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.
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