These poisons make the air of a crowded or unventilated room smell very
unpleasant to one who has just come in from the fresh air. Such air is
unfit to breathe.

~27. The Lungs Purify the Blood.~--We have learned that the blood
becomes dark in its journey through the body. This is because it loses
its oxygen and receives carbonic-acid gas. While passing through the
capillaries of the lungs, the blood gives out the carbonic-acid gas
which it has gathered up in the tissues, and takes up a new supply of
oxygen, which restores its scarlet hue.

~28. How the Air is Purified.~--Perhaps it occurs to you that with so
many people and animals breathing all the while, the air would after a
time become so filled with carbonic-acid gas that it would be unfit to
breathe. This is prevented by a wonderful arrangement of Nature. The
carbonic-acid gas which is so poisonous to us is one of the most
necessary foods for plants. Plants take in carbonic-acid gas through
their leaves, and send the oxygen back into the air ready for us to use
again.

~29.~ We have already learned that the oxygen taken in by the lungs is
carried to the various parts of the body by the little blood corpuscles.
The effect of strong liquors is to injure these corpuscles so that they
cannot carry so much oxygen as they ought to do. For this reason, the
blood of a drunkard is darker in color than that of a temperate person,
and contains more carbonic-acid gas. The drunkard's lungs may supply all
the air he needs, but his blood has been so damaged that he cannot use
it. Excessive smoking has a similar effect.