ass is
at all thick, are very liable to crack off on reheating, so that they
must be reintroduced (into the flame) with especial care. This
liability to breakage is reduced, but not eliminated, by the asbestos
annealing.

Figs. 18 and 19.

Sec. 25. When the bore is very fine, it is best to seal off the tubes,
and blow an incipient bulb near one end of each tube. These bulbs may
be cooled in asbestos, and cut across when cold by means of a scratch
touched at one end (Figs. 18 and 19) by a fine point of highly
incandescent glass. For details of this method see p. 46, Fig. 21.
Time is occasionally saved by blowing off the ends of the bulbs. The
details of this process will be described when the operation of making
thistle-headed tubes is dealt with.

Sec. 26. When the tubes are both of large diameter, long, and very thin
(cylinder tubes), a considerable amount of difficulty will be
experienced. On the whole, it is best to heat each end separately
till the glass thickens a little, anneal in the flame and in asbestos,
and then proceed as in Sec. 22. If the ends are not quite true, it will
be found that quite a thickness of glass may be "jumped" together at
one side of the tubes, while the edges are still apart at the other.
When this looks likely to happen, incline the tubes as if the joint
were a hinge, and bend back quickly; do not simply continue to push
the tubes together in a straight line, or an unmanageable lump of
glass will be formed on one side.

If in spite of these precautions such a lump does form, proceed as
follows. Take a rod of glass, at least one-eighth of an inch thick,
and warm it in the flame at one end. Heat the imperfect joint till it
softens all round, and then bring the flame right up to the thick
part, and heat that as rapidly and locally as possible. The oxygas
flame does this magnificently. Press the heated end of the glass rod
against the thick part, and pull off as much of the lump as it is
desired to remove, afterwards blowing the dint out by a judicious
puff. Finish off as before.

Sec. 27. Occasionally, when it is seen that in order to produce a joint
closed all round, one side of the tube would be too much thickened, it
is better to patch the open side. For this purpose take a glass rod
about one-sixteenth inch in diameter, and turn the flame to give its
greatest effect, still keeping rather an excess of air or oxygen. See
that the side of the joint already made i