dy rotation of
the glass during the heating is almost essential.

_Blowing a Bulb or Expanding a Join._--Prolonged heating is necessary in
order that the thick parts may be heated completely through. Blowing
should take place by stages, in order that the thin parts, which tend
to expand first, have time to cool. The thick parts can then be expanded
by further blowing and thus a bulb or expansion of even thickness can be
obtained.

_Cutting Glass._--The most useful method for general use is by means of
the file or glass-blowers' knife. Either file or knife must be kept
sharp by grinding. Neither file nor knife should be used on hot glass.
The diamond and wheel cutter are useful for cutting sheet-glass, and
when the diamond is employed a singing noise is an indication of a
satisfactory cut.

_Leading a Crack._--A crack may be led in any desired direction by means
of a bead of hot glass or a small gas flame. The glass which it is
desired to crack should be heated at a point slightly in advance of the
crack, which will extend in the direction of the source of the heat.

_Turning Out the End of a Tube._--This is done by heating the end of the
tube and rotating it against an iron rod. The rod must be kept polished
and free from rust, and it must not be allowed to become too hot while
in use, otherwise the glass will stick to it.

_Joining Unlike Glasses._--Joints between unlike glasses are often
unstable. When such joints are made it is desirable to blow them as thin
as possible, and to avoid the junction of unlike glasses in any complex
joint, such as an internal seal. A transitional portion of tubing may be
built up by the successive addition and interfusion of beads of one of
the glasses to the end of a sealed tube consisting of the other glass.

_Joining a Tube to a Very Thin Bulb._--The bulb may be thickened at the
point of union by fusing on a bead of glass and expanding this slightly.
A small central portion of the expanded part may then be perforated by
bursting and the tube joined on.

_Insertion of One Bulb Within Another._--A bulb may be divided into two
halves by leading a crack round it and the inner bulb is then
introduced. The two halves of the outer bulb may be fitted together
(care being taken to avoid any damage to the edges), and the bulb may be
completed by rotating the contacting edges before the blowpipe until
they are soft, and then expanding slightly by means of air-pressure.

_Annealing._--Fo