very apt to exhibit a phenomenon resembling
devitrification during working. It becomes covered with a white
incrustation, which seems to be comparatively rich in alkali.[27] This
incrustation is very easily removed by re-heating the whitened surface,
provided that the material has been kept scrupulously clean. If the
silica has been brought into the flame when dusty, or even after much
contact with the hands of the operator, its surface is very apt to be
permanently injured. _Too much attention cannot be given to cleanliness
by the workman._
[27] The rock crystal exhibits a yellow flame when first heated in the
oxy-gas flame, and most samples contain spectroscopic quantities of
lithium.
(3) When a heated tube or bulb of silica is to be expanded by blowing,
it is best not to remove it from the flame, for if that is done it will
lose its plasticity quickly unless it be large. The better plan is to
move it slightly from the "hot spot" into the surrounding parts of the
flame at the moment of blowing.
It is best to blow the bulb through an india-rubber tube attached to the
open end of the silica tube. At first one frequently bursts the bulbs
when doing this, but holes are easily repaired by stopping them with
plastic silica applied by the softened end of a fine rod of silica and
expanding the lump, after re-heating it, by blowing. After a few hours'
practice these mishaps gradually become rare.
I find it a good plan to interpose a glass tube packed with granulated
potash between the mouth and the silica tube. This prevents the interior
of the tube from being soiled. The purifying material must not be packed
so closely in the tube as to prevent air from passing freely through it
under a very low pressure.
It may be mentioned here that a finished tube usually contains a little
moisture, and a recognisable quantity of nitric peroxide. These may be
removed by heating the tube and drawing filtered air through it, but not
by washing, as it is difficult to obtain water which leaves no residue
on the silica.
=Making larger tubes and other apparatus of Silica.=--In order to
convert a small bulb of silica into a larger one or into a large tube,
proceed as follows:--Heat one end of a fine rod of silica and apply it
to the bulb so as to form a ring as shown in the figure. Then heat the
ring and the end of the bulb till it softens, and expand the end by
blowing. If this process is repeated, the bulb first becomes ovate and
then forms a short tube which can be lengthened at will, but the most
convenient way to obtain a very long tube is to make several shorter
tubes of the required diameter, and say 200 to 250 mm. in length, and to
join these end to end. It does not answer to add lumps of silica to the
end of the bulb, for the sides of the tube made in this way become too
thin, and blow-holes are constantly formed during the making of them.
These can be mended, it is true, but they spoil the appearance of the
work.
[Illustration]
Tubes made in the manner described above are thickened by adding rings
of silica and blowing them when hot to spread the silica. If a
combination of several jets is employed, very large tubes can be
constructed in this way. One of Messrs. Baird and Tatlock's workmen
lately blew a bulb about 5 cm. in diameter, and it was clear that he
could have converted it into a long cylindrical tube of equal diameter
had it been necessary to do so.
Very thin tubes of 1·5 cm. diameter, and tubes of considerable thickness
and of equal size, are easily made after some practice, and fine
capilliaries and millimetre tube can be made with about equal readiness.
If a very fine tube of even bore is required, it may be drawn from a
small thick cylinder after a little practice.
When a tube becomes so large that it cannot be heated uniformly on all
sides by rotating it in the flame, it is convenient to place a sheet of
silica in front of the flame a little beyond the object to be heated, in
order that the former may throw back the flame on those parts of the
tube which are most remote from the jet. A suitable plate may be made by
sticking together small lumps of silica rendered plastic by heat.
The silica tubes thus made can be cut and broken like glass, they can be
joined together before the flame, and they can also be drawn into
smaller tubes when softened by heat.
In order to make a side connection as in a T piece, a ring of silica
should be applied to the tube in the position fixed upon for the joint.
This ring must then be slightly expanded, a new ring added, and so on,
till a short side tube is formed. To this it is easy to seal a longer
tube of the required dimensions. It is thus possible to produce Geissler
tubes, small distilling flasks, etc. Solid rods of silica are easily
made by pressing together the softened ends of the fine rods or threads
previously mentioned. Such rods and small masses can be ground and
polished without annealing them.
=Quartz Fibres.=--These were introduced into physical work by Mr. Boys
in 1889. They may be made by attaching a fine rod of vitrified quartz to
the tail of a small straw arrow provided with a needle-point; placing
the arrow in position on a cross-bow, heating the rod of silica till it
is thoroughly softened and then letting the arrow fly from the bow, when
it will carry with it an extremely fine thread of silica. A little
practice is necessary to ensure success, but a good operator can
produce threads of great tenacity and great uniformity. Fuller accounts
of the process and of the various properties and uses of quartz fibres
will be found in Mr. Boys' lectures (Roy. Inst. Proc. 1889, and Proc.
Brit. Assn. 1890), and in Mr. Threlfall's Laboratory Arts.
INDEX.
Air-traps, 69.
Annealing, 23.
Apparatus needed for Glass-working, 11.
Appendix, 82.
Beginners, Failures of, 22.
Bellows, Position of, 3.
---- Various forms of, 7.
_See also_ Blower.
Bending Glass Tubes, 28.
Blower, Automatic, 8.
Blow-pipe, Cheap form of, 4.
---- Dimensions of, 4-5.
---- Fletcher's Automaton, 6.
---- Fletcher's Compound, 6.
---- Gimmingham's, 6.
---- Herapath's, 6.
---- Jets for the, 7.
---- Use of the, 8.
_See also_ Flames.
Blow-pipes, Use of several in combination, 21.
Brush Flame, 9.
---- Oxidising, 20.
Bulbs, Methods of blowing, 47.
Calibrating Apparatus, 76-81.
Camphorated Turpentine, 11.
Cetti's Vacuum Tap, 66.
Charcoal Pastils, 11.
Choking or Contracting the Bores of Tubes, 35.
Combining the Parts of Complicated Apparatus, 61.
Combustion Tube, how to work it, 25.
Contracting the Bore of a Tube, 35.
Cotton Wool for Annealing, 24.
Cutting Glass Tubes, 26, 27, 28.
Dividing a Line into Equal Parts, 75.
Electrodes, 38, 55.
Electrolysis, Making Apparatus for, 59.
Files for Cutting Glass, 27.
Flame, the Pointed, 8.
---- the Brush, 9.
---- the Oxidising Brush, 20.
---- the Smoky, 10.
Fletcher's Automaton Blow-pipe, 6.
Fletcher's Compound Blow-pipe, 6.
Funnels, Thistle-headed, 57.
Gimmingham's Blow-pipe, 6.
Gimmingham's Vacuum Tap, 68.
Glass, Annealing, 23.
---- Devitrification of, 15.
---- Method of Working with Lead, 17.
---- Method of Working with Soda, 22.
---- Nature of, 12.
---- Presenting to the Flame, 16.
Glass Tubes, Bending, 28.
---- Bordering, 31.
---- Characters of good, 14.
---- Choking, 35.
---- Cleaning, 15.
Glass Tubes, Cutting, 26, 27, 28.
---- Piercing, 37.
---- Purchase of, 12.
---- Sealing, 32.
---- Sealing Hermetically, 58.
---- Sizes of, 82.
---- Welding or Soldering, 39, 62.
---- Widening the Ends of, 36.
Graduating Apparatus, 70.
Grinding Stoppers, 51.
Herapath's Blow-pipe, 6.
Hofman's Apparatus for Electrolysis, 59.
Inside Joints, 43.
Jets for Blow-pipes, 7.
Joints, Air-tight, 64.
Lead Glass, Method of Working with, 17.
Lead Glass, Blackening of, 17.
Light, Effect of, in Working, 3.
Line, to Divide into Equal Parts, 75.
Mercury Joints, Various, 64.
Non-splintering Silica, Preparation of, from Quartz, 88.
Ozone Generator, To Make an, 44.
Pastils of Charcoal, 11.
Piercing Tubes, etc., 37.
Platinum Electrodes, Sealing in, 38, 55.
Pointed Flame, the, 9.
Quartz Fibres, 94.
Rounding Ends of Tubes, 31.
Sealing or Closing Openings in Tubes, 32.
Side-tubes, Fixing, 41.
Smoky Flame, 10.
Soda Glass, Method of Working, 22.
Soldering or Welding, 39, 62.
Spiral Tubes, 56.
Stoppers, Making and Grinding, 51.
Table for Glass-blower, 3.
Taps, Vacuum, 65.
Thistle-headed Funnels, 57.
Traps, Air, 69.
Tube, Combustion, how to work it, 25.
Tubes. _See_ Glass Tubes.
---- T-, 41.
---- U-, 56.
Turpentine, Camphorated, for Grinding, 11.
U-Tubes, 56.
Vacuum Taps, 65-68.
---- Tube, To Make a, 60.
Vitreous Silica, Apparatus required for Making, 89.
---- Behaviour under sudden changes of Temperature, 87.
---- Bulbs, etc., Making Joints on, 93.
---- Expansion of, 86.
---- Hardness of, 85.
---- Insulating Power of, 85.
---- Melting Point of, 85.
---- Permeability to Gases, 87.
---- Properties of, 84.
---- Rods, Making Joints on, 94.
---- Tubes, Method of Making, 90.
---- Tubes, Making Joints on, 94.
Welding or Soldering Tubes together, 39, 62.
White Enamel, Uses of, 39, 56.
Widening the Ends of Tubes, 36.
Working-place, 2.
Printed by T. and A. CONSTABLE, Printers to His Majesty
at the Edinburgh University Press, Scotland
