are heated, and not
to let the glass be thickened; bring the two ends together with
sufficient pressure to make them adhere, but not sufficient to compress
the glass to a thickened ring. Before the joint has time to cool too
much, adjust your blow-pipe for a pointed flame, if you are not already
working with that kind of flame, and allow the point of the flame to
play on any spot on the joint till it is heated to redness; rotate the
tube a little so as to heat the glass adjacent to that which is already
red-hot, and repeat this till the whole circumference of the rough joint
has been heated.[7] Repeat the operation last described, but, when each
spot is red-hot, blow gently into the open end of the tube so as to
slightly expand the viscous glass. Finally, rotate the whole joint in
the flame till the glass is softened, and blow gently as before into the
open end of the tube, still rotating it, in order that the joint may be
as symmetrical as possible. If in the last operation the diameter of the
joint becomes greater than that of the rest of the tube, it may be
cautiously re-heated and reduced by pulling it out, or this may be
secured by gently pulling apart the two ends, whilst the operator blows
it into its final shape.

[7] Some glass-blowers at once work on the glass as next described,
without this preliminary treatment. I find that some glass, usually soda
glass, will not always bear the necessary movements without breaking
unless first heated all round.

[Illustration: FIG. 14.]

When small tubes, or tubes of fine bore, are to be joined, in order to
prevent the fused glass from running together and closing the tube, it
is a good plan to border and enlarge the ends that are to be united, as
at _A_ (Fig. 14). Some glass-blowers prefer to border all tubes before
uniting them.

When a narrow tube is to be joined to one that is only slightly wider,
expand the end of the narrow tube till it corresponds in size to the
larger tube. If the tube be too narrow to be enlarged by inserting a
charcoal cone, seal one end and pierce it as directed (on p. 37).

For joining small thin-walled tubes Mr. Crookes recommends the use of a
small Bunsen flame.

In welding pieces of lead glass tube, take care that the heated glass is
perfectly free from reduced lead at the moment when the two ends of
viscous glass are brought into contact.

[Illustration: FIG. 15.]

_To join Tubes of Unequal Sizes End to End_ (Fig. 15).--Draw out t