Sometimes air rivers flow up and down so rapidly, pushing each other out of place that they make—what? See the leaves blowing on the trees. Wind! Wind is air that is in a hurry. You can feel it. Draw a quick breath. You felt a tiny wind in your nose, didn't you? You cannot feel air, but you can feel wind, or air in motion. You can feel the temperature of air, too. Your skin tells you if air is hot or cold. You can feel if it is damp or dry. You cannot smell air, but you can smell odors in it—the perfume of flowers, the freshness of rain, bad odors of decay, or smoke. You can train yourself to tell if the air in a room is fresh or stale. Doctors always come into a sick room with suspicious noses in the air. As we have to pay doctors for telling us when the air is bad, let us see if we can find out for ourselves.

When you were up on the step ladder, Robert, did you notice anything beside the heat? You felt smothered, then dizzy? You don't feel that way on the hottest summer day out of doors, do you? Let us see what was the matter with that air.

Put half an inch of water in a pie pan. Twist a bit of soft newspaper, light it with a match and drop it into a drinking glass. Let the glass fill with smoke, but while the paper is still burning turn the glass upside down in the pan of water. The flame goes out, leaving some paper unburned. The water rises in the glass, much higher than in the pan outside, and stays there. Something was burned out of the air, making room for the water to rise. The part that is gone is oxygen. If you had shut a live fly in the glass it would have died as quickly as the flame. Animals and fire cannot live without oxygen. By breathing air in you burn up oxygen. It would not take you very long to use up all the oxygen in a small room. Then, if you couldn't get any fresh air at all you would "go out" like the burning paper, and the fly in the glass.

Beside using up the oxygen in the air, when you breathe in, you make a poisonous gas when you breathe out. Fire does the same thing. Here is a bottle of clear lime water. It cost two cents at the drug store. Divide this lime water between two small fruit jars. In one of the jars hang an inch of lighted candle above the lime water, by a wire twisted around the candle and hooked over the top. Cover the top of the jar with a folded napkin. The flame goes out as soon as the oxygen in the jar is used up, of course. But something else happens, too. The clear lime water turns milky. The flame gave out a gas called carbon dioxide, or carbonic acid gas.

Put a tube of rubber or glass, or a big lemonade straw, into the lime water in the other jar. Blow your breath through the tube, into the water. You can tell when you have forced your breath in, for you make bubbles. Do this several times. This water turns milky, too, from the carbonic acid gas in your breath. Your breath is warm so it goes up to the ceiling. The air made by a gas flame, a heat register or radiator goes up, too. All this warm air has been used by fire and by people. The oxygen has been burned out of it, and carbonic acid has put into it. It would not feed a fire or a pair of lungs, and it becomes even poisonous. This is bad air. It will go out of doors if you help it; and be purified.

Beside helping to keep people well, to know about air may help them at any time to put out a fire. Fire cannot burn without oxygen, so it can be smothered. If your clothing catches fire roll up in a rug or heavy bed clothes. Keep the fire from getting air and it will go out.

Out of doors, nature is always purifying air. She does it by having plants and animals live together. Plants make oxygen and use up the carbonic acid gas that is poisonous to breathing animals. In the sea fishes and water plants help each other, in the same way. They get air out of the water. Seeds and worms and beetles get air out of the earth. If you plant seeds too deep, or pack the earth too hard above them, they will rot. So you must read the directions on your little paper of flower seeds and obey them, or you will have no flowers. You can kill little animals by stopping up their holes sometimes.

See how wonderfully this world is mixed up. Earth is solid, water is fluid, air a gas. But there is water and air in the earth, earth and air in the water, and earth dust and water vapor in the air. They all need each other, and plants and animals need all three. Air is the freest and sets everything else in motion.

Wind, or air in motion, is a great worker and wonder-worker. It tosses the tree tops, whirls the dust, carries vapor clouds to make rain, scatters seeds. It turns the long arms of windmills and sends sailing vessels flying over the water. It waits for nobody. It says how-do-you-do and good-by, and is gone. There isn't a bit of use to get out of humor with it, if it blows our hats off and turns umbrellas wrong side out. The wind can't help blowing. It is being pushed and jostled about itself. Besides it has the most important work in the world to do—keeping air in motion and purifying it. So get off the track. Wind has the right of way. See Air, page 33 ; Respiration, 1602.