The reason the balloon was drawn into the flask was that the steam in
the flask turned back to water as it cooled, and took very much less
space. This left a vacuum or empty space in the flask. What pushed the
balloon into the empty space?

[Illustration: FIG. 52. A toy balloon has been slipped over the mouth
of a flask that is filled with steam.]

[Illustration: FIG. 53. As the steam condenses and leaves a vacuum,
the air pressure forces the balloon into the flask.]

HOW STEAM MAKES AN ENGINE GO. The force of steam is entirely due to
the fact that steam takes so much more room than the water from which
it is made. A locomotive pulls trains across continents by using
this force, and by the same force a ship carries thousands of tons of
freight across the ocean. The engines of the locomotive and the ship
are worked by the push of steam. A fire is built under a boiler. The
water is boiled; the steam is shut in; the only way the steam can get
out is by pushing the piston ahead of it; the piston is attached to
machinery that makes the locomotive or ship move.

ONE THEORY ABOUT THE CAUSE OF VOLCANOES. The water that sinks deep
down into some of the hot parts of the earth turns to steam, takes
up more room, and forces the water above it out as a geyser. It is
thought by some scientists that volcanoes may be started by the water
in the ocean seeping down through cracks to hot interior parts of
the world where even the stone is melted; then the water, turning to
steam, pushes its way up to the surface, forcing dust and stone ahead
of it, and making a passage up for the melted stone, or lava. The
persons who hold this view call attention to the fact that volcanoes
are always in or near the sea. If this is the true explanation of
volcanoes, then we should have no volcanoes if steam did not take more
room than does the water from which it comes.

Here is a very practical fact about boiling water that many people do
not know; and their gas bills would be much smaller if they knew it.
Try this experiment:

[Illustration: FIG. 54. Will boiling water get hotter if you make it
boil harder?]

EXPERIMENT 38. Heat some water to boiling. Put the
boiling-point thermometer into the water (the thermometer
graduated to 110 deg. Centigrade and 220 deg. Fahrenheit), and note
the temperature of the boiling water. Turn up the gas and make
the water boil as violently as possible. Read the thermometer.
Does the water