will most nearly approach that of a true globe. If it be held in the
perpendicular position, with the mass of glass depending from it, the
form of the bulb will usually be somewhat elongated. If it be held
perpendicularly, with the mass of glass upwards, the resulting bulb will
be flattened.
When a bulb is not of a sufficiently regular form, it may sometimes be
re-made by re-collecting the glass, and re-blowing it. The greatest care
is needed at the earlier stages of re-heating to prevent the glass from
collapsing into a formless and unworkable mass. This is to be prevented
in all such cases by gently blowing it into shape from time to time
whilst gathering the glass.
[Illustration: FIG. 21.]
_To blow a Bulb between two Points_ (Fig 21).--Select a piece of
suitable tube, seal or cork one end, gather together a mass of glass at
the desired part, as directed for blowing a bulb at the end of a tube;
when a mass of glass has been collected of sufficient thickness, blow it
into shape from the open end of the tube by a rapid succession of short
blasts of air, till the expanding glass attains the desired dimensions.
The tube must be held horizontally, and must be rotated steadily during
the process. By slightly pressing together the glass while blowing, the
bulb will be flattened; by slightly drawing apart the two ends of the
tube, it will be elongated.
A pear-shaped bulb may be obtained by gently re-heating an elongated
bulb, say from _a_ to _a_, and drawing it out. It is easiest to perform
this operation on a bulb which is rather thick in the glass.
If the tubes _bb_ are to be small, and a globe of considerable size is
wanted, contract a tube as shown in Fig. 22, taking care that the narrow
portions of the tube are about the same axis as the wider portions, for
if this be not the case, the mouths of the bulb will not be
symmetrically placed; seal at _C_, cut off the wider tube at _B_, and
make the bulb, as previously described, from the glass between _AA_.
If, as probably will be the case, the contracted portions of the tube be
not very regular, they may be cut off, one at a time, near the bulb, and
replaced by pieces of tube of the size desired.
[Illustration: FIG. 22.]
When a bulb has to be blown upon a very fine tube, for example upon
thermometer tubing, the mouth should not be employed, for the moisture
introduced by the breath is extremely difficult to remove afterwards. A
small india-rubber bottle or reservoir, such as those which are used in
spray-producers, Galton's whistles, etc., securely attached to the open
end of the tube, should be used. With the help of these bottles bulbs
can be blown at the closed ends of fine tubes with ease, though some
care is necessary to produce them of good shape, as it is difficult to
rotate the hot glass properly when working in this way.
=Making and Grinding Stoppers.=--Apparatus which is to contain chemicals
that are likely to be affected by the free admission of air, needs to
have stoppers fitted to it. Making a good stopper is a much less tedious
process than is commonly supposed.
Suppose that the tube I. of Fig. 23 is to be stoppered at _A_, it must
be slightly enlarged by softening the end and opening it with a pointed
cone of charcoal; or a conical mouth for the stopper may be made by
slightly contracting the tube near one end, as at _B_, cutting off the
cylindrical end of the tube at the dotted line _C_, and then very
slightly expanding the end at _C_ with a charcoal cone after its edges
have been softened by heat. In either case the conical mouth should be
as long and regular as possible.
[Illustration: FIG. 23.]
For the stopper take a piece of rather thick tube, of such size that it
will pass easily, but not too easily, into _A_ or _B_. Expand this tube
at _D_, as shown in II., by softening the glass and gently compressing
it. The configuration of the enlarged tube as shown at _D_ may be
obtained by heating and compressing two or more zones of the tube that
are adjacent, one zone being less expanded than the other, so as to give
the sides of the imperfect stopper as nearly as possible the form shown
at _D_, which, however, is much less regular than may easily be
obtained. Seal off the head of the tube at _H_, and heat the glass till
it runs together into a nearly solid mass; compress this with a pair of
iron tongs to the flattened head _E_. In making _D_, aim at giving it a
form which will as nearly as possible correspond to that of the tube
into which it is to be ground, and make it slightly too large, so that
only the lower part at _D_ can be introduced into the mouth of _A_ or
_B_. Before it is ground, the stopper must be heated nearly to its
softening-point and annealed.
Moisten _D_ with a solution of camphor in recently distilled
turpentine, and dust the wet surface with finely-ground emery, then
gently grind it into its place till it fits properly. In this operation
the tail _G_, which should fit loosely into the tube _A_, will be of
assistance by preventing _D_ from unduly pressing in any direction on
_A_ in consequence of irregular movements. The stopper should be
completely rotated in grinding it. It must not be worked backwards and
forwards, or a well-fitting stopper will not be produced. Renew the
emery and camphorated turpentine frequently during the earlier part of
the grinding; when the stopper almost fits, avoid using fresh emery, but
continue to remove the stopper frequently at all stages of the
operation. That added at the earlier stages will be reduced to a state
of very fine division, and will therefore leave the stopper and mouth of
_A_ with smoother surfaces than fresh emery.[10]
[10] Mr. Gimmingham recommends giving stoppers a final polish with
rotten-stone (_Proceedings of the Royal Society_, p. 396, 1876).
NOTE.--The addition of camphor to the turpentine used for grinding glass
is very important. Notwithstanding its brittle nature, glass will work
under a file moistened with this solution almost as well as the metals.
Small quantities should be made at a time, and the solution should be
kept in a well-closed vessel, for after long exposure to the air it is
not equally valuable.
If the stopper is to fit a tube contracted like _B_, it must be
constructed from a piece of tube that will pass through the contraction
at _B_. The tail _GF_ will not do such good service as it does in the
case of a tube which has been opened out to receive its stopper, but it
will help to guide the stopper, and should be retained.
When the stopper has been ground into its place, melt off the tail at
_F_. The flame must be applied very cautiously, as glass which has been
ground is particularly apt to crack on heating. To avoid all risk of
this, the tail may simply be cut off, and its edges filed smooth with a
file moistened freely with camphorated turpentine.
The stoppers of bottles are not made exactly in the manner described
above, though, on occasion, a new stopper may be made for a bottle by
following those directions. Ill-fitting stoppers, which are very common,
can be very easily re-ground with emery and camphorated turpentine.
CHAPTER IV.
_MAKING THISTLE FUNNELS, U-TUBES, ETC.--COMBINING THE PARTS OF
COMPLICATED APPARATUS--MERCURY, AND OTHER AIR-TIGHT JOINTS--VACUUM
TAPS--SAFETY TAPS--AIR-TRAPS._
In Chapter III. the simpler operations used in making the separate parts
of which apparatus is composed have been described. In this Chapter
finished apparatus will be described, and the combination of the
separate parts into the more or less complicated arrangements used in
experiments will be so far explained as to enable the student to set up
such apparatus as he is likely to require. I have thought it would be
useful that I should add a short account of various contrivances that
have come much into use of late years for experimenting under reduced
pressure, such as safety taps, air-traps, vacuum joints, etc.
[Illustration: FIG. 24.]
=Electrodes.=--On page 38 (Fig. 13) is shown a simple form of electrode
sealed into a glass tube, which for many purposes answers very well. But
frequently, in order that there may be less risk of leakage between the
glass and the metal, the latter is covered for a considerable part of
its length with solid glass, which at one extremity is united to the
apparatus. In Fig. 24 _W_ is the metal core of the electrode, and _G_
the glass covering around it. The wire is fused into the glass, and the
glass is then united to the apparatus; a little white enamel should be
applied at one end and combined with the glass by fusion.
=U-Tubes.=--A U-tube is but a particular case of a bent glass tube. It
is scarcely possible when bending very large tubes in the manner
described on p. 29 to produce regular curves of sufficient strength.
To make a U-tube, or to bend a large tube, close one end of the tube
selected with a cork, soften and compress the glass in the flame at the
part where it is to be bent till a sufficient mass of glass for the bend
is collected, then remove the mass of glass from the flame, let it cool
a little, and simultaneously draw out the thickened glass, bend it to
the proper form, and blow the bend into shape from the open end of the
tube. Small irregularities may be partly corrected afterwards.
To make a good U-tube of large size, and of uniform diameter from end to
end, requires much practice, but to make a tolerably presentable piece
of apparatus in which the two limbs are bent round till they are
parallel, without any considerable constriction at the bend, can be
accomplished without much difficulty.[11]
[11] Large tubes may also be bent by rotating a sufficient length of the
tube in a large flame till it softens, and bending in the same manner as
in the case of smaller tubes, and after filling them with sand, closing
one end completely, and the other so that the sand cannot escape, though
heated air can do so.
=Spiral Tubes.=--These may be made by twisting a tube gradually softened
by heat round a metal cylinder. Spiral tubes made of small thin tubes
possess considerable elasticity, and have been used by Mr. Crookes for
making air-tight connections between separate pieces of apparatus when a
rigid connection would have been unnecessary and inconvenient. By the
use of such spiral tubes it is possible to combine comparatively free
movement with all the advantages attached to hermetically-sealed joints.
To make a flexible spiral tube, mount a copper cylinder on a screw, so
that the cylinder will travel in the direction of its axis when it is
rotated. Fix a fine glass tube to the cylinder, and direct a flame
towards the cylinder so as to heat and soften the glass, which will then
bend to the form of the cylinder. Gradually rotate the cylinder before
the source of heat, so that fresh portions of tube are successively
brought into position, softened, and bent. Useful spirals may also be
made by hand without a cylinder. As each length of tube is bent, a fresh
length may be united to it until the spiral is completed. The fine tubes
employed are prepared by heating and drawing out larger tubes.
[Illustration: FIG. 25.]
=Thistle Funnels= (Fig. 25).--Seal a moderately thick piece of small
glass tube at _A_, then heat a wide zone of it a little below _A_ by
rotating it horizontally in the blow-pipe flame till the glass softens,
and expand the glass to a bulb, as shown at _B_ of 1; during the
operation of blowing this bulb, the end _A_ must be directed to the
ground.
Soften the end _A_ and a small portion of _B_ as before, and, holding
the tube horizontally from the mouth, blow out the end _C_ as at 2. Heat
the end of _C_ gradually, till the glass softens and collapses to the
dotted line _dd_, and at once blow a steady stream of air into the open
end of the tube, rotating it steadily, till it is about to burst;
finally clean off the thin glass from round the edges of the funnel,
which should have the form shown at 3, and round them. An inspection of
a purchased thistle funnel will generally show that the head _B_ has
been formed from a larger tube sealed to _E_ at _f_.
[Illustration: FIG. 26.]
=Closing Tubes containing Chemicals= for experiments at high
temperatures.--Tubes of the hard glass used for organic analyses answer
best for this purpose; the operation of drawing out the end of such a
tube is practically identical with what has been described under the
head of choking, p. 35. A well-sealed tube presents the appearance of
that shown by Fig. 26.
In order to secure a thick end to the point of the tube _a_, about an
inch or so of the tube near the contracted part should be warmed a
little, if it is not already warm, at the moment of finally sealing it;
the contraction of the air in the tube, in consequence of the cooling of
the warm tube, will then ensure the glass at _a_ running together to a
solid end when it is melted in the flame.
If it will be necessary to collect a gas produced during a chemical
action from such a tube, make the contracted end several inches long,
and bend it into the form of a delivery tube. It will then be possible
to break the tip of this under a cylinder in a trough of liquid.
=In order to explain the construction of apparatus consisting of several
parts=, it will be sufficient to take as examples, two very well-known
instruments, and to describe their construction in detail. From what is
learned in studying these, the student will gather the information that
is wanted.
[Illustration: FIG. 27.]
1. _To make Hofman's Apparatus for the electrolysis of water_ (Fig. 27).
Take two tubes about 35 cm. in length, and 14 mm. in diameter for _AA_,
join taps _TT_ to the end _B_ of each of them, draw out the other end,
as shown at _D_, after sheets of platinum foil with wires attached to
them[12] have been introduced into the tubes, and moved by shaking to
_BB_. Then allow the platinum wires to pass through the opening _D_ left
for the purpose, and seal the glass at _D_ round the platinum as at _E_.
Pierce the tubes at _JJ_, and join them by a short piece of tube _K_,
about 14 mm. in diameter, to which the tube _T_, carrying the reservoir
_R_, has been previously united. _R_ may be made by blowing a bulb from
a larger piece of tube attached to the end of _T_. The mouth _M_ of the
reservoir being formed from the other end of the wide tube afterwards.
One of the taps can be used for blowing through at the later stages.
Each joint, especially those at _JJ_, must be annealed after it is
blown. Some operators might prefer to join _AA_ by the tube _K_ in the
first instance, then to introduce the electrodes at _E_ and _D_. In some
respects this plan would be rather easier than the other, but, on the
whole, it is better to make the joints at _JJ_ last in order, as they
are more apt to be broken than the others during the subsequent
manipulations.
[12] Red-hot platinum welds very well. The wire may be joined to the
sheet of foil by placing the latter on a small piece of fire-brick,
holding the wire in contact with it at the place where they are to be
united, directing a blow-pipe flame upon them till they are at an
intense heat, and smartly striking the wire with a hammer. The blow
should be several times repeated after re-heating the metal.
2. I have before me the vacuum tube shown by Fig. 28, in which the
dotted lines relate to details of manipulation only.
[Illustration: FIG. 28.]
It is usually possible to detect the parts of which a piece of apparatus
has been built up, for even the best-made joints exhibit evidence of
their existence. Thus, although I did not make the tube that is before
me, and cannot therefore pretend to say precisely in what order its
parts were made and put together, the evidence which it exhibits of
joints at the dotted lines _A_, _B_, _C_, _D_, _E_, _F_, enables me to
give a general idea of the processes employed in its construction, and
to explain how a similar tube might be constructed. I should advise
proceeding as follows:--
Join a piece of tube somewhat larger than _M_ to its end _A_, draw out
the other end of the larger tube, and blow a bulb _L_ as directed on p.
47. Then seal the electrode _R_ into the bulb _L_ (p. 55).
Blow a similar but larger bulb _N_ from a large piece of tube sealed
between two tubes of similar size to _M_, as described at p. 50. Cut off
one of the tubes at _B_, and join the bulb _N_ to _M_ at _B_. Form the
bulb _Q_ in the same manner as in the case of _L_, seal into it the
electrode _R_, and add the tube marked by the dotted lines at _F_.
Seal a narrow tube _P_ to the end of a larger tube, and blow out the
tube at the joint till the glass is thin and regular. Take a tube _O_,
of similar size to _M_, slightly longer than _P_, contract its mouth
slightly to meet the wide end of _P_ at _D_, and after loosely
supporting _P_ inside _O_ with a cork, or otherwise, close the end _N_
of _O_ by sealing or corking it, and join _P_ to _O_ at _D_. Cut off _O_
just above _D_ at _E_, and join it to the bulb _Q_, closing either _O_
or _F_ for the purpose. Cut off the end of _O_ at _C_ parallel to the
end of _P_, and connect _O_ to _N_, using _F_ for blowing the joint at
_C_. _F_ may be used subsequently for introducing any gas into the tube,
and, when a vacuum has been established, may be sealed before the
blow-pipe.
[Illustration: FIG. 29.]
=Modes of combining the Parts of Heavy Apparatus.=--It is often
necessary to connect pieces of apparatus which are too heavy to be
freely handled before the blow-pipe, and which, therefore, cannot be
welded together as described on p. 39, by some more effective method
than the ordinary one of connecting by india-rubber tubing. For example,
apparatus which is to be exhausted by a Sprengel air-pump must be
attached to the pump by a joint as perfectly air-tight as can be
obtained. This, indeed, often may be done by welding the apparatus to be
exhausted to the air-pump before the blow-pipe. But such a method is
open to the obvious objection that it is very troublesome to connect and
disconnect the parts as often as may be necessary, and that there is
some risk of accidental breakages. Nevertheless it may be done on
occasion, especially if there be no objection to the use of the
flexible spiral tubes already alluded to. When the use of a spiral
connecting-tube is not admissible the difficulty is considerably
increased. For example, the author has lately required to attach an
ozone generator, of the form shown by Fig. 19, which previously had been
cemented into a heavy copper jacket, to a pressure-gauge rigidly fixed
to a support, and of considerable size. The employment of a flexible
spiral connection was prohibited by the fact that it was necessary that
the volume of the connecting-tube should be but a small fraction of that
of the ozone generator, a condition which compelled the use of a tube of
almost capillary bore, and of inconsiderable length. At the same time
the frailness of such a connection made it necessary to fix the
generator and pressure-gauge rigidly to their supports, in order to
avoid the possibility of breakage by slight accidental movements of
either of them, and it was obviously necessary to fix the pieces of
apparatus in their final positions before joining them, lest the fine
tube which connected them should be fractured during adjustment. The
possibility of a strain being caused by the contraction that would occur
during the cooling down of the joint last made had to be provided for
also. The desired object was effected as follows. In Fig. 29 _A_
represents a section of the ozone generator at the point where the tube
to connect it to the gauge was fixed. _B_ represents the top of the
gauge, with the side tube _C_, which was to be connected with that from
_A_, viz. _D_. The ends of _C_ and _D_ were expanded as shown at _D_ (by
melting them and blowing them out), so that one of them, made rather
smaller than the other, could be overlapped by the larger one. _A_ and
_B_ being rigidly fixed in their final positions, with _C_ and _D_ in
contact, as shown in the figure, all openings in the apparatus were
closed, except one, to which was attached an india-rubber blowing-bottle
by means of a tube of india-rubber long enough to be held in the hand of
the operator, and to allow him to observe the operation of joining the
tubes at _D_. When everything was in readiness, a very small-pointed
flame from a moveable blow-pipe held in the hand was directed upon the
glass at _D_ till it melted and the two tubes united. To prevent the
fine tube when melted from running into a solid mass of glass, and so
becoming closed, a slight excess of pressure was maintained inside the
apparatus during the operation by forcing air into it with the
india-rubber blower from the moment at which _C_ and _D_ united. A point
of charcoal was kept in readiness to support the softened glass at _D_
in case it showed any tendency to fall out of shape.
The V-tube at _C_ served to prevent the subsequent fracture of the joint
in consequence of any strain caused by the contraction of the glass in
cooling.[13]
[13] For a method of joining soda glass to lead glass, see p. 81.
It is not difficult to connect several pieces of apparatus successively
in this manner, nor is this method only useful in such cases as that
just described. Pieces of apparatus of great length and weight may be
joined in a similar manner, irrespective of the size of the tubes to be
united.
The ends to be joined, prepared as before, so that one slightly overlaps
the other, must be held firmly in contact by clamps, and heated in
successive portions by a blow-pipe held in the hand of the operator,
each patch of glass being re-heated and gently blown, after a rough
joint has been made. Finally, a larger flame may be used to heat up the
whole joint for its final blowing. It is important to place the
apparatus so that the operator has free access to it on all sides. A
revolving table might be employed. An assistant to work the bellows is
necessary. Or, better still, air may be admitted to the blow-pipe from a
large gas-bag placed in some convenient position.
But in most cases one or other of the following air-tight joints can be
employed, and will be found to be very convenient:--
=Mercury Joints.=--The simplest form of mercury joint is shown at Fig.
30. _A_ and _B_ are the two tubes which are to be connected. A larger
tube or cup _F_ is attached to _A_ by the india-rubber tube _E_, and
placed on _A_ so that the end of _B_ may be brought into contact with
_A_ at _C_, and connected to it by a well-fitting piece of india-rubber
tube _C_. The cup _E_ is then brought into the position shown in Fig.
30, and mercury is introduced till the india-rubber tube at _C_ is
covered. As mercury and glass do not come into true contact, however,
such a joint, though said to give good results in practice, is not
theoretically air-tight, for air _might_ gradually find its way between
the liquid and the glass. By covering the mercury with a little
sulphuric acid or glycerine the risk of this occurring may be removed.
The same result may be attained by the use of glycerine in place of the
mercury in the cup _F_; but glycerine is less pleasant to work with than
mercury.[14]
[14] If the india-rubber tube _C_ be secured by wires, iron wire, not
copper wire, should be employed.
[Illustration: FIG. 30.]
When sulphuric acid is to be employed in such a joint, or when for any
other reason the use of an india-rubber tube is undesirable, the joint
may consist of a hollow stopper _B_ (Fig. 31), made of glass tube, and
ground to fit the neck of a thistle funnel _A_. _A_ and _B_ are joined
respectively to the pieces of apparatus to be connected, and connection
is made by placing _B_ in position in the neck of _A_; the joint is made
air-tight by introducing mercury with strong sulphuric acid above it
into the cup _A_. The joint may be rendered air-tight by introducing
sulphuric acid only into the cup. But this plan must not be adopted if
the interior of the apparatus is to be exhausted, as sulphuric acid is
easily forced between the ground glass surfaces by external pressure.
Mercury, however, will not pass between well-ground glass surfaces, and
is therefore to be employed for connecting apparatus which is to be
exhausted, and, if necessary, protected by a layer of strong sulphuric
acid to completely exclude air.
[Illustration: FIG. 31.]
Tubes placed horizontally may be joined by a glycerine or mercury joint
such as is shown in Fig. 32. The two tubes _A_ and _B_ are joined as
before by an india-rubber connection _C_, or one may be ground to fit
the other, and the joint is then enclosed within a larger jacketing-tube
_D_, with a mouth at _F_, which is filled with glycerine or mercury. _D_
is easily made by drawing out both ends of a piece of tube, leaving them
large enough to pass over the connection at _C_, however, and piercing
one side at _F_.
[Illustration: FIG. 32.]
=Vacuum Taps.=--It is not necessary to enter into a description of the
construction of ordinary glass taps, which can be purchased at very
reasonable prices. It may be remarked here, however, as a great many of
them are very imperfectly ground by the makers, that they may easily be
made air-tight by hand-grinding with camphorated turpentine and fine
emery, finishing with rotten-stone. A well-ground tap, which is well
lubricated, should be practically air-tight under greatly reduced
pressure for a short period; but when it is necessary to have a tap
which absolutely forbids the entrance of air into apparatus, one of the
following may be employed:--
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
(1.) _Mr. Cetti's Vacuum Tap_ (Fig. 34): This tap is cupped at _A_ and
sealed at _B_, and the cup _A_ is filled with mercury when the tap is in
use, so that if, for example, the end _C_ be attached to a flask, and
_D_ to an apparatus for exhausting the flask, it will be possible to
close the flask by turning off the tap _E_, and if no air be allowed
access through _D_, the vacuum produced in the flask at _C_ cannot be
affected by air leaking through the tap at _A_ or _B_.
A passage _F_ must be drilled from the bottom of the plug _E_ to meet
_G_, in order that when the plug is in position no residue of air shall
be confined within _B_, whence it might gradually leak into any
apparatus connected to it.
It is obvious, however, that this tap does not protect a flask sealed
to _C_ from the entrance of air through _D_, which, in fact, is the
direction in which air is most likely to effect an entrance. When using
one of these taps as part of an apparatus for supplying pure oxygen, I
have guarded against this by attaching a trap (Fig. 33) to the end _D_,
_C_ being joined to the delivery tube from the gas-holder. The structure
and mode of action of the trap are as follows:--
A narrow tube _G_ is joined to _D_ of Fig. 34, and terminates in the
wide tube _I_, which is connected above to _H_, and below to the
air-trap _J_. _J_ is connected at _K_, by a piece of flexible tube, to a
reservoir of mercury, from which mercury enters the air-trap, and
passing thence to _I_, can be employed for filling the V-trap _HLG_. The
air-trap _J_ is in the first instance filled with mercury, and then
serves to intercept any stray bubbles of air that the mercury may carry
with it. The particular form of the trap shown at _HLG_ was adopted
because with it the arm _LG_ is more readily emptied of mercury than
with any other form of trap made of small tube that I have tried. It has
been used in my apparatus in the following manner:--_H_ was connected
with a vessel to be filled with pure oxygen, the tap _E_ closed, and the
rise of mercury above _L_ prevented by a clamp on the flexible tube; the
vessel to be filled and the trap were then exhausted by a Sprengel pump,
and oxygen allowed to flow into the exhausted space by opening _E_, the
operation of exhausting the tubes and admitting oxygen being repeated as
often as necessary.
To prevent access of air to _E_ on disconnecting the vessel at _H_, the
mercury was allowed to flow into the trap till it reached to _MM_. _E_
was then closed, and _H_ exposed without danger of air reaching _E_, the
length of the arms of the trap being sufficient to provide against the
effects of any changes of temperature and pressure that could occur.
A delivery tube may be connected to _H_ and filled with mercury, by
closing _E_ and raising the mercury reservoir. All air being in that
way expelled from the delivery tube, and the supply of mercury cut off
by clamping the tube from the reservoir, oxygen can be delivered from
the tube by opening _E_, when it will send forward the mercury, and pass
into a tube placed to receive it without any risk of air being derived
from the delivery tube.
[Illustration: FIG. 35.]
(2.) _Gimmingham's Vacuum Tap_,[15] shown in Fig. 35, consists of three
parts. A tube _A_ is ground to fit the neck of _B_. _B_ is closed at its
lower end, and has a hole _d_ drilled through it; when _B_ is fitted to
_C_, _d_ can be made to coincide with the slit _e_. When _A_, _B_, _C_
are fitted together, if _d_ meet _e_, there is communication between any
vessels attached to _A_ and any other vessel attached to _C_, entrance
of external air being prevented by mercury being placed in the cups of
_C_ and _B_. The tap may be opened and closed at pleasure by rotating
_B_.
[15] From _Proceedings of Royal Society_, vol. XXV. p. 396.
If _A_ has to be removed, _C_ may be converted into a mercury joint _pro
tem._ by letting a little mercury from the upper cup fall into the tube
and cover _d_, the tap being closed. This mercury must be removed by a
fine pipette in order to use the tap again. It should be noted, however,
that though external air cannot enter by way of the ground glass joints,
there is no absolute protection against the passage of air between _A_
and _C_, or vessels joined to _A_ and _C_, even when the tap is closed.
The passage of air from _A_ to _C_ depends upon the grinding and
lubrication of the joint at _C_.
=Lubricating Taps.=--For general purposes resin cerate answers very
well. In special cases burnt india-rubber, or a mixture of burnt
india-rubber and vaseline will answer well, or vaseline may be used
alone. Sulphuric acid and glycerine are too fluid. When a lubricant is
wanted that will withstand the action of ether, the tap may be
lubricated by sprinkling phosphorus pentoxide upon it, and exposing it
to air till the oxide becomes gummy. The joint must then be protected
from the further action of the air if possible. For example, if a safety
tap be used the cup may be filled with mercury.
=Air-Traps.=--In Fig. 33, p. 66, an air-trap (_J_) is shown. An air-trap
is a device for preventing the mercury supplied to Sprengel pumps, etc.,
from carrying air into spaces that are exhausted, or are for any reason
to be kept free from air. Figs. 36 and 37 give examples of air-traps. In
the simpler of the two (Fig. 36) mercury flowing upwards from _C_ that
may carry bubbles of air with it passes through the bulb _A_, which is
_filled_ with mercury before use.[16] Any air which accompanies the
mercury will collect at _a_, the mercury will flow on through _b_. So
long as the level of the mercury in A is above _b_, the trap remains
effective.
[16] This may be done by clamping the tube which supplies mercury below
_C_, exhausting _A_, and then opening the clamped tube and allowing the
mercury to rise.
[Illustration: FIG. 36.]
[Illustration: FIG. 37.]
In the trap shown by Fig. 37, the tube _d_, which corresponds to _b_ in
Fig. 36, is protected at its end by the cup _E_. _E_ prevents the direct
passage of minute bubbles of air through _d_. This trap, like the other,
must be filled with mercury before it is used, and it will then remain
effective for some time.
CHAPTER V.
_GRADUATING AND CALIBRATING GLASS APPARATUS._
Although the subjects to which this concluding chapter is devoted do
not, properly speaking, consist of operations in glass-blowing, they are
so allied to the subject, and of such great importance, that I think a
brief account of them may advantageously be included.
=Graduating Tubes, etc.=--It was formerly the custom to graduate the
apparatus intended for use in quantitative work into parts of equal
capacity; for example, into cubic centimetres and fractions of cubic
centimetres. For the operations of volumetric analysis by liquids this
is still done. But for most purposes it is better to employ a scale of
equal divisions by length, usually of millimetres, and to determine the
relative values of the divisions afterwards, as described under
calibration. It rarely happens that the tube of which a burette or
eudiometer is made has equal divisions of its length of exactly equal
capacities throughout its entire length, and indeed, even for ordinary
volumetric work, no burette should be employed before its accuracy has
been verified. An excellent method for graduating glass tubes by
hand[17] has been described in Watts's _Dictionary of Chemistry_, and
elsewhere. Another excellent plan, which I have permission to describe,
has been employed by Professor W. Ramsay. It will be sufficient if I
explain its application to the operation of graduating a tube or strip
of glass in millimetre divisions.
[17] Originally suggested by Bunsen.
The apparatus required consists of a standard metre measure,[18] divided
into millimetres along each of its edges, with centimetre divisions
between them, a ruler adapted to the standard metre, as subsequently
explained, and a style with a fine point for marking waxed surfaces.
[18] Such measures can be obtained of steel for about _fifteen
shillings_ each. They are made by Mr. Chesterman of Sheffield. They can
be obtained also from other makers of philosophical instruments, at
prices depending upon their delicacy. Those of the greatest accuracy are
somewhat costly.
[Illustration: FIG. 38.]
Fig. 38 represents the standard measure, and the ruler.
At _AA_ are the millimetre divisions on the edges of the measure, the
longer transverse lines at _BB_ are placed at intervals of five
millimetres and of centimetres. The ruler is in the form of a
right-angled triangle; it is shown, by the dotted lines, in position on
the standard metre measure at _I_; and again, with its under surface
upwards, in the smaller figure at 2. It consists of a perfectly flat
sheet of metal, about ten centimetres in length from _C_ to _C_,
sufficiently thick to be rigid, and has a ledge, _DD_ in each figure,
which is pressed against the side of the measure when using it, to
ensure that the successive positions of the edge (_LL_) shall be
parallel to each other. At _GG_ are two small holes, into which fit
small screws with fine points. These must be in a line parallel to the
edge (_LL_), so that when the ruler is in position on the scale, the
points of the two screws, which project slightly, shall fall into
corresponding cuts on the divided scales (_AA_).
To graduate a strip of glass, or a glass tube (_HH_), the surface to be
marked must first be coated with wax, which should be mixed with a
little turpentine, and be applied to the surface of the glass,
previously made _warm_ and _dry_, by means of a fine brush, so as to
completely cover it with a thin, closely-adherent, and
evenly-distributed coat of wax, which must be allowed to cool.
Fix _HH_ firmly on a table, and fix the standard measure by the side of
_HH_. If the thickness of _HH_ be about equal to, but not greater than
that of the standard measure, this may be done by large drawing-pins.
If, however, a large tube or thick sheet of glass is to be graduated,
fix it in position by two strips of wood screwed to the table on each
side of it. One of these wooden strips, on which the measure may be
placed, may be about as broad as the standard measure, and of such
thickness that when the measure lies upon it beside the tube to be
graduated, the ruler, when moved along the measure, will move freely
above the tube, but will not be elevated more than is necessary to
secure free movement. The second strip of wood may be narrower, and of
the same thickness as the broader piece on which the standard measure
rests. In any case, let the standard measure and the object to be
graduated be very firmly secured in their places. Bring the ruler into
position at any desired part of the tube by placing the points of the
screws (_GG_) in corresponding divisions of the scales (_AA_). With the
style, which may be a needle mounted in a handle, make a scratch in the
wax along the edge of the ruler at _F_, move the ruler so that the
screws rest in the next divisions, and repeat the operation till the
required number of lines has been ruled. Longer marks may be made at
intervals of five and ten millimetres. Great care must be taken to hold
the needle perpendicularly, and to press it steadily against the edge
(_LL_) of the ruler in scratching the divisions.[19] The length of the
lines marking the millimetre divisions should not be too long; about 1
mm. is a good length. If they are longer than this, the _apparent_
distance between them is diminished, and it is less easy to read
fractions of millimetres. Before removing the scale to etch the glass,
carefully examine it to see that no mistakes have been made. If it is
found that any lines have been omitted, or that long lines have been
scratched in the place of short ones, remelt the wax by means of a
heated wire, and make new marks. Finally, mark the numbers on the scale
with a needle-point, or better, with a fine steel pen.
[19] To avoid variations of the position in which the needle is held
when marking the divisions, the edge (_LL_) should not be bevelled; and
an upright support may be placed upon the ruler, with a ring through
which the handle of the needle passes, thereby securing that the angle
formed by the needle and surface of the ruler is constant, and that
equal divisions are marked.
The marks on the wax should cut through it. When they are satisfactory,
they may be etched by one of the following processes:--
(1.) By moistening some cotton wool, tied to a stick, with solution of
hydrofluoric acid, and gently rubbing this over the scratched surface
for a minute or so; then washing away the acid with water, and cleaning
off the wax. This is the simplest method, but the marks made are
generally transparent, and therefore not very easy to read. The
simplicity of this method is a great recommendation, however.
(2.) Expose the tube to the fumes of hydrofluoric acid generated from a
mixture of powdered fluor-spar and strong sulphuric acid, in a leaden
trough. The marks produced in this way are usually opaque, and are
therefore very visible, and easily read.
After the above detailed account it will only be necessary to give an
outline of the other process of graduating tubes.
[Illustration: FIG. 39.]
The standard scale to be copied, _A_, which may in this case be another
graduated tube, or even a paper scale, and the object to be ruled, _B_,
are securely fixed, end to end, a little distance apart, in a groove
made in a board or in the top of a table. A stiff bar of wood, _C_, has
a point fixed at _D_, and a knife edge at _E_, _D_ is placed in any
division of _A_, _C_ is held firmly at _E_ and _D_, and a cut is made by
the knife through the wax on _B_, the point _D_ is then moved into the
next division, and the operation is repeated. To regulate the length and
position of the cuts, _B_ is usually held in position by two sheets of
brass projecting over the edges of the groove in which it lies; the
metal sheets have notches cut into them at the intervals at which longer
marks are to be made.
When the scale is completed, the equality of the divisions in various
parts of it may be, to some extent, verified as follows:--Adjust a
compass so that its points fall into two divisions 5, 10, or 20 mm.
apart. Then apply the points of the compass to various parts of the
scale. In every part the length of a given number of divisions should be
exactly the same. The individual divisions should also be carefully
inspected by the eye; they should be sensibly equal. If badly ruled,
long and short divisions will be found on the scale. Very often a long
and a short division will be adjacent, and will be the more easily
observed in consequence.
=To Divide a Given Line into Equal Parts.=--Occasionally it is necessary
to divide a line of given length into _x_ equal parts. For instance, to
divide the stem of a thermometer from the freezing-point to the
boiling-point into one hundred degrees.
The following outline will explain how a line may be so divided. Suppose
the line _AB_ (Fig. 40) is to be divided into nine equal parts. Adjust a
hinged rule so that the points _A_ and _B_ coincide with the inside
edges of the limbs, one of them, _A_, being at the ninth division
(_e.g._ the ninth inch) of _CE_. Then if lines parallel to _ED_ be drawn
from each division of the scale to meet _AB_, _AB_ will be divided into
nine equal parts.
[Illustration: FIG. 40.]
A very convenient and simple arrangement on this principle for dividing
a line into any number of equal parts with considerable accuracy, is
described by Miss S. Marks in the _Proceedings of the Physical Society_,
July 1885.[20] One limb of a hinged rule _D_ is made to slide upon a
plain rule fixed to it; the plain rule carries needles on its under
surface which hold the paper in position. The position of the divided
rule and line to be divided being adjusted, the hinged rule is gently
pushed forwards, as indicated by the arrow in Fig. 40, till division
eight coincides with the line _AB_. A mark is made at the point of
coincidence, and division seven on the scale is similarly brought to the
line _AB_, and so on. The inner edge of _EC_ should have the divisions
marked upon it, that their coincidence with _AB_ maybe more accurately
noted. The joint _E_ must be a very stiff one.
[20] Since this was printed I have observed that the above method is not
identical with that described by Miss Marks, but for ordinary purposes I
do not think it will be found to be inferior.
A line drawn of given length or a piece of paper may be divided into any
given number of equal parts, and will then serve as the scale _A_ of
Fig. 39, p. 74, the thermometer or other object to be graduated taking
the place of _B_.
Scales carefully divided according to any of the methods described will
be fairly accurate _if trustworthy instruments have been employed as
standards_.
It will be found possible when observing the volume of a gas over
mercury, or the height of a column of mercury in a tube, to measure
differences of one-sixth to one-eighth of a millimetre with a
considerable degree of accuracy. To obtain more delicate measurements a
vernier[21] must be employed.
[21] For the nature and use of the vernier, a treatise on Physics or
Physical Measurements may be consulted.
=To Calibrate Apparatus.=--The glass tubes of which graduated apparatus
is made are, as already stated, very rarely truly cylindrical
throughout their entire lengths. It follows that the capacities of equal
lengths of a tube will usually be unequal, and therefore it is necessary
to ascertain by experiment the true values of equal linear divisions of
a tube at various parts of it.
A burette may be calibrated by filling it with distilled water, drawing
off portions, say of 5 c.c. in succession, into a weighing bottle of
known weight, and weighing them.
Great care must be taken in reading the level of the liquid at each
observation. The best plan is to hold a piece of white paper behind the
burette, and to read from the lower edge of the black line that will be
seen. Each operation should be repeated two or three times, and the mean
of the results, which should differ but slightly, may be taken as the
value of the portion of the tube under examination.
If the weights of water delivered from equal divisions of the tube are
found to be equal, the burette is an accurate one, but if, as is more
likely, different values are obtained, a table of results should be
drawn up in the laboratory book showing the volume of liquid delivered
from each portion of the tube examined. And subsequently when the
burette is used, the volumes read from the scale on the burette must be
corrected. Suppose, for example, that a burette delivered the following
weights of water from each division of 5 c.c. respectively:--
C.C. Grams.
0 to 5 gave 4.90
5 " 10 " 4.91
10 " 15 " 4.92
15 " 20 " 4.93
20 " 25 " 4.94
25 " 30 " 4.95
30 " 35 " 4.96
35 " 40 " 4.97
40 " 45 " 4.98
45 " 50 " 4.99
and that in two experiments 20 c.c. and 45 c.c. respectively of a liquid
re-agent were employed. The true volumes calculated from the table would
be as 19.66 to 44.46.
If the temperature remained constant throughout the above series of
experiments, and if the temperature selected were 4 deg. C., the weights of
water found, taken in grams, give the volumes in cubic centimetres, for
one gram of water at 4 deg. C. has a volume of one cubic centimetre. If the
temperature at which the experiments were made was other than 4 deg. C., and
if great accuracy be desired, a table of densities must be consulted,
with the help of which the volume of any weight of water at a known
temperature can be readily calculated.
Pipettes which are to be used as measuring instruments should also have
the relation one to another of the volumes of liquid which they deliver
determined, and also the proportions these bear to the values found for
the divisions of the burettes in conjunction with which they will be
employed.
=To Calibrate Tubes for Measuring Gases.=--Prepare a small glass tube
sealed at one end and ground at the other to a plate of glass. The tube
should hold about as much mercury as will fill 10 mm. divisions of the
graduated tube. Fill this tube with mercury, removing all bubbles of air
that adhere to the sides by closing the open end of the tube with the
thumb, and washing them away with a large air-bubble left for the
purpose. If any persistently remain, remove them by means of a fine
piece of bone or wood. Then completely fill the tube with mercury,
removing any bubbles that may be introduced in the operation, and remove
the excess of mercury by placing the ground-glass plate on the mouth of
the tube, and pressing it so as to force out all excess of mercury
between the two surfaces. Clean the outside of the tube, and place it on
a small stand (this may be a small wide-mouthed glass bottle), with
which it has been previously weighed when empty, and re-weigh. Repeat
this operation several times. From the mean of the results, which should
differ one from another but very slightly, the capacity of the tube can
be calculated.
The purest mercury obtainable should be used. Since the density of pure
mercury at 0 deg. C. is 13.596, the weight of mercury required to fill the
tube at 0 deg. C., taken in grams, when divided by 13.596, will give the
capacity of the tube at 0 deg. C. in cubic centimetres. If the experiment be
not made at 0 deg. C., and if a very exact determination of the capacity of
the tube be required, the density of mercury must be corrected for
expansion or contraction.
Having now a vessel of known capacity, it can be employed for
ascertaining the capacities of the divisions of a graduated tube in the
following manner:--The graduated tube is fixed perpendicularly, mouth
upwards, in a secure position. The small tube of known capacity is
filled with mercury as previously described, and its contents are
transferred to the divided tube. The number of divisions which the known
volume of mercury occupies is noted after all air-bubbles have been
removed. This process is repeated until the divided tube is filled. A
table of results is prepared, showing the number of divisions occupied
by each known volume of mercury introduced.
In subsequently using the tube the volumes of the gases measured in it
must be ascertained from the table of values thus prepared.
In observing the level of the mercury, unless a cathetometer is
available, a slip of mirror should be held behind the mercury close to
the tube, in such a position that the pupil which is visible on the
looking-glass is divided into two parts by the surface of the mercury.
A correction must be introduced for the error caused by the meniscus of
the mercury. As the closed end of the tube was downwards when each
measured volume of mercury was introduced, and as the surface of mercury
is convex, the volume of mercury in the tube when it is filled to any
division _l_ (Fig. 41) is represented by _A_ of 1. But in subsequently
measuring a gas over mercury in the same tube, when the mercury stands
at the same division _l_, the volume of the gas will be as represented
by _B_ of 2, which is evidently somewhat greater than _A_. This will be
seen still more clearly in 3, where _a_ represents the boundary of the
mercury, and _b_ the boundary of the air, when the tube is filled to the
mark _l_ with mercury or a gas over mercury respectively.
[Illustration: FIG. 41.]
It is plain that when the level of the mercury in measuring a gas is
read at _l_, the volume of the gas is greater than the volume of the
mercury recorded, by twice the difference between the volume _A_ of
mercury measured, and that which would fill the tube to the level _l_,
if its surface were plane.
The usual mode of finding the true volume of a gas collected over
mercury is as follows:--
Place the graduated tube mouth upwards, introduce some mercury, and,
after removing all bubbles, note the division at which it stands. Then
add a few drops of solution of mercuric chloride; the surface of the
mercury will become level, read and record its new position. Then, in
any measurement, having observed that the mercury stands at _n_
divisions of the tube, add twice the difference between the two
positions of the mercury to _n_, and ascertain the volume which
corresponds to this reading from the table of capacities.
=To Calibrate the Tube of a Thermometer.=--Detach a thread of mercury
from half an inch to one inch in length from the body of the mercury.
Move it from point to point throughout the length of the tube, and note
its length in each position. If in one part it occupies a length of tube
corresponding to eight degrees, and at another only seven degrees, then
at the former point the value of each division is only seven-eighths of
those at the latter position.
From the results obtained, a table of corrections for the thermometer
should be prepared.
It is sometimes necessary to join soda glass to lead glass. In this case
the edge of the lead glass tube may be bordered with white enamel before
making the joint. Enough enamel must be used to prevent the lead and
soda glasses from mingling at any point. The enamel is easily reduced,
and must be heated in the oxidising flame. Dr. Ebert recommends _Verre
d'urane_ for this purpose. It is supplied by Herr Goetze of Leipzig
(Liebigstrasse).
CHAPTER VI.
_GLASS TUBING._
The diagrams given below show the sizes and thickness of the glass tubes
most frequently required. In ordering, the numbers of these diagrams may
be quoted, or the exact dimensions desired may be stated.
Glass tubes are usually sold by weight, and therefore the weight of tube
of each size that is wished for should be indicated, and also whether it
is to be of lead or soda glass.
[Illustration]
[Illustration]
CHAPTER VII.
_VITREOUS SILICA._
=Introductory.=--Vitreous Silica was made in fine threads by M. Gaudin
in 1839,[22] and small tubes of it were made in 1869 by M. A. Gautier,
but its remarkable qualities were not really recognised till 1889, when
Professor C. V. Boys rediscovered the process of making small pieces of
apparatus of this substance, and used the torsion of "quartz fibres" for
measuring small forces. More recently the author of this book has
devised a process for preventing the "splintering" of quartz which gave
so much trouble to the earlier workers, and jointly with Mr. H. G.
Lacell, has produced a variety of apparatus of much larger dimensions
than had been attempted =previously=. At the time of writing we can
produce by the processes described in the following pages tubes 1 to 1.5
cm. in diameter and about 750 cm. in length, globes or flasks capable of
containing about 50 cc., masses of vitreous silica weighing 100 grams or
more, and a variety of other apparatus.
[22] A brief summary of the history of this subject will be found in
_Nature_, Vol. 62, and in the Proceedings of the Royal Institution,
1901.
=Properties of Vitreous Silica.=--For the convenience of those who are
not familiar with the literature of this subject, I may commence this
chapter with a brief account of the properties and applications of
vitreous silica, as far as they are at present ascertained. Vitreous
silica is less hard than chalcedony, but harder than felspar. Tubes and
rods of it can be cut with a file or with a piece of sharpened and
hardened steel, and can afterwards be broken like similar articles of
glass. Its conducting power is low, and Mr. Boys has shown that fine
fibres of silica insulate remarkably well, even in an atmosphere
saturated with moisture. The insulating qualities of tubes or rods of
large cross sections have not yet been fully tested; one would expect
them to give good results provided that they are kept scrupulously
clean. A silica rod which had been much handled would probably insulate
no better than one of glass in a similar condition. The density of
vitreous silica is very near to that of ordinary amorphous silica. In
the case of a small rod not absolutely free from minute bubbles it was
found to be 2.21.
Vitreous silica is optically inactive, when homogeneous, and is highly
transparent to ultraviolet radiations.
The melting point of vitreous silica cannot be definitely stated. It is
plastic over a considerable range of temperature. Professor Callendar
has succeeded in measuring the rate of contraction of fine rods in
cooling from 1200 deg. to 1500 deg. C., so that its plasticity must be very
slight below the latter temperature. If a platinum wire embedded in a
thick silica tube be heated from without by an oxy-hydrogen flame the
metal may be melted at temperatures at which the silica tube will retain
its form for a moderate length of time, but silica softens to a marked
extent at temperatures a little above the melting point of platinum.
It has been observed by Boys, Callendar, and others that fine rods of
silica, and also the so-called "quartz fibres," are apt to become
brittle after they have been heated to redness. But I have not observed
this defect in the case of more massive objects, such as thick rods or
tubes; and as I have repeatedly observed that mere traces of basic
matter, such as may be conveyed by contact with the hand, seriously
injure the surface of silica, and have found that silica quickly becomes
rotten when it is heated to about 1000 deg. in contact with an infusible
base such as lime, I am disposed to ascribe the above-mentioned
phenomenon to chemical rather than to purely physical causes.[23] It is
certain, however, that silica apparatus must never be too strongly
heated in contact with basic substances. Silica is easily attacked by
alkalis and by lime, less readily by copper oxide, and still less by
iron oxide.
[23] In a recent communication Professor Callendar tells me that the
devitrification commences at the outside and is hastened by particles of
foreign matter.
The rate of expansion of vitreous silica has been studied by H. le
Chatelier, and more recently by Callendar. The former found its mean
coefficient of expansion to be 0.0000007 between 0 deg. and 10000 deg.,[24] and
that it contracted when heated above 700 deg..
[24] The silica blocks used were prepared by fusion in an electric
furnace; it is therefore probable that they were not quite pure.
Professor Callendar used rods of silica prepared by the author from
"Brazil crystal"; these were drawn in the oxy-gas flame and had never
been heated in contact with solid foreign matter, so that they
consisted, presumably, of very pure silica. His results differ in some
respects from those obtained by Le Chatelier, for he finds the mean
coefficient of expansion to be only 0.00000059, _i.e._ about one
seventeenth as great as that of platinum. Callendar found the rods of
silica expanded very regularly up to 1000 deg. but less regularly above that
temperature. Above 1200 deg. they contracted when heated.
The behaviour of vitreous silica under sudden changes of temperature is
most remarkable. Large masses of it may be plunged suddenly when cold
into the oxy-gas flame, and tubes or rods at a white heat may be thrust
into cold water, or even into liquid air, with impunity. As a
consequence of this, it is in one respect much more easily worked in the
flame than any form of glass. Difficult joints can be thrust suddenly
into the flame, or removed from it, at any stage, and they may be heated
unequally in different parts with impunity. It is safe to say that
joints, etc., in silica never crack whilst one is making them nor during
the subsequent cooling. They may be set aside in an unfinished state and
taken up again without any precautions. Therefore it is possible for an
amateur to construct apparatus in silica which he would be quite unable
to produce from glass.
The behaviour of vitreous silica with solvents has not yet been fully
investigated, but Mr. H. G. Lacell has this subject in hand. If it
behaves like the other forms of anhydrous silica it will withstand the
action of all acids except hydrofluoric acid. It is, of course, very
readily acted upon by solutions of alkalis and alkaline salts.
As regards the use of silica in experiments with gases, it must be
remarked that vitreous silica, like platinum, is slightly permeable to
hydrogen when strongly heated. One consequence of this is that traces of
moisture are almost always to be found inside recently-made silica tubes
and bulbs, however carefully we may have dried the air forced into them
during the process of construction. Owing to the very low coefficient of
expansion of silica, it is not possible to seal platinum wires into
silica tubes. Nor can platinum be cemented into the silica by means of
arsenic enamel, nor by any of the softer glasses used for such purposes.
I have come near to success by using kaolin, but the results with this
material do not afford a real solution of the problem, though they may
perhaps point to a hopeful line of attack. Possibly platinum wires might
be soldered into the tubes (see _Laboratory Arts_, R. Threlfall), but
this also is uncertain.
The process of preparing silica tubes, etc., from Lumps of Brazil
Crystal may be described conveniently under the following headings. I
describe the various processes fully in these pages, as those who are
interested in the matter will probably wish to try every part of the
process in the first instance. But I may say that in practice I think
almost every one will find it advantageous to start with purchased
silica tubes, just as a glass-worker starts with a supply of purchased
glass tubes. The manufacturer can obtain his oxygen at a lower price
than the retail purchaser, and a workman who gives much time to such
work can turn out silica tube so much more quickly than an amateur, that
I think it will be found that both time and money can be saved by
purchasing the tube. At the same time the beginner will find it worth
while to learn and practise each stage of the process at first, as every
part of the work described may be useful in the production of finished
apparatus from silica tubes.
This being so, I am glad to be able to add that a leading firm of
dealers in apparatus[25] has commenced making silica goods on a
commercial scale, so that the new material is now available for all
those who need it or wish to examine its properties.
[25] Messrs. Baird and Tatlock.
=Preparing non-splintering Silica from Brazil Pebble.=--The best variety
of native Silica is Brazil Pebble, which may be obtained in chips or
larger masses. These should be thoroughly cleaned, heated in boiling
water, and dropped into cold water, the treatment being repeated till
the masses have cracked to such an extent that they may be broken easily
by blows from a clean steel pestle or hammer.
The fragments thus produced must be hand-picked, and those which are not
perfectly free from foreign matter should be rejected. The pure and
transparent pieces must then be heated to a yellow-red heat in a covered
platinum dish in a muffle or reverberatory furnace and quickly plunged
into a deep clean vessel containing clean distilled water; this process
being repeated, if necessary, till the product consists of semi-opaque
friable masses, very much like a white enamel in appearance. After these
have been washed with distilled water, well drained and dried, they may
be brought into the hottest part of an oxy-gas flame safely, or pressed
suddenly against masses of white hot silica without any preliminary
heating, such as is necessary in the case of natural quartz. Quartz
which has not been submitted to the above preparatory process, splinters
on contact with the flame to such an extent that very few would care to
face the trouble and expense of working with so refractory a material.
But after the above treatment, which really gives little trouble, all
the difficulties which hampered the pioneer workers in silica disappear
as if by magic.
=Apparatus.=--Very little special apparatus need be provided for working
with silica, but it is absolutely essential to protect the eyes with
very dark glasses. These should be so dark as to render it a little
difficult to work with them at first. If long spells of work are
undertaken, two pairs of spectacles should be provided, for the glasses
quickly become hot enough to cause great inconvenience and even injury
to the eyes.
Almost any of the available oxy-gas burners may be used, but they vary
considerably in efficiency, and it is economical to obtain a very
efficient burner. The 'blow-through' burners are least satisfactory, and
I have long since abandoned the use of them. Some of the safety
'mixed-gas jets' have an inconvenient trick of burning-back, with sharp
explosions, which are highly disconcerting, if the work be brought too
near the nozzle of the burner. I have found the patent burner of Mr.
Jackson (Brin's Oxygen Company, Manchester) most satisfactory, and it
offers the advantage that several jets can be combined in a group easily
and inexpensively for work on large apparatus. The large roaring flames
such as are used, I understand, for welding steel are very expensive,
and not very efficient for the work here described.
=The method of making Silica Tubes.=--Before commencing to make a tube a
supply of vitreous silica in rods about one or two millimetres in
diameter must be prepared. To make one of these, hold a fragment of the
non-splintering silica described above in the oxy-gas flame by means of
forceps tipped with platinum so as to melt one of its corners, press a
small fragment of the same material against the melted part till the two
adhere and heat it from below upwards,[26] till it becomes clear and
vitreous, add a third fragment in a similar manner, then a fourth, and
so on till an irregular rod has been formed. Finally re-heat this rod in
sections and draw it out whilst plastic into rods or coarse threads of
the desired dimensions. If one works carefully the forceps do not suffer
much. I have had one pair in almost constant use for several years; they
have been used in the training of five beginners and are still
practically uninjured.
[26] This is to avoid bubbles in the finished glass.
The beginner should work with a gauge and regulator on the bottle of
oxygen, and should watch the consumption of oxygen closely. A large
expenditure of oxygen does not by any means necessarily imply a
corresponding output of silica, even by one who has mastered the initial
difficulties.
When a supply of the small rods of vitreous silica has been provided,
bind a few of them round a rod of platinum (diameter say, 1 mm.) by
means of platinum wires at the two ends and heat the silica gradually,
beginning at one end after slightly withdrawing the platinum core from
that end, till a rough tube about four or five centimetres in length has
been formed. Close one end of this, expand it, by blowing, into a small
bulb, attach a silica rod to the remote end of the bulb, re-heat the
bulb and draw it out into a fine tube. Blow a fresh bulb on one end of
this and again draw it out, proceeding in this way till you have a tube
about six or eight centimetres in length. All larger tubes and vessels
are produced by developing this fine tube suitably.
=Precautions.=--The following points must be carefully kept in mind,
both during the making of the first tube and afterwards:--
(1) The hottest spot in the oxy-gas flame is at a point very near the
tip of the inner cone of the flame, and silica can be softened best at
this hot spot. The excellence of a burner does not depend on the size of
its flame, so much as on the temperature of its "hot spot," and the
success of the worker depends on his skill in bringing his work exactly
to this part of the flame. Comparatively large masses of silica may be
softened in a comparatively small jet if the hot spot is properly
utilised.
(2) Silica is very apt to exhibit a phenomenon resembling
devitrification during working. It becomes covered with a white
incrustation, which seems to be comparatively rich in alkali.[27] This
incrustation is very easily removed by re-heating the whitened surface,
provided that the material has been kept scrupulously clean. If the
silica has been brought into the flame when dusty, or even after much
contact with the hands of the operator, its surface is very apt to be
permanently injured. _Too much attention cannot be given to cleanliness
by the workman._
[27] The rock crystal exhibits a yellow flame when first heated in the
oxy-gas flame, and most samples contain spectroscopic quantities of
lithium.
(3) When a heated tube or bulb of silica is to be expanded by blowing,
it is best not to remove it from the flame, for if that is done it will
lose its plasticity quickly unless it be large. The better plan is to
move it slightly from the "hot spot" into the surrounding parts of the
flame at the moment of blowing.
It is best to blow the bulb through an india-rubber tube attached to the
open end of the silica tube. At first one frequently bursts the bulbs
when doing this, but holes are easily repaired by stopping them with
plastic silica applied by the softened end of a fine rod of silica and
expanding the lump, after re-heating it, by blowing. After a few hours'
practice these mishaps gradually become rare.
I find it a good plan to interpose a glass tube packed with granulated
potash between the mouth and the silica tube. This prevents the interior
of the tube from being soiled. The purifying material must not be packed
so closely in the tube as to prevent air from passing freely through it
under a very low pressure.
It may be mentioned here that a finished tube usually contains a little
moisture, and a recognisable quantity of nitric peroxide. These may be
removed by heating the tube and drawing filtered air through it, but not
by washing, as it is difficult to obtain water which leaves no residue
on the silica.
=Making larger tubes and other apparatus of Silica.=--In order to
convert a small bulb of silica into a larger one or into a large tube,
proceed as follows:--Heat one end of a fine rod of silica and apply it
to the bulb so as to form a ring as shown in the figure. Then heat the
ring and the end of the bulb till it softens, and expand the end by
blowing. If this process is repeated, the bulb first becomes ovate and
then forms a short tube which can be lengthened at will, but the most
convenient way to obtain a very long tube is to make several shorter
tubes of the required diameter, and say 200 to 250 mm. in length, and to
join these end to end. It does not answer to add lumps of silica to the
end of the bulb, for the sides of the tube made in this way become too
thin, and blow-holes are constantly formed during the making of them.
These can be mended, it is true, but they spoil the appearance of the
work.
[Illustration]
Tubes made in the manner described above are thickened by adding rings
of silica and blowing them when hot to spread the silica. If a
combination of several jets is employed, very large tubes can be
constructed in this way. One of Messrs. Baird and Tatlock's workmen
lately blew a bulb about 5 cm. in diameter, and it was clear that he
could have converted it into a long cylindrical tube of equal diameter
had it been necessary to do so.
Very thin tubes of 1.5 cm. diameter, and tubes of considerable thickness
and of equal size, are easily made after some practice, and fine
capilliaries and millimetre tube can be made with about equal readiness.
If a very fine tube of even bore is required, it may be drawn from a
small thick cylinder after a little practice.
When a tube becomes so large that it cannot be heated uniformly on all
sides by rotating it in the flame, it is convenient to place a sheet of
silica in front of the flame a little beyond the object to be heated, in
order that the former may throw back the flame on those parts of the
tube which are most remote from the jet. A suitable plate may be made by
sticking together small lumps of silica rendered plastic by heat.
The silica tubes thus made can be cut and broken like glass, they can be
joined together before the flame, and they can also be drawn into
smaller tubes when softened by heat.
In order to make a side connection as in a T piece, a ring of silica
should be applied to the tube in the position fixed upon for the joint.
This ring must then be slightly expanded, a new ring added, and so on,
till a short side tube is formed. To this it is easy to seal a longer
tube of the required dimensions. It is thus possible to produce Geissler
tubes, small distilling flasks, etc. Solid rods of silica are easily
made by pressing together the softened ends of the fine rods or threads
previously mentioned. Such rods and small masses can be ground and
polished without annealing them.
=Quartz Fibres.=--These were introduced into physical work by Mr. Boys
in 1889. They may be made by attaching a fine rod of vitrified quartz to
the tail of a small straw arrow provided with a needle-point; placing
the arrow in position on a cross-bow, heating the rod of silica till it
is thoroughly softened and then letting the arrow fly from the bow, when
it will carry with it an extremely fine thread of silica. A little
practice is necessary to ensure success, but a good operator can
produce threads of great tenacity and great uniformity. Fuller accounts
of the process and of the various properties and uses of quartz fibres
will be found in Mr. Boys' lectures (Roy. Inst. Proc. 1889, and Proc.
Brit. Assn. 1890), and in Mr. Threlfall's Laboratory Arts.
INDEX.
Air-traps, 69.
Annealing, 23.
Apparatus needed for Glass-working, 11.
Appendix, 82.
Beginners, Failures of, 22.
Bellows, Position of, 3.
---- Various forms of, 7.
_See also_ Blower.
Bending Glass Tubes, 28.
Blower, Automatic, 8.
Blow-pipe, Cheap form of, 4.
---- Dimensions of, 4-5.
---- Fletcher's Automaton, 6.
---- Fletcher's Compound, 6.
---- Gimmingham's, 6.
---- Herapath's, 6.
---- Jets for the, 7.
---- Use of the, 8.
_See also_ Flames.
Blow-pipes, Use of several in combination, 21.
Brush Flame, 9.
---- Oxidising, 20.
Bulbs, Methods of blowing, 47.
Calibrating Apparatus, 76-81.
Camphorated Turpentine, 11.
Cetti's Vacuum Tap, 66.
Charcoal Pastils, 11.
Choking or Contracting the Bores of Tubes, 35.
Combining the Parts of Complicated Apparatus, 61.
Combustion Tube, how to work it, 25.
Contracting the Bore of a Tube, 35.
Cotton Wool for Annealing, 24.
Cutting Glass Tubes, 26, 27, 28.
Dividing a Line into Equal Parts, 75.
Electrodes, 38, 55.
Electrolysis, Making Apparatus for, 59.
Files for Cutting Glass, 27.
Flame, the Pointed, 8.
---- the Brush, 9.
---- the Oxidising Brush, 20.
---- the Smoky, 10.
Fletcher's Automaton Blow-pipe, 6.
Fletcher's Compound Blow-pipe, 6.
Funnels, Thistle-headed, 57.
Gimmingham's Blow-pipe, 6.
Gimmingham's Vacuum Tap, 68.
Glass, Annealing, 23.
---- Devitrification of, 15.
---- Method of Working with Lead, 17.
---- Method of Working with Soda, 22.
---- Nature of, 12.
---- Presenting to the Flame, 16.
Glass Tubes, Bending, 28.
---- Bordering, 31.
---- Characters of good, 14.
---- Choking, 35.
---- Cleaning, 15.
Glass Tubes, Cutting, 26, 27, 28.
---- Piercing, 37.
---- Purchase of, 12.
---- Sealing, 32.
---- Sealing Hermetically, 58.
---- Sizes of, 82.
---- Welding or Soldering, 39, 62.
---- Widening the Ends of, 36.
Graduating Apparatus, 70.
Grinding Stoppers, 51.
Herapath's Blow-pipe, 6.
Hofman's Apparatus for Electrolysis, 59.
Inside Joints, 43.
Jets for Blow-pipes, 7.
Joints, Air-tight, 64.
Lead Glass, Method of Working with, 17.
Lead Glass, Blackening of, 17.
Light, Effect of, in Working, 3.
Line, to Divide into Equal Parts, 75.
Mercury Joints, Various, 64.
Non-splintering Silica, Preparation of, from Quartz, 88.
Ozone Generator, To Make an, 44.
Pastils of Charcoal, 11.
Piercing Tubes, etc., 37.
Platinum Electrodes, Sealing in, 38, 55.
Pointed Flame, the, 9.
Quartz Fibres, 94.
Rounding Ends of Tubes, 31.
Sealing or Closing Openings in Tubes, 32.
Side-tubes, Fixing, 41.
Smoky Flame, 10.
Soda Glass, Method of Working, 22.
Soldering or Welding, 39, 62.
Spiral Tubes, 56.
Stoppers, Making and Grinding, 51.
Table for Glass-blower, 3.
Taps, Vacuum, 65.
Thistle-headed Funnels, 57.
Traps, Air, 69.
Tube, Combustion, how to work it, 25.
Tubes. _See_ Glass Tubes.
---- T-, 41.
---- U-, 56.
Turpentine, Camphorated, for Grinding, 11.
U-Tubes, 56.
Vacuum Taps, 65-68.
---- Tube, To Make a, 60.
Vitreous Silica, Apparatus required for Making, 89.
---- Behaviour under sudden changes of Temperature, 87.
---- Bulbs, etc., Making Joints on, 93.
---- Expansion of, 86.
---- Hardness of, 85.
---- Insulating Power of, 85.
---- Melting Point of, 85.
---- Permeability to Gases, 87.
---- Properties of, 84.
---- Rods, Making Joints on, 94.
---- Tubes, Method of Making, 90.
---- Tubes, Making Joints on, 94.
Welding or Soldering Tubes together, 39, 62.
White Enamel, Uses of, 39, 56.
Widening the Ends of Tubes, 36.
Working-place, 2.
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at the Edinburgh University Press, Scotland