ecome too much thickened at _A_, the fault may be corrected by removing
it from the flame and gently pulling the two ends apart till it is of
the proper size. If the bore at the contracted part of the tube should
become too much reduced, it may be enlarged by closing one end of the
tube with a small cork, and blowing gently into the open end after
sufficiently heating the contracted part. The tube should be rotated
during blowing or the enlargement produced may be irregular.

When the external diameter of the tube is to be increased as well as its
bore diminished, press together the ends of a tube heated at the part to
be contracted, as already described, and regulate the size of the bore
by blowing into the tube if at any time it threatens to become too much
contracted.

=Widening Tubes.=--Tubes may be moderately expanded at their extremities
by means of the charcoal cone (see Bordering, p. 31). They may be
slightly expanded at any other part by closing one end and gently
blowing into the open end of the tube, after softening the glass at the
part to be widened before the blow-pipe. But the best method of
obtaining a wide tube with narrow extremities (Fig. 11) is to join
pieces of narrow tube _AA_ to the ends of a piece of wider tube _B_ of
the desired dimensions. The method of performing this operation is
described under welding, on pp. 39-47.

[Illustration: FIG. 11.]

[Illustration: FIG. 12.]

=Piercing Tubes.=--The glass-blower very frequently requires to make a
large or small opening in some part of a tube or other piece of
apparatus. This is known as piercing. Suppose it is desired to make a
small hole at the point _a_ in _A_ (Fig. 12). When the tube has been
brought to the flame with the usual precautions, allow the end of the
pointed flame to touch it at _a_ till an area corresponding to the
desired size of the opening is thoroughly softened. Then expand the
softened glass by blowing to the form shown at _B_. Re-heat _a_, blow a
small globe as at _C_, and carefully break the thin glass, then smooth
the rough edges by rotating them in the flame till they form a mouth
like that of _D_. Instead of leaving the bulb to be broken at the third
stage _C_, it is a good plan to blow more strongly, so that the bulb
becomes very thin and bursts, the removal of the thin glass is then
accompanied by less risk of producing a crack in the thicker parts of
the glass. Openings may be made in a similar manner in the side