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.

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.

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.

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:

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 expand when they are heated:

Experiment 29. The brass ball and brass 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 pass 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 glass-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?

Fig. 45.

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 balls 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-glass 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?

What is coldness?

Here is an interesting and rather strange thing about heat and expansion. Although heat expands things, yet expansion does not heat them. On the contrary, if a thing expands without being heated from an outside source, it actually gets cold! You see, in order to expand, it has to push the air or something else aside, and it actually uses up the energy of its own heat to do this. You will understand this better after you do the next experiment.

Experiment 31. Wet the inside of a test tube. Hold the mouth of the test tube against the opening of a carbon dioxid tank. Open the valve of the tank with the wrench and let the compressed gas rush out into the test tube until the mouth of the test tube is white. Shut off the valve. Feel your test tube.

What has happened is this: The gas was tightly compressed in the tank. It was not cold; that is, it had some heat in it, as everything has. When you let it loose, it used up much of its heat in pushing the air in the test tube and all around it out of the way. In this way it lost its heat, and then it became cold. Cold means absence of heat, as dark means absence of light. So when the compressed gas used up its heat in pushing the air out of its way, it became so cold that it froze the water in your test tube.

One reason why it is always cold high up in the air. Even on hot summer days aviators who fly high suffer from the cold. You might think that they would get warmer as they went up nearer the sun; one reason that they get colder instead is this:

As you saw in the last experiment, a gas that expands gets very cold. Air is a kind of gas. And whenever air rises to where there is not so much air crowding down on it from above, it expands. So the air that rises high and expands gets very cold. Consequently mountains which reach up into this high, cold air are snow covered all the year round; and aviators who fly high suffer keenly from the cold. There are several reasons for this coldness of the high air. This is just one of them.

Application 26. Explain why air usually cools when it rises; why high mountain tops are always covered with snow.

Inference Exercise

Explain the following:

131. You should not fill a teakettle brim full of cold water when you are going to put it on the stove.

132. It is harder to erase an ink mark than a pencil mark.

133. Bearings of good watches, where there is constant rubbing on the parts, are made of very hard jewels.

134. You feel lighter for an instant when you are in an elevator which starts down suddenly.

135. When men lay cement sidewalks, they almost always make cracks across them every few feet.

136. To cool hot coffee one sometimes blows on it.

137. It is much easier to turn the latch of a door with the knob than with the spindle when the knob is off.

138. Patent-leather shoes do not soil as easily as plain leather shoes.

139. We use rubber bands to hold things together tightly.

140. As air goes up it usually cools.

Section 17. Freezing and melting.

When water freezes in a pipe, why does the pipe burst?

What is liquid air?

Why does not the wire in an electric lamp melt when it is red hot?

Suppose we looked at a piece of ice through the imaginary microscope that shows us the molecules. The ice molecules would be different from the iron molecules in size, but they would be vibrating back and forth in exactly the same way, only with less motion. It is because they have less motion that we say the ice is colder than the iron. Then let us suppose that the sun was shining on the ice while we watched the ice molecules.

First we should see movements of the ice molecules become gradually more rapid, just as the iron molecules did when the iron was warmed. Then, as they moved faster and faster, they would begin to bump into each other and go around every which way, each molecule bumping first into one neighbor, then into another, and bouncing back in a new direction after each collision. This is what causes the ice to melt. When its molecules no longer go back and forth in the same path all the time, the ice no longer keeps its shape, and we call it water—a liquid.

Almost all solid substances will melt when they are heated. Or, to put it the other way around, every liquid will freeze solid if it gets cold enough. Even liquid air (which is ordinary air cooled and compressed until it runs like water) can be frozen into a solid chunk. Some things will melt while they are still very cold; solid air, for instance, melts at a temperature that would freeze you into an icicle before you could count ten. Other things, such as stones, are melted only by terrific heat.

When the little particles of water that make up the clouds become very cold, they freeze as they gather and so make snowflakes. When the little particles of water in the air, that usually make dew, freeze while they are gathering on a blade of grass, we call it frost. When raindrops are carried up into colder, higher air while they are forming, they freeze and turn to hail. When snow or frost or hail or ice is heated, it melts and turns back to water.

But here is a strange fact: although heat usually expands things, water expands when it freezes. Like everything else, however, water also expands when it becomes hot, as you found when you made a kind of thermometer, using a flask of water and a glass tube. But if you should put that flask into a freezing mixture of ice and salt, you would find that when the water became very cold it would begin to expand a little immediately before it froze.

And it is very lucky for us that water does expand when it freezes, because if it did not, ice would be heavier than water is. But since the water expands as it freezes, ice weighs less than water and floats. And that is why lakes and oceans and rivers freeze over the top and do not freeze at the bottom. If they froze from the bottom up, as they would if the ice sank as it formed, every river and lake would be solid ice in the winter. All the harbors outside the tropics would probably be ice-bound all winter long. And the ice in the bottom of the lakes and rivers and in the ocean would probably never melt.

So in the case of freezing water, and in the case of a couple of metals, there is a point where coldness, not heat, makes things expand.

Experiment 32. Take a ketchup bottle with a screw cap and a cork that fits tightly. Fill it to the top with water; put a long pin beside the cork while you insert it, so that the water can be crowded out as the cork goes down; then when you have pushed the cork in tightly, pull out the pin. Screw the cap on the bottle so as to hold the cork fast. Put the bottle in a pail or box, and pack ice and salt around it. Within an hour you should be able to see what the freezing water does to the bottle.

Application 27. Explain why ice is lighter than water; why we have no snow in summer.

Inference Exercise

Explain the following:

141. Sealing wax is held over a candle flame before it is applied to a letter.

142. Automobile tires tighten upon a sudden change from cold weather to hot.

143. When paper has been rolled, it tends to curl up again after being unrolled.

144. Seats running across a car are much more comfortable when a car starts and stops, than are seats running along the sides.

145. You cannot siphon water from a low place to a higher one.

146. Candles get soft in hot weather.

147. Meteorites fall to the earth from the sky.

148. When you preserve fruit and pour the hot fruit into the jars, you fill the jars brim full and screw on the cap air-tight; yet a few hours later the fruit does not fill the jars; there is some empty space between the top of the fruit and the cover.

149. Water pipes burst in the winter when it is very cold.

150. When people want to make iron castings, they first melt the iron, then pour it into molds. They leave it in the molds until cold. After that the iron holds the shape of the molds. Explain why the iron changes from a liquid to a solid.

Section 18. Evaporation.

Why is it that when ink is spilled it dries up, but when it is in the bottle it does not dry up?

What put the salt into the ocean?

Why do you feel cold when you get out of the bathtub?

Wet clothes get dry when they are hung on the clothes-line. The water in them evaporates. It turns to invisible vapor and disappears into the air. Water and all liquids evaporate when they are long exposed to the air. If they didn't—well, let us imagine what the world would be like if all evaporation should suddenly stop:

You find that your face is perspiring and your hands as well. You wipe them on your handkerchief, but soon they are moist again, no matter how cool the weather. After wiping them a few more times your handkerchief becomes soaking wet, and you hang it up to dry. There may be a good breeze stirring, yet your handkerchief does not get dry. By this time the perspiration is running off your face and hands, and your underclothes are getting drenched with perspiration.

You hurry into the house, change your clothes, bathe and wipe yourself dry with a towel. When you find that your wet things are not drying, and that your dry ones are rapidly becoming moist, you hastily build a fire and hang your clothes beside it. No use, your clothes remain as wet as ever. If you get them very hot the moisture in them will boil and turn to steam, of course, but the steam will all turn back to water as soon as it cools a little and the drops will cling to your clothes and to everything around the room. You will have to get used to living in wet clothes. You won't catch cold, though, since there is no evaporation to use up your heat.

But the water problem outside is not one of mere inconvenience. It never rains. How can it when the water from the oceans cannot evaporate to form clouds? Little by little the rivers begin to run dry—there is no rain to feed them. No fog blows in from the sea; no clouds cool the sun's glare; no dew moistens the grass at night; no frost shows the coming of cold weather; no snow comes to cover the mountains. In time there is no water left in the rivers; every lake with an outlet runs dry. There are no springs, and, after a while, no wells. People have to live on juicy plants. The crops fortunately require very little moisture, since none evaporates from them or from the ground in which they grow. And the people do not need nearly as much water to drink.

Little by little, however, the water all soaks too deep into the ground for the plants to get it. Gradually the continents become great deserts, and all life perishes from the land.

All these things would really happen, and many more changes besides, if water did not evaporate. Yet the evaporation of water is a very simple occurrence. As the molecules of any liquid bounce around, some get hit harder than others. These are shot off from the rest up into the air, and get too far away to be drawn back by the pull of the molecules behind. This shooting away of some of the molecules is evaporation. And since it takes heat to send these molecules flying off, the liquid that is left behind is colder because of the evaporation. That is why you are always cold after you leave the bathtub until you are dry. The water that evaporates from your body uses up a good deal of your heat.

Gasoline evaporates more quickly than water. That is why your hands become so cold when you get them wet with gasoline.

Since heat is required to evaporate a liquid, the quickest way to dry anything is to warm it. That is why you hang clothes in the sun or by the stove to dry.

Try these experiments:

Experiment 33. Read a thermometer that has been exposed to the room air. Now dip it in water that is warmer than the air, taking it out again at once. Watch the mercury. Does the thermometer register a higher or a lower temperature than it did at the beginning? What is taking up the heat from the mercury?

Experiment 34. Put a few drops of water in each of two evaporating dishes. Leave one cold; warm the other over the burner, but do not heat it to boiling. Which evaporates more quickly?

Why the sea is salt. You remember various fairy stories about why the sea is salt. For a long time the saltness of the sea puzzled people. But the explanation is simple. As the water from the rains seeps through the soil and rocks, it dissolves the salt in them and continually carries some of it into the rivers. So the waters of the rivers always carry a very little salt with them out to sea. The water in the ocean evaporates and leaves the salt behind. For millions of years this has been going on. So the rivers and lakes, which have only a little salt in them, keep adding their small amounts to the sea, and once in the sea the salt never can get out. The oceans never get any fuller of water, because water only flows into the ocean as fast as it evaporates from the ocean. Yet more salt goes into the ocean all the time, washed down by thousands of streams and rivers. So little by little the ocean has been growing more and more salty since the world began.

Great Salt Lake and the Dead Sea, unlike most lakes, have no rivers flowing out of them to carry the salt and water away, but rivers flow into them and bring along small amounts of salt all the time. Then the water evaporates from Great Salt Lake and the Dead Sea, leaving the salt behind; and that is why they are so very salty.

When people want to get the salt out of sea water, they put the sea water in shallow open tanks and let the water evaporate. The salt is left behind.

Experiment 35. Dissolve some salt in warm water until no more will dissolve. Pour the clear liquid off into an evaporating dish, being careful not to let any solid particles of the salt go over. Either set the dish aside uncovered, for several days, or heat it almost to boiling and let it evaporate to dryness. What is left in the dish?

Application 28. Some girls were heating water for tea, and were in a hurry. They had only an open stew pan to heat the water in.

"Cover the pan with something; you'll let all the heat out!" Helen said.

"No, you want as much heat to go through the water as possible. Leave the lid off so that the heat can flow through easily," said Rose.

"The water will evaporate too fast if the lid is off, and all the heat will be used up in making it evaporate; it will take it much longer to get hot without the lid," Louise argued.

"That's not right," Rose answered. "Boiling water evaporates fastest of all. We want this to boil, so let it evaporate; leave the lid off."

What should they have done?

Application 29. Two men were about to cross a desert. They had their supply of water in canvas water bags that leaked just enough to keep the outside of the bags wet. Naturally they wanted to keep the water as cold as possible.

"I'm going to wrap my rubber poncho around my water bag and keep the hot desert air away from the water," said one.

"I'm not. I'm going to leave mine open to the air," the other said.

Which man was right? Why?

Inference Exercise

Explain the following:

151. When you go up high in an elevator, you feel the pressure of the air in your ears.

152. Water is always flowing into Great Salt Lake; it has no outlet; yet it is getting more nearly empty all the time.

153. A nail sinks while a cork floats in water.

154. Steep hillsides are paved with cobblestones instead of asphalt.

155. If you place one wet glass tumbler inside another you can pull them apart only with difficulty, and frequently you break the outer one in the attempt.

156. Sausages often break their skins when they are being cooked.

157. A drop of water splashed against a hot lamp chimney cracks it.

158. When you shoot an 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.

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.

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?

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:

Experiment 38. Heat some water to boiling. Put the boiling-point thermometer into the water (the thermometer graduated to 110° Centigrade and 220° 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 become appreciably hotter over the very hot fire than it does over the low fire, if it is boiling in both cases? But in which case is more steam given off? Will a very hot fire make the water boil away more rapidly than a low fire?

When you are cooking potatoes, are you trying to keep them very hot or are you trying to boil the water away from them? Which are you trying to do in making candy, to keep the sugar very hot or to boil the water away from it?

All the extra heat you put into boiling water goes toward changing the water into steam; it cannot raise the water's temperature, because at the moment when water gets above the boiling point it ceases to be water and becomes steam. This steam takes up much more room than the water did, so it passes off into the air. You can tell when a teakettle boils by watching the spout to see when the steam³ pours forth from it in a strong, steady stream. If the steam took no more room than the water, it could stay in the kettle as easily as the water.

Footnote 3: What you see is really not the steam, but the vapor formed as the steam condenses in the cool room. The steam itself is invisible, as you can tell by looking at the mouth of the spout of a kettle of boiling water. You will see a clear space before the white vapor begins. The clear space is steam.

Distilling. When liquids are mixed together and dissolved in each other, it looks as if it would be impossible to take them apart. But it isn't. They can usually be separated almost perfectly by simply boiling them and collecting their vapor. For different substances boil at different temperatures just as they melt at different temperatures. Liquid air will boil on a cake of ice; it takes the intense heat of the electric furnace to boil melted iron. Alcohol boils at a lower temperature than water; gasoline boils at a lower temperature than kerosene. And people make a great deal of practical use of these facts when they wish to separate substances which have different boiling temperatures. They call this distilling. You can do some distilling yourself and separate a mixture of alcohol and water in the following manner:

Experiment 39. First, pour a little alcohol into a cup—a few drops is enough—and touch a lighted match to it. Will it burn? Now mix two teaspoonfuls of alcohol with about half a cup of water and enough blueing to color the mixture. Pour a few drops of this mixture into the cup and try to light it. Will it burn?

Fig. 55.

Fig. 55. By distillation clear alcohol can be separated from the water and red ink with which it was mixed.

Now pour this mixture into a flask. Pass the end of the long bent glass rod (the "worm") through a one-hole rubber stopper that will fit the flask (Fig. 55). Put the flask on a ring stand and, holding it steady, fasten the neck of the flask with a clamp that is attached to the stand. Put the stopper with the worm attached into the flask, and support the worm with another clamp. Put a dry cup or beaker under the lower end of the worm. Set a lighted burner under the flask. When the mixture in the flask begins to boil, turn the flame down so that the liquid will just barely boil; if it boils violently, part of the liquid splashes up into the lower end of the worm.

As the vapor rises from the mixture and goes into the worm, it cools and condenses. When several drops have gone down into the cup, try lighting them. What is it that has boiled and then condensed: the water, the alcohol, or the blueing? Or is it a mixture of them?

Alcohol is really made in this way, only it is already mixed in the water in which the grains fermented and from which people then distil it. Gasoline and kerosene are distilled from petroleum; there is a whole series of substances that come from the crude oil, one after the other, according to their boiling points, and what is left is the foundation for a number of products, including paraffine and vaseline.

Experiment 40. Put some dry, fused calcium chlorid on a saucer and set it on the plate of the air pump. This is to absorb the moisture when you do the experiment. (This calcium chlorid is not the same as the chlorid of lime which you buy for bleaching or disinfecting.) Fill a flask or beaker half full of water and bring it to a boil over a Bunsen burner. Quickly set the flask on the plate of the air pump. The water will stop boiling, of course. Cover the flask and the saucer of calcium chlorid with the bell jar immediately, and pump the air out of the jar. Watch the water.

The water begins to boil again because water will boil at a lower temperature when there is less air pressure on its surface. So although the water is too cool to boil in the open air, it is still hot enough to boil when the air pressure is partially removed. It is because of this that milk is evaporated in a vacuum for canning; it is not necessary to make it so hot that it will be greatly changed by the heat, if the boiling is done in a vacuum. On a high mountain the slight air pressure lets the water boil at so low a temperature that it never becomes hot enough to cook food.

Application 30. Two college students were short of money and had to economize greatly. They got an alcohol lamp to use in cooking their own breakfasts. They planned to boil their eggs.

"Let's boil the water gently, using a low flame," one said; "we'll save alcohol."

"It would be better to boil the eggs fast and get them done quickly, so that we could put the stove out altogether," the other replied.

Which was right?

Application 31. Two girls were making candy. They put a little too much water into it.

"Let us boil the candy hard so that it will candy more quickly," said one.

"Why, you wasteful girl," said the other. "It cannot get any hotter than the boiling point anyhow, so you can't cook it any faster. Why waste gas?"

Which girl was right?

Inference Exercise

Explain the following:

161. Warm air rises.

162. The lid of a teakettle rattles.

163. Heating water makes a steam engine go.

164. When an automobile with good springs and without shock absorbers goes over a rut, the passengers do not get a jolt, but immediately afterward bounce up into the air.

165. Comets swing around close to the sun, then off again into space; how do they get away from the sun?

166. When you wish to pour canned milk out, you need two holes in the can to make it flow evenly.

167. Liquid air changes to ordinary air when it becomes even as warm as a cake of ice.

168. Skid chains tend to keep automobiles from skidding on wet pavement.

169. A warm iron and a blotter will take candle grease out of your clothes.

170. Candies like fudge and nougat become hard and dry when left standing several days open to the air.

Section 20. Conduction of heat and convection.

Why does a feather comforter keep you so warm?

When you heat one end of a nail, how does the heat get through to the other end?

How does a stove make the whole room warm?

Here is a way to make heat run a race. See whether the heat that goes through an iron rod will beat the heat that goes through a glass rod, or the other way round:

Experiment 41. Take a solid glass rod and a solid iron rod, each about a quarter inch in diameter and about 6 inches long. With sealing wax or candle grease stick three ball bearings or pieces of lead, all the same size, to each rod, about an inch apart, beginning 2 inches from the end. Hold the rods side by side with their ends in a flame, and watch the balls fall off as the heat comes along through the rods. The heat that first melts off the balls beats.

What really happens down among the molecules when the heat travels along the rods is that the molecules near the flame are made to move more quickly; they joggle their neighbors and make them move faster; these joggle the ones next to them, and so on down the line. Heat that travels through things in this way is called conducted heat. Anything like iron, that lets the heat travel through it quickly, is called a good conductor of heat. Anything like glass, that allows the heat to travel through it only with difficulty, is called a poor conductor of heat, or an insulator of heat.

A silver spoon used for stirring anything that is cooking gets so hot all the way up the handle that you can hardly hold it, while the handle of a wooden spoon never gets hot. Pancake turners usually have wooden handles. Metals are good conductors of heat; wood is a poor conductor.

An even more obvious example of the conducting of heat is seen in a stove lid; your fire is under it, yet the top gets so hot that you can cook on it.

When anything feels hot to the touch, it is because heat is being conducted to and through your skin to the sensitive little nerve ends just inside. But when anything feels cold, it is because heat is being conducted away from your skin into the cold object.

Air carries heat by convection. One of the poorest conductors of heat is air; that is, one particle of air can hardly give any of its heat to the next particle. But particles of air move around very easily and carry their heat with them; and they can give the heat they carry with them to any solid thing they bump into. So when air can move around, the part that is next to the stove, for instance, becomes hot; this hot air is pushed up and away by cold air, and carries its heat with it. When it comes over to you in another part of the room, some of its heat is conducted to your body. When air currents—or water currents, which work the same way—carry heat from one place to another like this, we say that the heat has traveled by convection.

Since heat is so often carried to us by convection,—by warm winds, warm air from the stove, warm ocean currents, etc.,—it seems as if air must be a good conductor of heat. But if you shut the air up into many tiny compartments, as a bird's feathers do, or as the hair on an animal's back does, so that it cannot circulate, the passage of heat is almost completely stopped. When you use a towel or napkin to lift something hot, it is not so much the fibers of cotton which keep the heat from your hand; it is principally the very small pockets of air between the threads and even between the fibers of the threads.

Inference Exercise

Explain the following:

171. If the axle of a wheel is not greased, it swells until it sticks fast in the hub; this is a hot box.

172. When you have put liquid shoe polish on your shoes, your feet become cold as it dries.

173. The part of an ice-cream freezer which holds the cream is usually made of metal, while that which goes outside and contains the ice and salt is usually made of wood.

174. The steam in a steam radiator rises from a boiler in the basement to the upper floors.

175. When you throw a ball, it keeps going for a while after it leaves your hand.

176. Clothes keep you warm, especially woolen clothes.

177. The Leaning Tower of Pisa does not fall over.

178. It is almost impossible to climb a greased pole.

179. Heat goes up a poker that is held in a fire.

180. A child can make a bicycle go rapidly without making his feet go any faster than if he were walking.