The "why" of it
The glass becomes soft when heated because
it becomes almost a liquid, and if it is heated sufficiently it
comes entirely a liquid. In this respect it acts very much as pitch,
rosin, and wax act when heated by the sun or by a fire.
The end of a glass tube becomes smooth, or
closes entirely, when heated, for the following reason: The surface
FIG. 11
MAKING A GLASS BUBBLE
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of any liquid tries to take the smallest possible area (this is
explained in detail under "Surface Tension" in the Gilbert book on
"Experimental Mechanics"), for example, a small particle of water
takes the shape of a drop, a sphere, and the surface of a sphere
has the least area for a given amount of water. Now when the end of
the glass tube is heated it comes a liquid, and the surface of this
liquid contracts the glass into a smooth rounded surface of least
area. If the tube is heated still more, the surface contracts still
more and closes the end.
Experiment 6. Fun blowing glass bubbles.
Smooth one end of a piece of No. 2 tube and
allow it to cool. Close the other end in the blowpipe flame, turn
it slowly, put the smooth end into your mouth quickly, and blow as
hard as you can (Fig. 11). Do you get a fine big glass bubble which
bursts with a pop?
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