Internal Seals, Air-Traps, Spray Arresters,
Filter-Pumps--Sprays, Condensers; Plain, Double Surface, and
Spherical--Soxhlet Tubes and Fat Extraction
Apparatus--Vacuum Tubes, Electrode Work, Enclosed
Thermometers, Alarm Thermometers, Recording Thermometers,
"Spinning" Glass.
_Internal Seals._--It is convenient to class those cases in which a
glass tube passes through the wall of another tube or bulb under the
heading of "Internal Seals." These are met with in barometers, spray
arresters, and filter pumps, in condensers and some forms of vacuum
tube. The two principal methods of making such seals will be considered
first and their special application afterwards.
_An Air Trap on a Barometer Tube._--This involves the use of the first
method, and is perhaps the simplest example that can be given. Fig. 9,
_a_, _a1_ and _a2_, show the stages by which this form of internal seal
is made. For the first trials, it is well to work with fairly
thick-walled tubing, which should be cut into two pieces, each being
about eight inches long.
[Illustration: Fig. 9]
First seal the end of one tube as described on page 13, heat the sealed
end and expand to a thick walled bulb. Fuse the end of the other tube,
attach a piece of glass rod to serve as a handle, and draw out; cut off
the drawn-out portion: leaving an end like _a_.
Now heat a small spot at the end of the bulb, blow, burst out, and
remove the thin fragments of glass. Heat a zone on the other tube at the
point where the drawn-out portion commences and expand as shown by _a1_.
The next stage is to join the tubes. Heat the ragged edges of the
burst-out portion until they are thoroughly rounded. At the same time
heat the drawn-out tube to just below softening point. Then, while the
rounded edges of the burst-out portion are still soft, insert the other
tube; rotate the join in the blowpipe flame until it is quite soft, and
expand by blowing. If necessary, re-heat and expand again. The finished
seal, which should be slightly annealed by smoking in a sooty flame, is
shown by _a2_.
_A Spray Arrester._--This is made by the second method, in which the
piece of tube which projects inside the bulb is fused in position first
and the outer tube is then joined on. The various stages of making are
illustrated by _b_, _b1_ and _b2_, Fig. 9.
A bulb is blown between two tubes by the method given on page 22, the
larger tube is then cut off and the small piece of tube introduced into
the bulb after having been shaped as shown in by _b_, Fig. 9. The
opening in the bulb is sealed as shown by _b1_. The sealed part is now
heated and the bulb inclined downwards until the inner tube comes in
contact with the seal and is fused in position. This operation requires
some practice in order to prevent the inner tube either falling through
the soft glass or becoming unsymmetrical. The end of the bulb, where the
inner tube comes in contact with it, is now perforated by heating and
blowing, thus giving the form shown by _b2_, and the outer tube is
joined on. The finished spray arrester is shown by _b3_. Practice alone
will give the power to produce a symmetrical and stable piece of work.
_Two Forms of Filter Pump._--That illustrated by _d_, Fig. 9, is made by
the method explained under "An Air Trap on a Barometer Tube." That
illustrated by _c_ is made by the method explained under "A Spray
Arrester." No new manipulation is involved, and the construction should
be clear from a study of the drawings.
_Multiple and Branched Internal Seals._--A fuller consideration of these
will be found on page 39, but one general principle may well be borne in
mind; that, as far as is possible, a tube having both ends fastened
inside another tube or bulb should be curved or have a spiral or bulb at
some point in its length, otherwise any expansion or contraction will
put great strain on the joints.
_Sprays._--A spray which is easy to make, easy to adjust, and easy to
clean after use is shown by _e_, Fig. 9. The opening on the top of the
bulb is made by melting on a bead of glass, expanding, bursting, and
fusing the ragged edges. The two branches which form the spray producing
junction are made by the method used for an exhaustion branch and
described on page 18.
A spray which can be introduced through the neck of a bottle is shown by
_h_, Fig. 9. The various stages in making this are illustrated by _f_,
and _g_. If the inner tube is made by drawing out from a larger piece of
glass so that two supporting pieces are left on each side of the place
where it is intended to make the final bend, that bend can be made in a
flat-flame gas burner without causing the inner tube to come in contact
with the walls of the outer tube. Care must be taken when joining on the
side piece that the inner tube is not heated enough to fuse it. The
small hole in the side of the outer tube is produced by heating and
bursting.
_A Liebig's Condenser._--This consists of a straight glass tube passing
through an outer cooling jacket. In practice it is better to make the
jacket as a separate piece, and to effect a water-tight junction by
means of two short rubber tubes. It may, however, be made with two
internal seals of the class described under "A Spray Arrester." There is
much less risk of these seals cracking if the inner tube is made in the
form of a spiral or has a number of bulbs blown on it in order to give a
certain amount of elasticity.
_A Double-Surface Condenser._--Fig. 10 shows a condenser of this nature
which is supplied by Messrs. Baird and Tatlock. It may be built up in
stages as shown by _a_, _b_, and _c_, but the work involved requires
considerable skill, and the majority of laboratory workers will find it
cheaper to buy than to make.
[Illustration: Fig. 10]
_A Spherical Condenser._--Such a condenser as that shown by _f_, Fig 10,
involves a method which may find application in a number of cases. The
outer bulb is blown from a thick piece of tubing which has been inserted
in a smaller piece (see _d_, Fig. 6); then the inner bulb by similar
method. It is now necessary to introduce the smaller bulb into the
larger, and for this purpose the larger bulb must be cut into halves. A
small but deep cut is made with the file or glass-blowers' knife in the
middle of the larger bulb, and at right angles to the axis of the tube
on which it is blown. A minute bead of intensely heated glass is now
brought in contact with the cut in order to start a crack. This crack
may now be led round the bulb as described on page 30. If the work is
carried out with care, it is possible to obtain the bulb in two halves
as shown by _d_, and these two halves will correspond so exactly that
when the cut edges are placed in contact they will be almost air-tight.
The two tubes from the smaller bulb should be cut to such a length that
they will just rest inside the larger, and the ends should be expanded.
Place the inner bulb in position and fit the two halves of the outer
bulb together, taking great care not to chip the edges. If the length of
the tubes on the inner bulb has been adjusted properly, the inner bulb
will be supported in position by their contact with the tubes on the
outer bulb. Now rotate the cracked portion of the outer bulb in front
of a blowpipe flame and press the halves together very gently as the
glass softens. Expand slightly by blowing if necessary. If a small
pin-hole develops at the joint it is sometimes possible to close this
with a bead of hot glass; but if the bulb has been cut properly there
should be no pin-holes formed. The condenser is finished by joining on
the side tubes and sealing the inner tube through by the methods already
given. In order to blow bulbs large enough to make a useful condenser,
it will be convenient to employ the multiple-jet blowpipe described on
page 4.
_A Soxhlet-Tube or Extraction Apparatus._--This involves the
construction of a re-entrant join where the syphon flows into the lower
tube. It is of considerable value as an exercise and the complete
apparatus is easy to make.
A large tube is sealed at the bottom and the top is lipped, as in making
a test-tube. A smaller tube is then joined on by a method similar to
that given on page 18, but without making a perforation in the bottom of
the large tube. Heating and expanding by air pressure, first through the
large tube, then through the smaller tube and then again through the
large tube, will give a satisfactory finish to this part of the work.
[Illustration: Fig. 11]
The syphon tube is now joined on to the large tube as shown by _a_, Fig.
11, care being taken to seal the other end of the syphon tube before
joining. The details of the final and re-entrant joint of the syphon
tube are shown at the lower part of _a_. This join is made by expanding
the sealed end of the syphon tube into a small, thick-walled bulb, and
the bottom of this bulb is burst out by local heating and blowing; the
fragments of glass are removed and the edges made smooth by melting. A
similar operation is carried out on the side of the tube to which the
syphon tube is to be joined. This stage is shown by _a_. Now heat the
syphon tube at the upper bend until it is flexible, and press the bulb
at its end into the opening on the side of the other tube. Hold the
glass thus until the syphon is no longer flexible. The final join is
made by heating the two contacting surfaces, if necessary pressing the
edges in contact with the end of a turn-pin, fusing together and
expanding. The finished apparatus is shown by _c_.
_Electrodes._--A thin platinum wire may be sealed into a capillary tube
without any special precautions being necessary. The capillary tube may
be drawn out from the side of a larger tube by heating a spot on the
glass, touching with a glass rod and drawing the rod away; or the
exhaustion branch described on page 18 may be used for the introduction
of an electrode. It is convenient sometimes to carry out the exhaustion
through the same tube that will afterwards serve for the electrode. The
electrode wire is laid inside the branch before connecting to the
exhaustion pump. When exhaustion is completed the tube is heated until
the soft glass flows round the platinum and makes the seal air-tight.
The branch is now cut off close to the seal on the pump side, a loop is
made in the projecting end of the platinum wire, and the seal is
finished by melting the cut-off end.
Platinum is usually employed for such work, but if care is taken to
avoid oxidation it is not impossible to make fairly satisfactory seals
with clean iron or nickel wire. Hard rods of fine graphite, such as are
used in some pencils, may also be sealed into glass, but it seems
probable that air would diffuse through the graphite in the course of
time.
Another method for the introduction of an electrode is illustrated by
_d_, _e_, _f_ and _g_, Fig. 11. In this case the bulb or thin-walled
tube into which the electrode is to be sealed is perforated by a quick
stab with an intensely heated wire--preferably of platinum--which is
then withdrawn before the glass has had time to harden, and thus a
minute circular hole is made. The electrode is coated with a layer of
similar glass, or of the specially made enamel which is sold for this
purpose, inserted into the bulb or tube by any convenient opening, and
adjusted by careful shaking until the platinum wire projects through the
small hole. The bulb or tube is then fused to the coating of the
electrode and the whole spot expanded slightly by blowing. The
appearance of the finished seal is shown by _g_. It is well to anneal
slightly by smoking.
_Thermometers._--Apart from the notes on page 20 with respect to the
blowing of a suitable bulb on capillary tubing there is little to say in
connection with the glass working needed in making a plain thermometer.
The size desirable for the bulb will be determined by the bore of the
capillary tube, the coefficient of expansion of the liquid used for
filling, and the range of temperature for which the thermometer is
intended.
Filling may be carried out as follows:--Fit a small funnel to the open
end of the capillary by means of a rubber tube, and pour into the funnel
rather more than enough of the liquid to be used than is required to
fill the bulb. Mercury or alcohol will be used in practice, most
probably. Warm the bulb until a few air bubbles have escaped through
the liquid and then allow to cool. This will suck a certain amount of
liquid into the bulb. Now heat the bulb again, and at the same time heat
the capillary tube over a second burner. The liquid will boil and sweep
out the residual air, but it is necessary to heat the capillary tube as
well in order to prevent condensation. Allow the bulb and tube to cool,
then repeat the heating once more. By this time the bulb and tube should
be free from air, and cooling should give a completely filled
thermometer. Remove the funnel and heat the thermometer to a few degrees
above the maximum temperature for which it is to be used; the mercury or
other filling liquid will overflow from the top, and, as the temperature
falls, will recede, thus allowing the end of the capillary to be drawn
out. Reheat again until the liquid rises to the top of the tube, then
seal by means of the blowpipe flame. The thermometer is now finished
except for graduation; this is dealt with on page 75.
_An Alarm Thermometer._--A thermometer which will complete an electric
circuit when a certain temperature is reached may be made by sealing an
electrode in the bulb and introducing a wire into the top, which in this
case is not sealed. Naturally, this thermometer will be filled with
mercury. There is considerable difficulty in filling such a bulb without
causing it to crack.
Several elaborations of this form are made, in which electrodes are
sealed through the walls of the capillary tube, thus making it possible
to detect electrically the variation of temperature when it exceeds any
given limits.
_An Enclosed or Floating Thermometer._--The construction of this type of
thermometer is shown by _h_ and _i_, Fig 11. It is made in the following
stages:--A bulb is blown on the drawn-out end of a thin-walled tube as
shown by _h_. A small bulb is blown on the end of a capillary tube,
burst, and turned out to form a lip which will rest in the drawn-out
part of the thin-walled tube but is just too large to enter the bulb.
The capillary tube is introduced and sealed in position, care being
taken to expand the joint a little. The thermometer is filled and the
top of the capillary tube closed by the use of a small blowpipe flame. A
paper scale having the necessary graduations is inserted, and the top
of the outer tube is closed as shown by _i_.
_A Maximum and Minimum Thermometer._--If a small dumb-bell-shaped rod of
glass or metal is introduced into the capillary tube of a horizontally
placed, mercury-filled thermometer in such a position that the rising
mercury column will come in contact with it, the rod will be pushed
forward. When the mercury falls again the rod will be left behind and
thus indicate the maximum temperature attained. If a similar
dumb-bell-shaped rod is introduced into an alcohol-filled thermometer
and pushed down until it is within the alcohol column, it will be drawn
down by surface tension as the column falls; but the rising column will
flow passed it without causing any displacement; thus the minimum
temperature will be recorded.
Six's combined maximum and minimum thermometer is shown by _b_, Fig. 11.
In this case both maximum and minimum records are obtained from a
mercury column, although the thermometer bulb is filled with alcohol. It
is an advantage to make the dumb-bell-shaped rods of iron, as the
thermometer can then be reset by the use of a small magnet, another
advantage consequent on the use of metal being that the rods can be
easily adjusted, by slight bending, so as to remain stationary in the
tubes when the thermometer is hanging vertically, and yet to move with
sufficient freedom to yield to the pressure of the recording column.
The thermometer may be filled by the following method:--When the
straight tube has been made the first dumb-bell is introduced and shaken
down well towards the lower bulb, the tube is now bent to its final
shape and the whole thermometer filled with alcohol as described on page
44. Now heat the thermometer to a little above the maximum temperature
that it is intended to record, and pour clean mercury into the open bulb
while holding the thermometer vertically. Allow to cool, and the mercury
will be sucked down. The second dumb-bell is now introduced, sufficient
alcohol being allowed to remain in the open bulb to about half fill it,
and the alcohol in this bulb is boiled to expel air. The tube through
which the bulb was filled in now sealed.
_Clinical Thermometers._--The clinical thermometer is a maximum
thermometer of a different type. In this case there is a constriction
of the bore at a point just above the bulb. When the mercury in the bulb
commences to contract, the mercury column breaks at the constriction and
remains stationary in the tube, thus showing the maximum temperature to
which it has risen.
_Vacuum Tubes._--There are so many forms of these that it is scarcely
practicable or desirable to give detailed instructions for making them;
but an application of the various methods of glass-working which have
already been explained should enable the student to construct most of
the simpler varieties. An interesting vacuum tube is made which has no
electrodes, but contains a quantity of mercury. When the tube is rocked
so as to cause friction between the mercury and the glass sufficient
charge is produced to cause the tube to glow.
_A Sprengel Pump._--This, in its simplest form, is illustrated by _a_,
Fig. 12. Such a form, although highly satisfactory in action, needs
constant watching while in action, as should the mercury funnel become
empty air will enter the exhausted vessel. Obviously, the fall-tube must
be made not less than thirty inches long; the measurement being taken
from the junction of the exhaustion branch with the fall-tube to the top
of the turned-up end.
[Illustration: Fig. 12]
_A Macleod Pump._--One form of this is illustrated by _b_, Fig. 12. It
has the advantage that the mercury reservoir may be allowed to become
empty without affecting the vacuum in the vessel being exhausted.
_"Spinning" Glass._--By the use of suitable appliances, it is quite
possible to draw out a continuous thread of glass, which is so thin as
to have almost the flexibility and apparent softness of woollen fibre; a
mass of such threads constitutes the "glass wool" of commerce.
The appliances necessary are:--a blowpipe capable of giving a
well-formed flame of about six or eight inches in length, a wheel of
from eighteen inches to three feet in diameter and having a flat rim of
about three inches wide, and a device for rotating the wheel at a speed
of about three hundred revolutions per minute.
A very satisfactory arrangement may be made from an old bicycle; the
back wheel having the tyre removed and a flat rim of tin fastened on in
its place. The chain drive should be retained, but one of the cranks
removed and a handle substituted for the remaining pedal. The whole
device is shown by Fig. 13.
[Illustration: Fig. 13]
The procedure in "spinning" glass is as follows:--First melt the end of
a glass rod and obtain a large mass of thoroughly softened glass, now
spin the wheel at such a speed that its own momentum will keep it
spinning for several seconds. Touch the end of the melted rod with
another piece of glass and, without withdrawing the original rod from
the blowpipe flame, draw out a thread of molten glass and twist it round
the spinning wheel. If this is done properly, the thread of glass will
grip on the flat rim, and by continuing to turn the wheel by hand it is
possible to draw out a continuous thread from the melted rod, which must
be advanced in the blowpipe flame as it is drawn on the wheel. If the
rod is not advanced sufficiently the thread will melt off, if it is
advanced too much, so as to heat the thick part and allow the glass to
become too cool at the point of drawing out, then the thread will become
too thick, but it is easy after a little practice to obtain the right
conditions. Practice is necessary also in order to find the right speed
for the wheel.
When sufficient glass has been "spun," the whole "hank" of thin thread
may be removed by drawing the thumb-nail across the wheel at any point
on its flat rim, thus breaking the threads, and allowing the "hank" to
open.
_Brushes for Use with Strong Acids._--Glass wool, if of fine enough
texture to be highly flexible, can be used to make acid-resisting
brushes. A convenient method for mounting the spun glass is to melt the
ends of the threads together into a bead, and then to fuse the bead on
to a rod; thus giving a brush. If a pointed brush is necessary, the
point may be ground on an ordinary grindstone or carborundum wheel by
pressing the loose end of the spun glass against the grinding wheel with
a thin piece of cardboard.
When using brushes of this description, it is well to bear in mind the
fact that there is always a liability of a few threads of glass breaking
off during use.