ecessary 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.

SUMMARY.

1. Our bodies need air, just as a candle or a fire does.

2. A small animal shut up in a close jar soon dies for want of air. We
need the oxygen which the air contains.

3. Oxygen causes a sort of burning in our bodies.

4. The burning in our bodies keeps us warm, and destroys some of the
waste matters.

5. The breathing organs are the windpipe and bronchial tubes, the
voice-box, the epiglottis, the nostrils, the soft palate, the lungs, the
air-cells, the pleura, the diaphragm, and the chest walls.

6. 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. The lungs purify the blood. While passing through the lungs, the
color of the blood changes from purple to bright red.

12. Plants purify the air by removing the carbonic-acid gas.

13. Alcohol and tobacco injure the blood corpuscles so that they cannot
take up the oxygen from the air which the lungs receive.

CHAPTER XIV.

HOW TO KEEP THE LUNGS HEALTHY.

~1. Pure Air Necessary.~--A person may go without eating for a month, or
without drinking for several days, and still live; but a strong man will
die in a few moments if deprived of air. It is very important that we
breathe plenty of pure air. There are many ways in which the air becomes
impure.

~2. Bad Odors.~--Anything which rots or decays will in so doing produce
an unpleasant odor. Bad odors produced in this way are very harmful and
lik