f point is to get
a very even and not too thick ring at the junction, and consequently
the extra thickening produced by making a rim on B is rather a
drawback. The method consists in cutting off from B the length which
it is desired to insert, slipping this into A (which may be an
otherwise closed bulb, for instance), and then gradually melting up
the open end of A till the piece of B inside will no longer fall out.
By holding the joint downwards so that the inserted portion of B rests
on the edges of the opening, a joint may be made with the minimum
thickening.

The external part of B, previously heated, is then applied, and the
joint subjected to a "general" heat and blown out. Very nice joints
may be made by this method, and it is perhaps the better one where the
external part of B is to be less in diameter than the inserted part.
It was in this manner that the writer was taught to make glass
velocity pumps, one of which, of a good design, is figured as an
example.

In all cases good annealing should follow this operation. If the
inserted part of the inner tube (B) is anything like an inch in
diameter, and especially if it is of any length, as in some forms of
ozone apparatus, or in a large Bunsen's ice calorimeter, the
arrangements for supporting the inner part must be very good. A
convenient way of proceeding when the inner tube is well supported is
to make the mouth of A only very little larger than the diameter of B,
so that B will only just slip in. Then the mouth of A and the zone of
B may be heated together, and B blown out upon A. This, of course,
must be arranged for, if necessary, by temporarily stopping the inner
end of B.

The inner support of B should be removed as soon as practicable after
the joint is made, or, at all events, should not be perfectly rigid; a
tightly-fitting cork, for instance, is too rigid. The reason is, of
course, that in cooling there may be a tendency to set B a little to
one side or the other, and if it is not free to take such a set, the
joint most probably will give way. Good annealing both with flame and
asbestos is a sine qua non in all inserted work.

Fig. 34.

Sec. 35. Bending Tubes.

I have hitherto said nothing about bending tubes, for to bend a tube
of a quarter of an inch in diameter, and of ordinary thickness, is
about the first thing one learns in any laboratory, while to bend
large tubes nicely is as difficult an operation as the practice of
glass-b