breathe out the same
quantity. Our lungs hold, however, very much more than this amount. A
man, after he has taken a full breath, can breathe out a gallon of air,
or more than ten times the usual amount. After he has breathed out all
he can, there is still almost half a gallon of air in his lungs which he
cannot breathe out. So you see the lungs hold almost a gallon and a half
of air.

~24.~ Do you think you can tell why Nature has given us so much more
room in the lungs than we ordinarily use in breathing? If you will run
up and down stairs three or four times you will see why we need this
extra lung-room. It is because when we exercise vigorously the heart
works very much faster and beats harder, and we must breathe much faster
and fuller to enable the lungs to purify the blood as fast as the heart
pumps it into them.

~25. The Two Breaths.~--We have learned that the air which we breathe
out contains something which is not found in the air which we breathe
in. This is carbonic-acid gas. How many of you remember how we found
this out? We can also tell this in another way. If we put a candle down
in a wide jar it will burn for some time. If we breathe into the jar
first, however, the candle will go out as soon as we put it into the
jar. This shows that the air which we breathe out contains something
which will put a candle out. This is carbonic-acid gas, which is a
poison and will destroy life.

~26. Other Poisons.~--The air which we breathe out also contains other
invisible poisons which are very much worse than the carbonic-acid gas.
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
n