11. Why do we need to breathe at all?
Because the air contains a gas called oxygen, and a constant supply of this gas must be taken into the blood, or else we cannot remain in good health. When we breathe, the air passes down into the lungs and there meets with the blood.
12. And as the oxygen gas passes inwards, another gas, which has to be got rid of, passes outwards and is breathed out into the air. Thus the air we breathe out is different from the air we breathe in. It has lost the gas which is necessary for our life and health, and it contains a gas which is hurtful to us.
13. If I live in a room that does not get fresh air, the air in it will soon become close and bad, because every time I breathe I take some of the oxygen out of it and put injurious gas in its place.
14. If a small animal, such as a bird or a rabbit, is put under a glass bell so that no air can get in, it uses up all the oxygen. In a few minutes it becomes faint; it is unable to stand up, and unless it gets fresh air it will soon die.
15. You may have heard the story of the Black Hole of Calcutta. One hundred and forty-six English prisoners were shut up in a small cell. They could not get enough fresh air to breathe, and in the morning one hundred and twenty-three of them were found dead.
III
16. The third health lesson I have to learn is this: I must take plenty of exercise. To make the body strong we must use it. The parts that are most used become the strongest, and those used least will be the weakest.
17. The arms of the blacksmith are very strong because he uses them so much. The boy who works and plays in the open air grows strong and healthy, but the boy who sits indoors and does not take exercise often grows up to be a weak and unhealthy man. It is best to take our exercise in the open air and the sunlight.