117. If an electric fan is not fastened in place and has not a heavy base, it will move backward while it is going.

118. Doors with springs on them will close after you.

119. When you jump down on the end of a springboard, it throws you into the air.

120. You move your hands backward to swim forward.

NOTE. There are really two kinds of elasticity, which have nothing to do with each other. Elasticity of _form_ is the tendency of a thing to go back to its original shape, as rubber does. If you make a dent in rubber, it springs right back to the shape it had before. Elasticity of _volume_ is the tendency of a substance to go back to its original _size_, as lead does. If you manage to squeeze lead into a smaller s.p.a.ce, it will spring right back to the same size as soon as you stop pressing it on all sides. But a dent in lead will stay there; it has little elasticity of form.

Air and water--all liquids, in fact--have a great deal of elasticity of _volume_, but practically no elasticity of form.

They do not tend to keep their shape, but they do tend to fill the same amount of s.p.a.ce. Putty and clay likewise have very little elasticity of form; when you change their shape, they stay changed.

Jelly and steel and gla.s.s have a great deal of elasticity of _form_. When you dent them or twist them or in any way change their shape, they go right back to their first shape as soon as they can.

When we imagined a man with an "elastic touch," we were imagining a man who gave everything he touched perfect elasticity of _form_. It is elasticity of _form_ that most people mean when they talk about elasticity.

CHAPTER FOUR

HEAT

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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 gla.s.s, except for the little opening into the tube above.

When it expands it must have more room, and the only s.p.a.ce into which it can move is up in the tube. So it rises in the tube.

[Ill.u.s.tration: 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:

[Ill.u.s.tration: 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 gla.s.s 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 gla.s.s 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 expand when they are heated:

EXPERIMENT 29. The bra.s.s ball and bra.s.s ring shown in Figure 43 are called the expansion ball and ring. Try pushing the ball through the ring. Now heat the ball over the flame for a minute or two--it should not be red hot--and try again to pa.s.s it through the ring.

Heat both ball and ring for a short time. Does heating expand the ring?

EXPERIMENT 30. Go to the electric apparatus (described on page 379) and turn on the switch that lets the electricity flow through the long resistance wire. Watch the wire as it becomes hot.

_APPLICATION 24._ A woman brought me a gla.s.s-stoppered bottle of smelling salts and asked me if I could open it. The stopper was in so tightly that I could not pull it out. I might have done any of the following things: Tried to pull the stopper out with a pair of pliers; plunged the bottle up to the neck in hot water; plunged it in ice-cold water; tried to loosen the stopper by tapping it all around. Which would have been the best way or ways?

[Ill.u.s.tration: FIG. 45. But notice how it sags when it is hot.]

_APPLICATION 25._ I used to buy a quart of milk each evening from a farmer just after he had milked. He cooled most of the milk as soon as it was strained, to make it keep better. He asked me if I wanted my quart before or after it was cooled.

Either way he would fill his quart measure brim full. Which way would I have received more milk for my money?

INFERENCE EXERCISE

Explain the following:

121. Billiard b.a.l.l.s will rebound from each other and from the edges of the table again and again and finally stop.

122. In washing a tumbler in hot water it is necessary to lay it in sidewise and wet it all over, inside and out, to keep it from cracking; if it is thick in some parts and thin in others, like a cut-gla.s.s tumbler, it is not safe to wash it in hot water at all.

123. The swinging of the moon around the earth keeps the moon from falling to the earth.

124. A fire in a grate creates a draft up the chimney.

125. Telegraph wires and wire fences put up in the summer must not be strung too tightly.

126. Candy usually draws in somewhat from the edge of the pan as it hardens.

127. A meat chopper can be screwed to a table more tightly than you can possibly push it on.

128. A floor covered with linoleum is more easily kept clean than a plain wood floor.

129. Rough seams on the inside of clothes chafe your skin.

130. You can take the top off a bottle of soda pop with an opener that will pry it up, but you cannot pull it off with your fingers.

SECTION 16. _Cooling from expansion._

We get our heat from the sun; then why is it so cold up on the mountain tops?