MOTHER: It is a pretty pink color and seems much like a very fine sponge. If we could go inside we should find the passages divided again and again, till there are thousands and thousands of tiny air tubes, each ending in a little pouch quite like a bunch of grapes, only you should think of the grapes as being as small as a grain of sand. When the lungs are full of air, they grow larger, and when we breathe it out, they grow small. ELMER: That is like a pair of bellows. [Illustration: _A pair of bellows._] MOTHER: Very much the same, and the bellows will help us understand how we breathe. Try to think of a little tree with its trunk, limbs, and leaves all hollow. If air were blown through the trunk, it would make every leaf puff out, and when no air was blown in, they would fall together again. It is the same with our lungs. They keep swelling out and falling together about eighteen times every minute. AMY: But how is the blood washed in air, mother? MOTHER: Perhaps it would be better to say it is aired, the same as we hang a garment in the sunshine and wind to make it fresh and sweet. You will remember that the blood takes oxygen, which is a part of the air, to every part of the body-house, and this makes it warm. In exchange the muscles give the blood a poison called carbonic acid gas. This gives the blood a dark, purplish color, and it must carry away the gas and get more oxygen before it can do any more work in mending the body. PERCY: But I would like to know how it gets into the bath room. MOTHER: The right side of the heart, which has nothing but soiled blood in it all the time, sends it to the lungs in a hurry, and it fills the thousands of hair-like veins which are in every part of the lungs. The walls of the veins are so thin that the oxygen in the lungs soaks through into the blood, and the poison in the blood goes through into the air, and is breathed out of the body. Do you understand it now? PERCY: I think so. MOTHER: If I should tie a piece of bladder over a glass of milk and place the glass in a bucket of water, the milk would come through into the water, and the water would pass into the milk, even though they were in separate dishes. Another way to show how the blood is cleansed would be to say that blood and air keep running near together, each in its own room, and as they pass they say, “Good-day;” air washes blood so it becomes bright and clean, and blood makes air very dirty with its poison gas; and, after trading in this way, both hurry along as fast as they came in. ELMER: It must be that good air is needed more than good food. MOTHER: Why, yes; for while we need to eat only two or three times a day, we must take in air more than twenty-five thousand times. If we could not breathe for six or seven minutes, we would die, while we could live without food quite a number of days. How thankful we ought to be for pure, fresh air! And there is so much of it that we can have it without money and without price.