Section 15. Heat makes things expand.
How does a thermometer work? What makes the mercury rise in it?
Why does heat make things get larger?
When we look at objects through a microscope, they appear much larger and in many cases we are able to see the smaller parts of which they are made. If we had a microscope so powerful that it made a tiny speck of dust look as big as a mountain (of course no such microscope exists), and if we looked through this imaginary microscope at a piece of iron, we should find to our surprise that the particles were not standing still. The iron would probably look as if it were fairly alive with millions of tiny specks moving back and forth, back and forth, faster than the flutter of an insect's wings.
These tiny moving things are molecules. Everything in the world is made of them. It seems strange that we should know this, since there really are no microscopes nearly powerful enough to show the molecules to us. Yet scientists know a great deal about them. They have devised all sorts of elaborate experiments—very accurate ones—and have tested the theories about molecules in many ways. They have said, for instance, "Now, if this thing is made of molecules, then it will grow larger when we make the molecules move faster by heating it." Then they heated it—in your next experiment you will see what happened. This is only one of thousands of experiments they have performed, measuring over and over again, with the greatest care, exactly how much an object expanded when it was heated a certain amount; exactly how much heat was needed to change water to steam; exactly how far a piece of steel of a certain size and shape could bend without breaking; exactly how crystals form—and so on and so on. And they have always found that everything acts as if it were made of moving molecules. 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.
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.
