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 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.

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.