The second is a very slow, dull gas, called nitrogen; and nothing will burn in it at all. Pure oxygen would be too active for us to live in, so it is mixed with nitrogen.
When we breathe, the air goes down into our lungs, which are something like sponges, inside our chests.
These sponges have in them an immense quantity of little blood-vessels, and great numbers of little air-vessels; so that the blood almost touches the air; there is only a very, very thin skin between them.
Through that skin, the blood sends away the waste and useless things it has collected from all parts of the body, and takes in the fresh oxygen which the body wants.
You have often heard man's life compared to a candle. I will show you some ways in which they are much alike.
When a candle or lamp burns, if we keep it from getting any new air, it soon uses all the lively gas, or oxygen, and then it goes out. This is easily shown by placing a glass jar over a lighted candle.
If the candle gets only a little fresh air, it burns dim and weak. If we get only a little fresh air, we are sickly and weak.
The candle makes another kind of gas. It is called carbonic acid gas, which, is unhealthy and not fit for breathing. The heat of our bodies also makes this gas, and we throw it off in our breath.
Oxygen and carbon, in a separate condition, make up a good part of our flesh, blood, and bones; but when they are joined together, and make carbonic acid gas, they are of no further use to us.
You might go to a store and buy sand and sugar; but if they became mixed together as you brought them home, you would not be able to use either one of them, unless some clever fairy could pick them apart for you.