air gun, the air is compressed at
first; then when it is released it springs out to its original
volume and throws the bullet ahead of it.

159. Leather soles get wet through in rainy weather, while
rubbers remain perfectly dry on the inside.

160. When you want to clean a wooden floor, you scrub it with
a brush.

SECTION 19. _Boiling and condensing._

What makes a geyser spout?

How does a steam engine go?

Once more let us imagine we are looking at molecules of water through
our magical microscope. But this time suppose that the water has been
made very hot. If we could watch the molecules smash into each other
and bound about more and more madly, suddenly we should see large
numbers of them go shooting off from the rest like rifle bullets,
and they would fly out through the seemingly great spaces between the
slower molecules of air. This would mean that the water was boiling
and turning to steam.

Here are a couple of experiments that will show you how much more room
water takes when it turns to steam than while it remains just water:

EXPERIMENT 36. Pour a half inch of water into the bottom of a
test tube. Put a cork in the test tube so tightly that it will
not let any steam pass it, but not too tightly. Hold the test
tube with a test-tube clamp at arm's length over a flame,
pointing the cork away from you. Wait for results.

The reason the cork flew out of the test tube is this: Steam takes
a great deal more room than water does,--many times as much room; so
when the water in the test tube turned to steam, the steam had to get
out and pushed the cork out ahead of it.

[Illustration: FIG. 51. In a minute the cork will fly out.]

EXPERIMENT 37. Pour about half an inch of water into the
bottom of a flask. Bring it to a vigorous boil over the burner
and let it boil half a minute. Now take the flask off the
flame and quickly slip the mouth of a toy balloon over the
mouth of the flask. Watch what happens. If things go too
slowly, you can speed them up by stroking the outside of the
flask with a cold, wet cloth.

When the balloon has been drawn into the flask as far as it
will go, you can put the flask back on the burner and heat the
water till it boils. When the balloon has been forced out of
the flask again and begins to grow large, take the flask off
the burner. Do this before the balloon explodes.