The air is transparent, or clear, like glass; that is, it lets the light come through it to your eyes. Sometimes glass is not clear, and you can not see things plainly through it. So, also, the air is sometimes not clear, as when there is dust flying in it, or when there is a fog.
Though you can not see air, you can see what it does when it is in motion. You can see it move the trees and other things. This I will tell you about in the next chapter.
The air is a thing which is necessary to our life. If it be shut out in any way from our lungs, great distress is immediately produced; and if it be shut out only for a few minutes, death occurs. I have told you in Part II., in the chapter on breathing, why it is that breathing air is so necessary to life.
Air necessary to life.
Air is as necessary to the life of plants as it is to the life of animals. In animals the air is used by lungs, but in plants it is used by the leaves. This I have told you about in the chapter on the uses of leaves, in Part I.
Nothing can burn without air.
Air is needed for another thing. Nothing can burn without air. It is the air that makes wood, and coal, and oil, and gas burn when fire is put to them.
The air that is all around the earth does not reach to the sun, and moon, and stars. It extends about forty-five miles above the earth. Beyond this there is no air. You will want to know how this was found out, as no one has ever been so far from the earth. I will not explain this to you now, for you are not old enough to understand it.
Questions.—What is a room full of when the furniture is all taken out? Tell about the box of books and about the foot-ball. What is said about life-preservers? Why does a boat float on the water? How are life-boats made? Can you see air? Can you feel it when it is still? What is wind? What is said about the transparency of air? What is said about its being necessary to the life of animals? What about its being necessary to the life of plants? What else is air needed for? How high does the air extend?