large, it may be necessary to incline the end _B_ downwards; but as that
is apt to result in an excess of glass accumulating towards _d_, avoid
doing so if possible by rotating the glass steadily and rapidly. If at
any time the glass shows indications of collapsing, it must be removed
from the flame and gently blown into shape, during which operation it
may be rotated in the perpendicular position; indeed, to promote a
regular distribution of the glass by allowing it plenty of time to
collect, it is well from time to time to remove the heated mass of glass
from the flame, and slightly expand it by blowing. Finally, when a
regular mass of glass, such as is shown at _C_ (Fig. 20) has been
obtained, remove it from the flame, and blow it to its final dimensions.
A succession of gentle puffs _quickly_ succeeding each other should be
employed, in order that the progress of the bulb may be more easily
watched and arrested at the right moment. During the process of blowing,
the hot glass must be steadily rotated.

To collect the glass for blowing a bulb of lead glass, employ the flame
described on pp. 17-22 for heating lead glass.

If the tube be held horizontally whilst the globe is blown, its form
will most nearly approach that of a true globe. If it be held in the
perpendicular position, with the mass of glass depending from it, the
form of the bulb will usually be somewhat elongated. If it be held
perpendicularly, with the mass of glass upwards, the resulting bulb will
be flattened.

When a bulb is not of a sufficiently regular form, it may sometimes be
re-made by re-collecting the glass, and re-blowing it. The greatest care
is needed at the earlier stages of re-heating to prevent the glass from
collapsing into a formless and unworkable mass. This is to be prevented
in all such cases by gently blowing it into shape from time to time
whilst gathering the glass.

[Illustration: FIG. 21.]

_To blow a Bulb between two Points_ (Fig 21).--Select a piece of
suitable tube, seal or cork one end, gather together a mass of glass at
the desired part, as directed for blowing a bulb at the end of a tube;
when a mass of glass has been collected of sufficient thickness, blow it
into shape from the open end of the tube by a rapid succession of short
blasts of air, till the expanding glass attains the desired dimensions.
The tube must be held horizontally, and must be rotated steadily during
the process. By slightly pressing together