[i]

THE METHODS OF GLASS BLOWING

AND OF

WORKING SILICA

[ii]

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.

[iii]

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

[v]

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
[vi]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.

[vii]

CONTENTS

[1]

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[2]
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[3]
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[4]
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.

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[5]
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[6]
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[7]
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.

Fig. 2

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[8]
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.

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.

Fig. 3.

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[9]
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.

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[10]
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[11]
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.

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.

[12]

CHAPTER II.

VARIETIES OF GLASS AND THEIR MANAGEMENT.

All the varieties of glass that are ordinarily met with contain
silica (SiO₂) 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]—

In purchasing glass tubes, it is well to lay in a considerable[13]
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[14]
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[15]
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]

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[16]
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[17]
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[18]
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[19]
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.

[20]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.

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[21]
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[22]
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[23]
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[24]
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.

[25]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.

[2]
For details of the composition of the various glasses, some work on
glass-making may be consulted.

[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.

[4]
See Principles of Glass-making, p. 31.

[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.

[26]

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[27]
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[28]
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.

Fig. 6.

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.[29]
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.

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[30]
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.

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[31]
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[32]
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.

Fig. 8.

[33]
(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[34]
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.

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.

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[35]
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.

Fig. 9.

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.

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.

Fig. 10.

[36]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.

Fig. 11.

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[37]
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.

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[38]
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.

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[39]
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[40]
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.

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.

Fig. 15.

In welding pieces of lead glass tube, take care that the
heated glass is perfectly free from reduced lead at the[41]
moment when the two ends of viscous glass are brought into
contact.

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,[42]
must then be heated till they melt, and united by pressing
them together. The joint may then be finished as before.

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
[43]
on the convex side of the angle; or, A of Fig. 16 may be
bent as desired after attaching B in the ordinary
way.

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]

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.

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).

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[44]
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[45]
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.

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[46]
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.

[47]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.

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[48]
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[49]
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[50]
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.

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[51]
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.

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.

Fig. 23.

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[52]
softened by heat. In either case the conical mouth should
be as long and regular as possible.

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[53]
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]

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[54]
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.

[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.

[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.

[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.

[9]
This can be obtained from Messrs. Powells, Whitefriars Glassworks.

[10]
Mr. Gimmingham recommends giving stoppers a final polish with
rotten-stone (Proceedings of the Royal Society, p. 396, 1876).

[55]

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.

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[56]
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]

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,[57]
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.

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[58]
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.

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.

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[59]
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.

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[60]
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.

Fig. 28.

2. I have before me the vacuum tube shown by Fig. 28, in
which the dotted lines relate to details of manipulation only.

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[61]
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.

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[62]
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[63]
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]

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.

Fig. 30.

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[64]
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.

Fig. 31.

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]

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[65]
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.

Fig. 32.

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.

Fig. 33.

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[66]
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:—

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[67]
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:—

Fig. 35.

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[68]
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.

(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.

Fig. 36.

Fig. 37.

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[69]
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.

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.

[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.

[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.

[13]
For a method of joining soda glass to lead glass, see p. 81.

[14]
If the india-rubber tube C be secured by wires, iron wire, not
copper wire, should be employed.

[15]
From Proceedings of Royal Society, vol. XXV. p. 396.

[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.

[70]

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[71]
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.

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.

Fig. 38.

Fig. 38 represents the standard measure, and the ruler.

[72]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[73]
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.

The marks on the wax should cut through it. When they[74]
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.

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.

[75]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.

Fig. 40.

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.

A very convenient and simple arrangement on this[76]
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.

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.

To Calibrate Apparatus.—The glass tubes of which
graduated apparatus is made are, as already stated, very[77]
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:—

[78]
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[79]
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[80]
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.

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[81]
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).

[17]
Originally suggested by Bunsen.

[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.

[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.

[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.

[21]
For the nature and use of the vernier, a treatise on Physics or
Physical Measurements may be consulted.

[82]

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.

[83]

[84]

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 c.c., masses of
vitreous silica weighing 100 grams or more, and a variety
of other apparatus.

Properties of Vitreous Silica.—For the convenience
of those who are not familiar with the literature of this[85]
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.[86]
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.

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°.

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.

[87]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[88]
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.

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[89]
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.

[90]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.

[91]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[92]
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.

(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[93]
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.

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[94]
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[95]
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.

[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.

[23]
In a recent communication Professor Callendar tells me that the
devitrification commences at the outside and is hastened by particles of
foreign matter.

[24]
The silica blocks used were prepared by fusion in an electric
furnace; it is therefore probable that they were not quite pure.

[25]
Messrs. Baird and Tatlock.

[26]
This is to avoid bubbles in the finished glass.

[27]
The rock crystal exhibits a yellow flame when first heated in the
oxy-gas flame, and most samples contain spectroscopic quantities of
lithium.

[97]

INDEX.

Printed by T. and A. Constable, Printers to His Majesty
at the Edinburgh University Press, Scotland