Experiment 9.--Hold the glass tubing between the thumb and
forefinger of each hand, resting it against the second finger.
Heat it in the upper flame, slowly at first, then strongly, but
heat only a very small portion in length, and keep it in constant
rotation with the right hand. Hold it steadily, and avoid
twisting it as the glass softens. The yielding is detected by the
yellow flame above the glass and by an uneven pressure on the
hands. Pull it a little as it yields, then heat a part just at
one side of the most softened portion. Rotate constantly without
twisting, and soon it can be separated into two closed tubes. No
thread should be attached; but if there be one, it can be broken
off and the end welded. The bottom can be made more symmetrical
by heating it red-hot, then blowing, gradually, into the open
end, this being inserted in the mouth. The parts should be
annealed by holding above the flame for a short time, to cool
slowly.

For hard glass--Bohemian--or large tubes, the blast-lamp or
blowpipe is needed. In the blast-lamp air is forced out with
illuminating gas. This gives a high degree of heat. Bulbs can be
made in the same way as ignition-tubes, and thistle-tubes are
made by blowing out the end of a heated bulb, and rounding it
with charcoal.

19. To Bend Glass Tubing.

Experiment 10.--Hold the tube in the upper flame. Rotate it so as
to heat all parts equally, and let the flame spread over 3 or 4
cm. in length. When the glass begins to yield, without removing
from the flame slowly bend it as desired. Avoid twisting, and be