up" his
glass as soon as it gets really soft.

If his object is to make one joint--at any cost--then let him be
careful to use two bits of exactly the same kind of glass, and only
get the temperature up to the viscous stage. If the joint be then
pulled out till it is comparatively thin, it will probably stand (if
of soda glass); certainly, if of lead glass, though in this case it
may not be sound. In any case the joint should be annealed in the
asbestos box if practicable, otherwise (unless between narrow tubes)
with the asbestos rag. Care must be taken that the asbestos is dry.

Sec. 22. To weld two Tubes of different Sizes.

To do this, the diameter of the larger tube must be reduced to that of
the smaller. The general procedure described in drawing down must be
followed, with the following modification. In general, a greater
length of the tube must be heated, and it must be made hotter. The
tube is to be gradually drawn in the flame with constant turning till
the proper diameter and thickness of glass are attained.

Fig. 16.

For this operation time must be allowed if the operator's hands are
steady enough to permit of it; the shoulder should form partly by the
glass sinking in and partly by the process of drawing the hot glass
out. A shoulder properly prepared is shown in the sketch. Beginners
generally make the neck too thin on large tubes, and too thick on
smaller ones. There ought to be no great difference in thickness of
glass between the neck on the larger tube, and the smaller tube. The
diameters should be as nearly as possible alike.

Having drawn down the larger tube to a neck, take it out of the flame,
and as it cools pull and turn till the neck is of the right thickness
and is perfectly straight, i.e. make the final adjustment outside the
flame, and to that end have the neck rather too thick (as to glass)
before it is taken out. It is not necessary to wait till the neck
gets cold before the end can be cut off. Make a scratch as before--this
will probably slightly damage the temper of the file knife, but
that must be put up with. Hold the tube against the edge of the
table, so that the scratch is just above the level of the rim, and
strike the upper part a smart blow with the handle of the glass knife
rather in the direction of its length. [Footnote: A bit of hoop iron
nailed against the side of the table is a very convenient arrangement,
and it need not project appreciably above the