cules. Their experiments have been so careful
and scientists have found out so much about what _seem_ to be
molecules,--how large they are, what they probably weigh, how fast
they move, and even what they are made of,--that almost no one has any
doubt left that fast-moving molecules make up everything in the world.

[Illustration: FIG. 41. A thermometer.]

To go back, then: if we looked at a piece of iron under a microscope
that would show us the molecules,--and remember, no such powerful
microscope could exist,--we should see these quivering particles, and
nothing more. Then if some one heated the iron while we watched the
molecules, or if the sun shone on it, we should see the molecules move
faster and faster and separate farther and farther. That is why heat
expands things. When the molecules in an object move farther apart,
naturally the object expands.

_Heat is the motion of the molecules._ When the molecules move faster
(that is, when the iron grows hotter), they separate farther and the
iron swells.

[Illustration: FIG. 42. A thermometer made of a flask of water. It
does not show the exact degree of heat of the water, but it does show
whether the water is hot or cold.]

HOW WE CAN TELL THE TEMPERATURE BY READING A THERMOMETER. The mercury
(quicksilver) in the bulb of the thermometer like everything else
expands (swells) when it becomes warm. It is shut in tightly on all
sides by the glass, except for the little opening into the tube above.
When it expands it must have more room, and the only space into which
it can move is up in the tube. So it rises in the tube.

[Illustration: FIG. 43. Will the hot ball go through the ring?]

Water will do the same thing. You can make a sort of thermometer,
using water instead of mercury, and watch the water expand when you
heat it. Here are the directions for doing this:

[Illustration: FIG. 44. When the wire is cold, it is fairly tight.]

EXPERIMENT 28. Fill a flask to the top with water. Put a piece
of glass tubing through a stopper, letting the tube stick 8 or
10 inches above the top of the stopper. Put the stopper into
the flask, keeping out all air; the water may rise 2 or 3
inches in the glass tube. Dry the flask on the outside and put
it on a screen on the stove or ring stand, and heat it. Watch
the water in the tube. What effect does heat have on the
water?

Here are two interesting experiments that show how solid things exp