CHAPTER II AIR 8. What is Air Like? - One of the hard things for man to understand was the air. It fanned his cheek, turned his windmills, and drove his ships, but he could not find out much about it until a little over a hundred years ago. We can see why this was true. Air is invisible to us, except when we see its quivering motion over a hot stove or on a hot day in summer. What man could not see, he found it hard to handle and to understand. Thus, when he used up some of the air, he did not know that he had done so, because more air rushed in from all sides to fill the empty space. Observe how the surrounding water rushes in when we dip some of it out of a tub or pail; air would rush in in the same way. When men grew used to this idea, they did not try to study all the air at once, but they took vessels of air, such as tanks or bottles, stoppered them in some way, and then studied this enclosed portion of air by itself. In this way they learned a great deal more about the air. 9. Does Air Take Up Room? Have you ever thought, as you filled a glass with water from a pitcher or faucet, whether anything was flowing out of the glass while the water flowed in? Have you ever poured water out of a small-mouth bottle? Try it and see whether anything happens which shows that something goes into the bottle as water flows out. Suppose we push a glass, held upside down, into a deep dish of water; does the water rise up to fill the glass? Why? Let us perform the following experiment (Fig. 3): Put a funnel stem loosely into a small-mouth bottle, such as a ketchup or vinegar bottle, and quickly fill the funnel with water. Does the water run in as a stream, or by spurts? Now fit the funnel stem tightly into the mouth of the bottle. You can use a one-hole stopper for this purpose, or you can wind around the stem a strip of wet muslin, about an inch wide, until Will the water the joint is tight. Finally, fill the funnel rapidly with How does the water run in now? Why? Carry out another experiment (Fig. 4): FIG. 3. run into this bottle in a stream, or by spurts? water. Fill a bottle with water and close the mouth of the bottle tightly with your hand; then turn the bottle upside down in a pan of water. If the mouth of the bottle is under "water, you can remove your hand without the water falling out. If you then blow air into the bottle through a bent tube, you can collect the air in the bottle; for as the air bubbles rise into the bottle, they push the water out. In this way we can collect a gas 66 over water." From these experiments FIG. 4. Air from Lungs Collecting the air of your breath "over water." we can see that air takes up room, or occupies space, just as water does. If we want to pour water into a |