Fig. 50.A view of the Dead Sea.

Fig. 50.Fig. 50.A view of the Dead Sea.

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?

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?

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.

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?

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 thetest tube turned to steam, the steam had to get out and pushed the cork out ahead of it.

Fig. 51.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.

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 asit 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?

Fig. 52.Fig. 52.A toy balloon has been slipped over the mouth of a flask that is filled with steam.

Fig. 53.Fig. 53.As the steam condenses and leaves a vacuum, the air pressure forces the balloon into the flask.

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 waterthat many people do not know; and their gas bills would be much smaller if they knew it. Try this experiment:

Fig. 54.Fig. 54.Will boiling water get hotter if you make it boil harder?

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 steam3pours 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 ofthese 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 stopperwith 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?

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 stopperwith 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 isnotthe 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 toboil 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?

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?

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.

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?

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:

Fig. 56.Fig. 56.The metal balls are fastened to the iron and glass rods with drops of wax.

Experiment 41.Take a solid glass rod and a solid iron rod, each about a quarter inch in diameter and about 6inches 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.

Fig. 57.Fig. 57.Does the heat travel faster through the iron or through the glass?

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 iscalledconductedheat. Anything like iron, that lets the heat travel through it quickly, is called agood conductorof heat. Anything like glass, that allows the heat to travel through it only with difficulty, is called apoor conductorof heat, or aninsulatorof 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, whichwork the same way—carry heat from one place to another like this, we say that the heat has traveled byconvection.

Fig. 58.Fig. 58.Convection currents carrying the heat of the stove about the room.

Since heat is so often carried to us by convection,—by warm winds, warm air from the stove, warm ocean currents, etc.,—itseemsas 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.

Fig. 59.Fig. 59.Diagram of a hot-water heater. What makes the water circulate?Cold the absence of heat.Cold is merely the absence of heat; so if you keep the heat from escaping from anything warm, it cannot become cold; while if you keep the heat from reaching a cold thing it cannot become warm. A blanket is just as good for keeping ice from melting, by shutting the heat out, as it is for keeping you warm, by holding heat in.Application 32.Explain why ice is packed in straw or sawdust; why a sweater keeps you warm.Select from the following list the good conductors of heat from the poor conductors (insulators): glass, silver, iron, wood, straw, excelsior, copper, asbestos, steel, nickel, cloth, leather.

Fig. 59.Fig. 59.Diagram of a hot-water heater. What makes the water circulate?

Cold the absence of heat.Cold is merely the absence of heat; so if you keep the heat from escaping from anything warm, it cannot become cold; while if you keep the heat from reaching a cold thing it cannot become warm. A blanket is just as good for keeping ice from melting, by shutting the heat out, as it is for keeping you warm, by holding heat in.

Application 32.Explain why ice is packed in straw or sawdust; why a sweater keeps you warm.Select from the following list the good conductors of heat from the poor conductors (insulators): glass, silver, iron, wood, straw, excelsior, copper, asbestos, steel, nickel, cloth, leather.

Application 32.Explain why ice is packed in straw or sawdust; why a sweater keeps you warm.

Select from the following list the good conductors of heat from the poor conductors (insulators): glass, silver, iron, wood, straw, excelsior, copper, asbestos, steel, nickel, cloth, leather.

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.

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.

Section 21.How heat gets here from the sun; why things glow when they become very hot.

If we were to go back to our imaginary switchboard we should find a switch, between the heat and the light switches, labeledRadiation. Suppose we turn it off:

Instantly the whole world becomes pitch dark; so does the sky. We cannot see the sun or a star; no electric lights shine; and although we can "light" a match, it gives no light. The air above the burning match is hot, and we can burn our fingers in the invisible flame, but we can see nothing whatever.

Yet the world does not get cold. If we leave the switch off for years, while the earth remains in darkness and we all live like blind people, it never gets cold. Winter and summer are alike, day and night are just the same. Gradually, after many ages, the ice and snow in the north and in the far south begin to melt as the warmth from the rest of the world is conducted to the polar regions. And the heat from the interior of the earth makes all the parts of the earth's surface warmer. Winds almost stop blowing. Ocean currents stop flowing. The land receives less rainfall, until finally everything turns to a desert; almost the only rain is on the ocean. Animals die even before the rivers dry up, for the flesh eaters are not able to see their prey, and since, without light, all green things die, the animals that live on plants soon starve. Men have to learn to live on mushrooms, which grow in the dark. The world is plunged into an eternal warm, pitch-black night.

Fig. 60.Fig. 60.It is by radiation that we get all our heat and light from the sun.

Turning off the radiation would cause all these things to happen, because it is by radiation that we get all our heat from the sun and all our light from any source. And it is by radiation that the earth loses heat into space in the night and loses still more heat into space during the winter.

We do not get our heat from the sun by conduction; we cannot, because there is nothing between us and the sun to conduct it. The earth's air, in amounts thick enough to count, goes up only a hundred miles or so. It is really just a thin sort of blanket surrounding the earth. The sun is 93,000,000 miles away. Between us and the sun there is empty space. There are no molecules to speak of in that whole vast distance. So if heat traveled only by conduction,—that is, if radiation stopped,—we should be so completely shut offfrom the sun that we should not know there was such a thing.

But even if we filled the space between us and the sun with copper or silver, which are about the best conductors of heat in the world, it would take the heat from the sun years and years to be conducted down to us. Yet we know that the sun's heat really gets to us in a few minutes. This is because heat can travel in a very much quicker way than by conduction. Itradiatesthrough space, just as light does. And it can come the whole 93,000,000 miles from the sun in about 8 minutes. This is so fast that if it were going around the world instead of coming from the sun, it would go around 7-1/2 times before you could say "Jack Robinson,"—really, because it takes you at least one second to say "Jack Robinson."

We are not absolutely sure how heat gets here so fast. But what most scientists think nowadays is that there is a sort of invisible rigid stuff, not made of molecules or of anything but just itself, calledether. (This ether, if there really is such a thing, is not related at all to the ether that doctors use in putting people to sleep. It just happens to have the same name.) The ether is supposed to fill all space, even the tiny spaces between molecules. The fast moving particles of the sun joggle the ether up there, and make ripples that spread out swiftly all through space. When those ripples strike our earth, they make the molecules of earth joggle, and that is heat. The ripples that spread out from the sun are calledether waves.

But the important and practical fact to know is that there is a kind of heat, calledradiant heat, that canpass through empty space with lightning-like quickness. And when this radiant heat strikesthings, it is partly absorbed and changed to the usual kind of heat.

This radiant heat is closely related to light. As a matter of fact, light is only the special kind of ether waves that affect our eyes. Radiant heat is invisible. The ether waves that are visible we call light. In terms of ether waves, the only difference between light and radiant heat is that the ripples in light are shorter. So it is no wonder that when we get a piece of iron hot enough, it begins to give off light; and we say it is red hot. What happens to the ether is this: As the molecules of iron go faster and faster (that is, as the iron gets hotter and hotter), they make the ripples in the ether move more frequently until they get short enough to belightinstead of radiant heat. Objects give off radiant heat without showing it at all; the warmth that you feel just below a hot flatiron is mainly radiant heat.

When anything becomes hot enough to glow, we say it isincandescent. That is why electric lamps are calledincandescent lamps. The fine wires—called thefilament—in the lamp get so hot when the electricity flows through them that they glow or become incandescent, throwing off light and radiant heat.

It is the absorbing of the radiant heat by your hand that makes you feel the heat the instant you turn an electric lamp on. Try this experiment:

Experiment 42.Turn on an incandescent lamp that is cold. Feel it with your hand a second, then turn it off at once. Is the glass hot? (The lamp you use should be an ordinary 25, 40, or 60 watt vacuum lamp.)

The radiant heat from the incandescent filament in the lamp passed right out through the vacuum of the lamp, and much of it went on through the glass to your hand. You already know what a poor conductor of heat glass is; yet it lets a great deal of radiant heat pass through it, just as it does light. As soon as the lamp stops glowing, the heat stops coming; the glass is not made hot and you no longer feel any heat. In one way the electric filament shining through a vacuum is exactly like the sun shining through empty space: the heat from both comes to us by radiation.

If a lamp glows for a long time, however, the glass really does become hot. That is partly because there is not a perfect vacuum within it (there is a little gas inside that carries the heat to the glass by convection), and it is partly because the glass does not let quite all of the radiant heat and light go through it, but absorbs some and changes it to the regular conducted heat.

One practical use that is made of a knowledge of the difference between radiant and conducted heat is in the manufacture of thermos bottles.

Experiment 43.Take a thermos bottle apart. Examine it carefully. If it is the standard thermos bottle, with the name "thermos" on it, you will find that it is made of two layers of glass with a vacuum between them. The vacuum keeps anyconductedheat from getting out of the bottle or into it. But, as you know,radiantheat can flash right through a vacuum. So to keep it from doing this the glass is silvered, making a mirror out of it. Just as a mirror sends light back to where it comes from, it sends practically all radiant heat back to where it comes from. Heat, therefore, cannot get into the thermos bottle or out of it either byradiation or conduction. And that is why thermos bottles will keep things very hot or ice-cold for such a long time.Fig. 61.Fig. 61.How a thermos bottle is made. Notice the double layer of glass in the broken one.Fill the thermos bottle with boiling water, stopper it, and put it aside till the next day. See whether the water is still hot.

Fig. 61.Fig. 61.How a thermos bottle is made. Notice the double layer of glass in the broken one.

Fill the thermos bottle with boiling water, stopper it, and put it aside till the next day. See whether the water is still hot.

If we could make the vacuum perfect, and surround all parts of the bottle, even the mouth, with the perfect vacuum, and if the mirror were perfect, things put into a thermos bottle would stay boiling hot or icy cold forever and ever.

Why it is cool at night and cold in winter.It is the radiation of heat from the earth into space that makes the earth cooler at night and cold in winter. Much of the heat that the earth absorbs from the sun in the daytime radiates away at night. And since it keeps onradiating away until the sun brings us more heat the next day, it is colder just before dawn than at midnight, more heat having radiated into space.

For the same reason it is colder in January and February than in December. It is in December that the days are shortest and the sun shines on us at the greatest slant, so that we get the least heat from it; but we still have left some of the heat that was absorbed in the summer. And we keep losing this heat by radiation faster than we get heat from the sun, until almost spring.

Application 33.Distinguish between radiant and conducted heat in each of the following examples:(a) The sun warms a room through the window. (b) A room is cooler with the shades down than up, when the sun shines on the window. (c) But even with the shades down a room on the sunny side of the house is warmer than a room on the shady side. (d) When a mirror is facing the sun, the back gets hot. (e) If you put your hand in front of a mirror held in the sun, the mirror reflects heat to your hand. (f) If you put a plate on a steam radiator, the top of the plate gradually becomes hot. (g) If anything very hot or cold touches a gold or amalgam filling of a sensitive tooth, you feel it decidedly. (h) The handle of your soup spoon becomes hot when the bowl of it is in the hot soup. (i) The moon is now very cold, although it probably was once very hot.

Application 33.Distinguish between radiant and conducted heat in each of the following examples:

(a) The sun warms a room through the window. (b) A room is cooler with the shades down than up, when the sun shines on the window. (c) But even with the shades down a room on the sunny side of the house is warmer than a room on the shady side. (d) When a mirror is facing the sun, the back gets hot. (e) If you put your hand in front of a mirror held in the sun, the mirror reflects heat to your hand. (f) If you put a plate on a steam radiator, the top of the plate gradually becomes hot. (g) If anything very hot or cold touches a gold or amalgam filling of a sensitive tooth, you feel it decidedly. (h) The handle of your soup spoon becomes hot when the bowl of it is in the hot soup. (i) The moon is now very cold, although it probably was once very hot.

Explain the following:181. Trees bend in the wind, then straighten up again. Why do they straighten up?182. A cloth saturated with kerosene and placed in the bottom of a clock will oil the clockworks above it.183. In cold weather the doorknobinsidethe front door is cold.184. It is cool in the shade.185. Clothes get hot when you iron them.186. Potatoes fried in deep fat cook more quickly than those boiled in water.187. If you hold your hand near a vacuum electric lamp globe that is glowing, some of the heat will go out to your hand at once.188. Rubbing silver with fine powder polishes it.189. A mosquito can suck your blood.190. A hot-water tank becomes hot at the top first, then gradually heats downward. When you light the gas under an ordinary hot-water heater, the hot water circulates to the top of the boiler, while the cold water from the boiler pushes into the bottom part of the heater, as shown in Figure 59. What causes this circulation?

Explain the following:

181. Trees bend in the wind, then straighten up again. Why do they straighten up?

182. A cloth saturated with kerosene and placed in the bottom of a clock will oil the clockworks above it.

183. In cold weather the doorknobinsidethe front door is cold.

184. It is cool in the shade.

185. Clothes get hot when you iron them.

186. Potatoes fried in deep fat cook more quickly than those boiled in water.

187. If you hold your hand near a vacuum electric lamp globe that is glowing, some of the heat will go out to your hand at once.

188. Rubbing silver with fine powder polishes it.

189. A mosquito can suck your blood.

190. A hot-water tank becomes hot at the top first, then gradually heats downward. When you light the gas under an ordinary hot-water heater, the hot water circulates to the top of the boiler, while the cold water from the boiler pushes into the bottom part of the heater, as shown in Figure 59. What causes this circulation?

Section 22.Reflection.

How is it that you can see yourself in a mirror?What makes a ring around the moon?Why can we see clouds and not the air?Why is a pair of new shoes or anything smooth usually shiny?

How is it that you can see yourself in a mirror?

What makes a ring around the moon?

Why can we see clouds and not the air?

Why is a pair of new shoes or anything smooth usually shiny?

If we turn off a switch labeledReflection of Lighton our imaginary switchboard, we think at first that we have accidentally turned offRadiationagain, for once more everything instantly becomes dark around us. We cannot see our hands in front of our faces. Although it is the middle of the day, the sky is jet black. But this time we see bright stars shining in it. And among them is the sun, shining as brightly as ever and dazzling our eyes when we look at it. But its light does no good. When we look down from the sky toward the earth, everything is so black that we should think we were blind if we had not just seen the stars and sun.

Groping our way along to an electric lamp, we turn it on. It shines brightly, but it does not make anythingaround it light; everything stays absolutely invisible. It is as if all things in the world except the lights had put on some sort of magic invisible caps.

We can strike a match and see its flame. We can see a fire on the hearth. We may feel around for the invisible poker, and when we find it, we may put it in the fire. When it becomes hot enough, it will glow red and become visible. We can make a match head glow by rubbing it on a wet finger. We can even see a firefly, if one comes around. But only those things which are glowing of themselves, like flames, and red-hot pokers, and fireflies, will be visible.

The reason why practically everything would be invisible if there were no reflection of light is this: When you look at anything, as a man, for instance, what you really see is the light that hits him and bounces back (reflects) into your eyes. Suppose you go into a dark room and turn on an electric light. Instantly ripples of light flash out from the lamp in every direction. As soon as they strike the object you are looking at, they reflect (bounce back) from it to your eyes. When light strikes your eyes, you see.

Of course, when you look at an electric lamp, or a star, or the sun, or anything that is incandescent (so hot that it shines by its own light), you can see it, whether reflection exists or not. But most things you look at do not shine by their own light. This book that you are reading simply reflects the light in the room to your eyes; it would not give any light in a dark room. The paper reflects a good deal of light that strikes it, so it looks very light; the print reflects practically none ofthe light that strikes it, so it looks dark, or black, just as a keyhole looks black because it does not reflect any light to your eyes. But without reflection, the book would be entirely invisible. The only kind of print you could read if there were no reflection would be the electric signs made out of incandescent lamps arranged to form letters.

What the ring around the moon is; what sunbeams are.The reason you sometimes see a ring around the moon is that some of the moonlight reflects from tiny droplets of water in the air, making them visible. In the same way, the dust in the air of a room becomes visible when the sun shines through it and is reflected by each speck of dust; we call it asunbeam. But we are not really looking directly at the sunlight; we are seeing the part of the sunlight that is reflected by the dust specks.

Have you ever noticed that when you stand a little to one side of a mirror where you cannot see your own image in it, you can sometimes see that of another person clearly, while he cannot see his own image but can see yours? It is easy to understand this by comparing the reflection of the light from your face to his eye and from his face to your eye, to the bouncing of a ball from one person to another. Suppose you and a friend are standing a little way apart on sandy ground where you cannot bounce a ball, but that between you there is a plank. If each of you is standing well away from the plank, neither one of you can possibly bounce the ball on it in such a way that he can catch it himself. Yet you can easily bounce it to your friend and he can bounce it to you.

Fig. 62.Fig. 62.The ball bounces from one boy to the other, but it does not return to the one who threw it.

The mirror is like that plank; it is something that will reflect (bounce) the light directly. The light from your face goes into the mirror, just as you may throw the ball against the plank, and the light is reflected to your friend just as the ball is bounced to him; so he sees your image in the mirror. If he can see you, you can see him, just as when you bounce the ball to him he can bounce it to you. But you may be unable to see yourself, just as you may be unable to bounce the ball on the plank so that you yourself can catch it.

In other words, when light strikes against something it bounces away, just as a rubber ball bounces from a smooth surface. If you throw a ball straight down, it comes straight up; if light shines straight down on a flat, smooth surface, it reflects straight up. If you throw a ball down at a slant, it bounces up at the same slant in the opposite direction; if light strikes a smooth surfaceat a slant, it reflects at the same slant in the opposite direction.

Fig. 63.Fig. 63.In the same way, the light bounces (reflects) from one boy to the other. It does not return to the point from which it started and neither boy can see himself.

But to reflect light directly and to give a clear image, the surface the light strikesmustbe extremely smooth, just as a tennis court must be fairly smooth to make a tennis ball rebound accurately. Any surface that is smooth enough will act like a mirror, although naturally, if it lets most of the light go through, it will not reflect as well as if it sends all the light back. A pane of glass is very smooth, and you can see yourself in it, especially if there is not much light coming through the glass from the other side to mix up with your reflection. But if the pane of glass is silvered so thatno light can get through, you have a real mirror; most of the light that leaves your face is reflected to your eyes again.

Why smooth or wet things are shiny.When a surface is very smooth, we say it is shiny or glossy. Even black shoes, if they are polished, become smooth enough to reflect much of the light that strikes them; of course the parts where the light is being reflected do not look black but white, as any one who has tried to paint or draw a picture of polished shoes knows. Anything wet is likely to be shiny, because the surface of water is usually smooth enough to reflect light rather directly.

If a surface is uneven, like a pool with ripples on it, the light reflects unevenly, and you see a distorted image; your face seems to be rippling and moving in the water.

Fig. 64.Fig. 64.How should the mirror be placed?Application 34.Some boys were playing war and were in a ditch that they called a trench. They wanted to make a simple periscope so that they could look out of the ditch at the "enemy" without being in danger. They had an old stovepipe and a mirror. Practically all of them agreed that if the mirror were fixed in the top of the stovepipe and if they looked up through the bottom, they would be able to see over the side of the ditch. But they had an argument as to how the mirror should be placed. Each drew a diagram to show how he thought the mirror should be arranged, using dotted lines to show how the light would come from the enemy to their eyes. Three of the diagrams are shown in Figure 64.The boy who drew the first said: "If you want to see the enemy, the mirror's got to face him. Then it will reflect the light down to your eyes."The boy who drew the second said: "No, the light would just go back to him again. The mirror must slant so that the light that strikes it at a slant will be reflected to your eye at the same slant.""How could it get to your eye at all," the third boy said, "if the mirror didn't face you? You've got to have the mirror reflect right down toward your face. Then all the light that strikes it will come down to you."Which arrangement would work?

Fig. 64.Fig. 64.How should the mirror be placed?

Application 34.Some boys were playing war and were in a ditch that they called a trench. They wanted to make a simple periscope so that they could look out of the ditch at the "enemy" without being in danger. They had an old stovepipe and a mirror. Practically all of them agreed that if the mirror were fixed in the top of the stovepipe and if they looked up through the bottom, they would be able to see over the side of the ditch. But they had an argument as to how the mirror should be placed. Each drew a diagram to show how he thought the mirror should be arranged, using dotted lines to show how the light would come from the enemy to their eyes. Three of the diagrams are shown in Figure 64.The boy who drew the first said: "If you want to see the enemy, the mirror's got to face him. Then it will reflect the light down to your eyes."

The boy who drew the first said: "If you want to see the enemy, the mirror's got to face him. Then it will reflect the light down to your eyes."

The boy who drew the second said: "No, the light would just go back to him again. The mirror must slant so that the light that strikes it at a slant will be reflected to your eye at the same slant.""How could it get to your eye at all," the third boy said, "if the mirror didn't face you? You've got to have the mirror reflect right down toward your face. Then all the light that strikes it will come down to you."Which arrangement would work?

The boy who drew the second said: "No, the light would just go back to him again. The mirror must slant so that the light that strikes it at a slant will be reflected to your eye at the same slant."

"How could it get to your eye at all," the third boy said, "if the mirror didn't face you? You've got to have the mirror reflect right down toward your face. Then all the light that strikes it will come down to you."

Which arrangement would work?

Explain the following:191. Your hands do not get wet when you put them into mercury.192. When beating hot candy, we sometimes put it in a pan of water.193. Electric stoves frequently have bright reflectors.194. We put ice in thetopof a refrigerator.195. You can jack up the back part of an automobile when you could not possibly lift it up.196. The sun shines up into your face and sunburns you when you are on the water.197. People in the tropics dress largely in white.198. Menthol rubbed into your skin makes it feel very cold afterward.199. We feel the heat of the sun almost as soon as the sun rises.200. You can shoot a stone far and hard with a sling shot.

Explain the following:

191. Your hands do not get wet when you put them into mercury.

192. When beating hot candy, we sometimes put it in a pan of water.

193. Electric stoves frequently have bright reflectors.

194. We put ice in thetopof a refrigerator.

195. You can jack up the back part of an automobile when you could not possibly lift it up.

196. The sun shines up into your face and sunburns you when you are on the water.

197. People in the tropics dress largely in white.

198. Menthol rubbed into your skin makes it feel very cold afterward.

199. We feel the heat of the sun almost as soon as the sun rises.

200. You can shoot a stone far and hard with a sling shot.

Section 23.The bending of light: Refraction.

How do glasses help your eyes?On a hot day, how is it that you see "heat waves" rising from the street?What makes the stars twinkle?

How do glasses help your eyes?

On a hot day, how is it that you see "heat waves" rising from the street?

What makes the stars twinkle?

Light usually travels in straight lines. If the light from an object comes from straight in front of you, you know that the object is straight in front of you. But you can bend light so that it seems to come from a different place, thus making things seem to be where they are not.

Experiment 44.Hold a triangular glass prism vertically (straight up and down) in front of one eye, closing the other eye. Look through the prism, turning it or your head around until you see a chair through it. Watch only the chair through the prism. When you are sure you know just where it is, try to sit down in it.Now look for a pencil or a piece of chalk through the prism, in the same way. When you think you know where it is, try to pick it up.

Now look for a pencil or a piece of chalk through the prism, in the same way. When you think you know where it is, try to pick it up.

The reason the chalk and chair seem to be where they are not is that the prism bends the light that comes from them and makes the light seem to come from somewhere else.

As you already know, when you look at a chair you see the light that reflects from it. You judge wherethe chair is by the direction from which the light is coming when it reaches your eye. But if the light is bent on its way, so that it comes to your eye as it ordinarily comes from an object off to one side, naturally you think the thing you are looking at is off to one side. Maybe the diagram (Fig. 65) will make this clearer.

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