ol 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 no