Now as we breathe _out_, we send into the air the tiny atoms of this waste stuff. It is carbonic acid gas. As we breathe _in_, we take from the fresh air a gas called oxygen. That goes to our lungs, and lo! it makes the blood fresh and clean, and red once more. So you can see, that when many people breathe in one room they will use up all the good clean air. At the same time they will load the air of the room with the gas they breathe out. That is why the window is opened. We wish to sweep away the bad air, and let in good air. But at this rate, as all men and other animals breathe out carbonic acid gas, why does not all the air in the world get bad? Why, when they all use oxygen, do they not use up all the oxygen that is in the world? Just here the plants come in to help. Carbonic acid gas is bad for men, it is food to plants. Oxygen is needed by animals, but plants want to get rid of it. Animals breathe out a form of carbon and breathe in oxygen. Plants do just the other thing. They breathe out oxygen and take in carbonic acid gas. The air, loaded with this, comes to the plant. At once all the little leaf-mouths are wide open to snatch out of the air the carbonic acid gas. And, as the plants are very honest little things, they give where they take away. They take carbon from the air, and breathe into the air a little oxygen. Where did they get that? The air they breathe has both carbon and oxygen in it. So they keep what they want,--that is, carbon,--and send out the oxygen. Now it is only the green part of the plant that does this fine work for us. It is the green parts, chiefly the leaves, that send out good oxygen for us to breathe. It is the green leaf that snatches from the air those gases which would hurt us. It is the green leaf that changes the harmful form of carbon into good plant stuff, which is fit for our food. How does it do that? Let us see. What makes a leaf green? Bobby who crushed a leaf to see, told me “a leaf was full of green paint.” Inside the green leaves is a kind of green paste, or jelly. Now it is this “leaf-green” that does all the work. The “leaf-green” eats up carbon. The “leaf-green” turns carbon into nice safe plant material. It is “leaf-green” that sets free good oxygen for us. “Leaf-green” is a good fairy, living in every little cell in the leaf. Leaf-green is a fairy which works only in the day-time. Leaf-green likes the sun. Leaf-green will not work in the dark, but goes to bed and goes to sleep! In such simple lessons as these, I can tell you only a little of what _is_. The deep “how” and “why” of things I cannot explain. Even the very wisest men do not know all the how and the why of the “leaf-green” fairy. I have told you these few things that you may have wonders to think of when you see green leaves. After this lesson, will you not care more for seeds and leaves than you ever did before? LESSON VIII. THE COLOR OF PLANTS.