_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 glass 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 glass. 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 glass or frosted glass. 246. Opera glasses are of value to those sitting at a distance from the stage. 247. In order to see clearly through opera glasses, 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 passing out of your throat makes the vocal cords vibrate. These and your tongue and lips make the air in front of you vibrate.