When we breathe we use our lungs like a pair of bellows.
7.

A man's lungs hold nearly one and a half gallons of air.
8.

In ordinary breathing we use less than a pint of air, but when necessary we can use much more.
9.

The air we breathe out contains carbonic-acid gas and another invisible poison.
10.

A candle will not burn in air which has been breathed, and animals die when confined in such air.
11.