ADVANCED EXERCISES
EXERCISE NO. 8
SEALING A TUBE THROUGH ANOTHER TUBE
_First Method--Making a Gas-washing Tube_
This first method can be used whenever one can work through an open end
opposite to the end of the tube where the joint is to be made. To
illustrate it, take a piece of rather thin-walled tubing, about 3/4 inch
in diameter, and some pieces of rather strong tubing a little less than
1/4 inch in diameter. Draw off the large tube in a short cone, then draw
off the tail as in the making of the bulb on the end of the tube, blow
out the little lump slightly, shrink the whole cone a little and blow
gently to form a rounded end like that on a test-tube, with walls about
the thickness of those of the rest of the tube. Cut this tube to a
suitable length, say about six inches, and provide two corks which will
fit the open end of it. Now cut a piece of the small tubing of the
proper length to form the piece which is to be inside the large tube.
For practice purposes, this piece should be about an inch shorter than
the large tube. Flange one end of this tube a little, and anneal the
flange well in the smoky flame. Bore one of the corks so that a piece of
the small tubing will fit it, and cut a couple of notches in the side of
this cork so that air can pass between it and the glass. Pass a short
piece of the small tubing through this cork, and attach the flanged
piece of small tube to this by means of a short piece of rubber tubing,
so that when the whole is inserted in the large tube it is arranged as
in _a_, Fig. 11. The piece of glass tubing projecting out through the
cork is now cut off so as to leave an end about 1/2 inch long when the
cork is firmly seated and the inner tube pushed into contact with the
center of the end of the large tube, as shown in the drawing. Care
should be taken that the little rubber tube which joins the two pieces
is arranged as in the figure; _i.e._, most of it on the piece of tubing
which passes through the cork, and very little on the other piece, so
that when the cork is removed after the small tube has been sealed
through the large one, the rubber tube may easily come with it. Select a
short piece of the small tubing of suitable length for the piece which
is to be on the outside of the large tube as a continuation of the
piece inside, and another piece for the delivery tube. A small bulb may
be blown in the latter at a point about 2-1/2 inches from the closed
end, and the open end cut off about 1-1/2 inches from the bulb. A cork
or cork-boring of suitable size to stopper the small tube is prepared,
and laid ready with the other (unbored) cork for the large tube.
[Illustration: FIG. 11.--Gas-washing tube.]
When everything is in readiness, the rounded end of the large tube is
slowly heated until it softens and joins firmly to the small tube
inside. After it has shrunk down well, it is blown out to its original
size, placing the whole end of the large tube, cork and all, in the
mouth. Now with a fine-pointed flame the glass covering the end of the
small tube is heated to the softening temperature, and then is blown out
to an excrescence by blowing on the end of the small tube which passes
through the cork. The end of this excrescence is heated and blown off in
the usual way, so as to leave the small tube sealed on the inside of the
large one and opening through it into this short tube which has been
blown out. The end of the small tube which passes through the cork is
now closed with the cork prepared for it, and the short outer tube is
joined to the tube that has just been blown out, so that the joint
appears like _b_, Fig. 11. Use the first method (Exercise No. 1) for
this joint. Reheat the whole of the end of the tube nearly to the
softening temperature, anneal it a little, and allow to cool a few
seconds until well set. Now remove the cork, short glass tube and rubber
tube from the open end of the large tube and insert the solid cork in
their place. Warm the joint and the whole of that end of the tube again
carefully up to about the softening point, then seal on the side tube
for the delivery of the gas in the usual way, taking care that the whole
of the end and the joint are kept warm meanwhile. When thoroughly
sealed, the delivery tube is bent up parallel to the tube through which
the gas enters, and then out at right angles to it, as shown in _c_. The
whole of the end of the tube is now cautiously reheated and then cooled
slowly to anneal it.
The cork may now be removed from the open end of the large tube, this
end heated in a large flame, caught together with a scrap of glass
tubing and drawn off into a cone so that the base of the cone is about
opposite the end of the inner tube. The lump of glass is drawn off the
point of this cone and it is reblown to form a rounded end, as
previously described.
After this cools, the tube through which the gas enters may be heated at
the proper point and bent at right angles to form the finished apparatus
as shown in _d_. The ends of the small tube are cut off square and
fire-polished.
=Discussion.=--After the joint has once been made, great care must be
taken that it is kept hot during all the subsequent manipulations, and
if it becomes somewhat cool at any time it must be reheated very slowly.
It is obvious that the rate of heating and cooling of the inner tube
will be slower than that of the outer tube, and this will readily
produce stresses which tend to crack the tube at the joint. The amount
of heating and cooling which such a joint will stand depends upon its
form. The beginner should examine such a joint on regular factory-made
apparatus, and note the uniformity of wall-thickness and the "clean-cut"
appearance of the joint, as a model for his imitation. A ragged joint,
where the line of joining of the inner and outer tubes wavers instead of
going squarely around the tube, is almost sure to crack during the
cooling and heating unless extra precautions are taken with it. The
presence of a small lump of glass at any point on the joint affords an
excellent starting place for a crack, as do also the points on a ragged
joint where the inner tube comes farther down on the outer tube than at
other points.
In order to insure a joint which is square and not ragged, it is
essential that the angle between the inner and outer tubes at the joint
be very nearly a right angle. For this reason the two tubes should not
be of too near the same size, or if this cannot be avoided, a small bulb
should be blown on the end where the joint is to be made. If this bulb
be made with the same wall-thickness as the rest of the tube, and
somewhat pear-shaped, it may be drawn out to the same size as the rest
of the tube, if necessary, after the joint has been made.
This method is used wherever possible in preference to the second method
(Exercise No. 9), as it is easier to get a good joint with it. It may
also be used where it is desired to seal the tube through the side of a
tube, or for a tube sealed through the wall of a bulb, as in a Geissler
potash bulb or similar apparatus. Where there is not space to join the
inner tube to the blowing tube by a rubber tube, this joint may be made
with a small piece of gummed paper, which can readily be broken when
desired.
EXERCISE NO. 9
SEALING A TUBE THROUGH ANOTHER TUBE
_Second Method--Making a Suction Pump_
Select a piece of tubing 3/8 to 1/2 inch in diameter, with walls about
1/16 inch or a little less in thickness, heat a place about 4 inches
from one end and draw it out so that when cut off at the proper point it
will look like _a_, Fig. 12; the open end of the drawn out part being
small enough to slip inside another piece of the original tube. A small
thick-walled bulb is now blown as indicated by the dotted lines, and
annealed. A piece of the original tubing is now prepared, 7 or 8 inches
long, with one end cut square off and the other closed. A piece of
1/4-in tubing about 2 inches long, and drawn out at one end to a tail
several inches long is also prepared, to form the inlet tube for the
air. Another piece of the 3/8-inch tube is prepared, about 4 inches
long, and provided with a tail drawn out as indicated in _b_, so that
when cut off at about 2-1/2 or 3 inches from the main tube its inner
diameter may be slightly less than that of the narrowest point of the
tube _a_. A small thick-walled bulb is blown at the point indicated by
the dotted lines, and annealed. Care must be taken in drawing the
capillary and blowing the bulb in both _a_ and _b_ that the capillary
tubes are in the axis of the main tube, and in the same straight line
with it.
[Illustration: FIG. 12.--Suction pump.]
The open end of the 8-inch piece of tube and the bulb of the piece _a_
are now warmed together, the end of the tube only moderately and the
bulb to about its softening temperature. The tube _a_ is now inserted in
the open end of the large tube, and the bulb softened with a suitable
flame and pressed into good contact with the tube. It is then reheated,
including the joint, blown a little and pulled out to form a straight
tube in line with the main tube. By warming the joint a little, and
proper rotation, the capillary may be brought into the same straight
line with the rest of the tube.
Keeping this joint hot, a place about an inch from it on the tube _a_ is
warmed, and the piece of 1/4-inch tubing previously prepared is sealed
on at that point. The joint is then well annealed and allowed to cool.
The tube _a_ is now cut at such a place that when _b_ is inserted in the
open end the point will come near the end of the constriction of _a_, as
shown in _c_. Care is taken to get a clean square cut. The side tube is
now cut off about an inch from the main tube and corked. Tube _b_ is
sealed into the open end of _a_, in the same way as _a_ was sealed into
the large tube, and the joint carefully annealed.
=Discussion.=--As in the first method, the secret of success lies in
getting a square joint, and having the inner tube leave the outer one at
nearly right angles. All the remarks about annealing, lumps, etc., made
under the previous method apply here.
This method may be applied in sealing a small tube into the end of a
large one, the latter being either drawn to a cone and cut off at the
desired diameter, or else given a rounded end like a test-tube and a
hole the proper size blown in the center of it. A suitable thick-walled
bulb is to be blown on the small tube, as in the case described above.
This method is also used in making the Kjeldahl trap (_a_, Fig. 13), the
small tube to be inserted being first drawn, the thick bulb blown at its
point of union with the main tube, and then the small tube bent and cut.
The large bulb is best made with rather heavy wall, being either blown
in the middle of a tube, and one piece of the tube drawn or cut off, or
else made on the end of a tube. In the latter case a drop of glass must
be put on the point where the joint is to be, so as to get a hole of the
proper size with enough glass around it to prevent it from growing
larger when it is heated. The author prefers to blow the bulb in the
middle of the tube, draw off one end of the bulb, and blow out the
desired hole where the tube was drawn off. The whole bulb must generally
be reheated and blown a little at the end of the process, and well
annealed.
[Illustration: FIG. 13.--_a_, Kjeldahl trap; _b_, suction pump on
smaller tubing.]
The suction pump can also be made on 1/4-inch tubing, and one joint
saved if desired, by constricting the tube to form the raceway for the
water and air, as shown in _b_, Fig. 13. (See page 10 for method.) But
it is more difficult to make a square joint on such small tubing.