Maybe you will understand color better if it is explained in another way. Suppose I throw b.a.l.l.s of all colors to you, having trained you to keep all the b.a.l.l.s except the red ones. I throw you a blue ball; you keep it. I throw a red ball; you throw it back. I throw a green ball; you keep it. I throw a yellow ball; you keep it. I throw two b.a.l.l.s at once, yellow and red; you keep the yellow and throw back the red. I throw a blue and yellow ball at the same time; you keep both b.a.l.l.s.

Now suppose I change this a little. Instead of throwing b.a.l.l.s, I shall throw lights to you. You are trained always to throw red light back to me and always to keep (absorb) all other kinds of light. I throw a blue light; you keep it, and I get no light back. I throw a red light; you throw it back to me. I throw a green light; you keep it, and I get no light back. I throw a yellow light; you keep it, and I get no light back. I throw two lights at the same time, yellow and red; you keep the yellow and throw back only the red. But yellow and red together make orange; so when I throw an orange light, you throw back the red part of it and keep the yellow.

Now if we suppose that instead of throwing lights to _you_ I throw them to molecules of dye which are "trained" to throw back the red lights and keep all the other kinds (absorb them and change them to heat), we can understand what the dye in a red sweater does. The dye is not really trained, of course, but for a reason which we do not entirely understand, some kinds of dye always throw back (reflect) any red that is in the light that shines on them, but they keep all other kinds of light, changing them to heat. Other dyes or coloring matter always throw back any green that is in the light that shines on them, keeping the other colors. Blue coloring matter throws back only the blue part of the light, and so on through all the colors.

So if you throw a white light, which contains all the colors, on a "red" sweater, the dye in the sweater picks out the red part of the white light and throws that back to your eyes (reflects it to you) but it keeps the rest of the colors of the white light, changing them to heat; and since only the red part of the light is reflected to your eyes, that is the only part of it that you can see; so the sweater looks red. The "green" substance (chlorophyll) in gra.s.s acts in the same way; only it throws the green part of the sunlight back to your eyes, keeping the rest; so the part of the light that reaches you from the gra.s.s is the green light, and the gra.s.s looks green.

Anything white, like a piece of paper, reflects all the light that strikes it; so if all the colors (white light) strike it, all are reflected to your eyes and the object looks white.

You have looked at people under the mercury-vapor lights in photo-postal studios, have you not? The lights are long, inclined tubes which glow with a greenish-violet light. No matter how good the color of a person is in ordinary light, in that light it is ghastly.

[Ill.u.s.tration: FIG. 94. A mercury-vapor lamp.]

Go into the kitchen tonight, light a burner of the gas stove, turn out the light and sprinkle salt on the blue gas flame. The flame will leap up, yellow. Look at your hands, at some one"s lips, at a piece of red cloth, in this light. Does anything look red?

The reason why nothing looks pink or red in these two kinds of light is this: The light given by glowing salt vapor or mercury vapor has no red in it; if you tried to make a "rainbow" from it with a prism, you would find no red or orange color in it. A thing looks red when it absorbs all the parts of the light that are not red and reflects the red light to your eyes. If there is no red in the light to reflect, obviously a thing cannot look red in that light.

When you look through a piece of colored gla.s.s, the case is somewhat different. A piece of blue gla.s.s, for instance, acts as a sort of strainer. The coloring matter in it lets the blue light through it, but it holds back (absorbs) the other kinds of light. So if you look through a piece of blue gla.s.s you see everything blue; that is, only the blue part of the light from different objects can reach your eyes through this kind of gla.s.s. Anything that is transparent and colored acts in a similar way.

WHY THE SKY IS BLUE. And that is why the sky looks blue. Air holds back all colors of light except blue; that is, it holds them back a little. A room full of air holds the colors back hardly at all. A few miles of air hold them back more; mountains in the distance look bluish because only the blue light from them can reach you through the air. The hundred or more miles of air above you hold back a considerable amount of the other colors of light, letting through much more of blue than of any other color. So the sky looks blue; that is, when the air scatters the sunlight above you, it is chiefly the blue parts of the sunlight that it allows to reach your eyes.

WHY BODIES OF WATER LOOK GREEN IN SOME PLACES AND BLUE IN OTHERS.

Water acts in a similar way, but it lets the green light through instead of the blue. A little water holds back (absorbs) the other colors so slightly that you cannot notice the effect in a gla.s.s of water. But in a swimming tank full of water, or in a lake or an ocean, you can notice it decidedly when you look straight down into the water itself.

When you look at a smooth body of water at a slant on a clear day, the blue sky is reflected to you and the water looks blue instead of green. And it may even look blue when you look straight down in it if it is too deep for you to see the bottom and the sky is reflected from the surface.

WHY THE SKY IS OFTEN RED AT SUNSET. Dust lets more of red and yellow light through than of any other color, and for this reason there is much red and yellow in the sunset. Just before the sun sets, it shines through the low, dusty air. The dust filters out most of the light except the red and yellow. The red light and yellow light are reflected by the clouds (for the clouds are themselves "white"; that is, they reflect all the colors that strike them), and you have the beautiful sunset clouds. Sometimes there is a purple in the sunset, and even green. But since the air itself is blue (that is, it lets mostly blue light go through), it is easy to see how this blue can combine with the red or yellow that the dust lets through, to form purple or green.

But we could not have sunset colors or all the colors we see on earth, if it were not that the sunlight is mostly white--that it contains all colors. And that, too, is why we can have a rainbow.

HOW RAINBOWS ARE FORMED. You already know fairly well how a rainbow is formed, since you made an imitation of one with a prism. A rainbow appears in the sky when the sun shines through the rain; the plain white light of the sun is divided up into red, orange, yellow, green, blue, indigo, and violet. As the white light of the sun pa.s.ses through the raindrops, the violet part of the light is bent more than any of the rest, the indigo part is bent not quite so much, and so on to the red, which is bent least of all. So all the colors fan out from the single beam of white light and form a band of color, which we call the rainbow.

[Ill.u.s.tration: FIG. 95. Explain why the breakers are white and the sea green or blue.]

HOW WE CAN TELL WHAT THE SUN AND STARS ARE MADE OF. When a gas or vapor becomes hot enough to give off light (when it is incandescent), it does not give off white light but light of different colors. An experiment will let you see this for yourself.

EXPERIMENT 53. Sprinkle a little copper sulfate (bluestone) in the flame of a Bunsen burner. What color does it make the flame?

Copper vapor always gives this greenish-blue light when it is heated.

The photographer"s mercury-vapor light gave a greenish-violet glow.

When you burn salt or soda in a gas flame, you remember that you get a clear yellow light. By breaking up these lights, somewhat as you broke up the sunlight with the prism, chemists and astronomers can tell what kind of gas is glowing. The instrument they use to break up the light into its different colors is called a _spectroscope_, and the band of colors formed is called the _spectrum_. With the spectroscope they examine the light that comes from the sun and stars and by the colors of the spectra they can tell what these far-distant bodies are made of.

_APPLICATION 39._ If you were going to the tropics, would it be better to wear outside clothes that were white or black?

_APPLICATION 40._ A dancer was to dance in a spotlight on the stage. The light was to change colors constantly. She wanted her robe to reflect each color that was thrown on it. Should she have worn a robe of red, yellow, white, green, or blue?

_APPLICATION 41._ If you looked through a red gla.s.s at a purple flower (purple is red mixed with blue), would the flower look red, blue, purple, black, or white?

INFERENCE EXERCISE

Explain the following:

241. Mercury is separated from its ore by heating the ore so strongly that the mercury rises from it as a vapor.

242. Hothouses are built of gla.s.s.

243. A "rainbow" is sometimes seen in the spray of a garden hose.

244. Your feet become hot when your shoes are being polished.

245. Doors into offices usually have windows of ground gla.s.s or frosted gla.s.s.

246. Opera gla.s.ses are of value to those sitting at a distance from the stage.

247. In order to see clearly through opera gla.s.ses, you have to adjust them.

248. It is warm inside an Eskimo"s hut although it is built of ice and snow.

249. It is usually cooler on a lawn than on dry ground.

250. Black clothes are warmer in the sunlight than clothes of any other color.

CHAPTER SIX

SOUND

SECTION 28. _What sound is._

What makes a dictaphone or a phonograph repeat your words?

What makes the wind howl when it blows through the branches of trees?

Why can you hear an approaching train better if you put your ear to the rail?

If you were to land on the moon tonight, and had with you a tank containing a supply of air which you could breathe (for there is no air to speak of on the moon), you might _try_ to speak. But you would find that you had lost your voice completely. You could not say a word. You would open and close your mouth and not a sound would come.

Then you might try to make a noise by clapping your hands; but that would not work. You could not make a sound. "Am I deaf and dumb?" you might wonder.

The whole trouble would lie in the fact that the moon has practically no air. And sound is usually a kind of motion of the air,--hundreds of quick, sharp puffs in a second, so close together that we cannot feel them with anything less sensitive than the tiny nerves in our ears.

If you can once realize the fact that sound is a series of quick, sharp puffs of air, or to use a more scientific term, _vibrations_ of air, it will be easy for you to understand most of the laws of sound.

A phonograph seems almost miraculous. Yet it works on an exceedingly simple principle. When you talk, the breath pa.s.sing out of your throat makes the vocal cords vibrate. These and your tongue and lips make the air in front of you vibrate.