If there is an opening near the top of the room, the bad air goes out; but if there is no opening, it by and by grows cold and heavy, and comes down again.  Then we have to breathe it.

If you open the window at the top, it will let out the bad air, and you will not feel a draught.  It is not often so very cold that you cannot bear the window open, even a little way from the top, and that is the best way of airing a room.

This is just as necessary by night as by day.  People who shut in the bad air, and shut out the good air, all night long, can never expect to awake refreshed, feeling better for their sleep.

What becomes of the carbonic acid gas which the body throws off through our breath?  Can any thing pick the carbon and oxygen in it apart, and make them fit for us to use again?

Yes.  Every plant, every green leaf, every blade of grass, does that for us.  When the sun shines on them, they pick the carbon out and send back the oxygen for us to breathe.  They keep the carbon and make that fit for us and animals to eat.

The grass makes the carbon fit for sheep and cows, and then we eat their flesh or drink their milk; and the corn makes the carbon fit to eat; so do potatoes, and all the other vegetables and fruits which we eat.  Is not this a wonderful arrangement?

But perhaps you think, considering what an amazing number of people there are in the world, besides all the animals—­for all creatures that breathe, spoil the air just as we do—­there can hardly be trees and plants enough to set all the air right again.

Round about cities and large towns there are certainly more people than there are trees, but in many other parts of the world there are a great many more trees than there are people.

I have heard of forests in South America so thick and so large, that the monkeys might run along the tops of the trees for a hundred miles.  So you see there are plenty of trees in the world to do the work.

But then, how does all the bad air leave the towns and cities where men live, and get to the forests and meadows?

The air is constantly moving about; rising and falling, sweeping this way or that way, and traveling from place to place.

Not only the little particles out of our breath, but any thing that gives the air any smell, does it some harm.  Even nice smells, like those of roses, are unhealthy, if shut up in a room for some time.

Dirty walls, ceilings, and floors give the air a musty, close, smell; so do dirty clothes, muddy boots, cooking, and washing.  Some of these ought not to be in the house at all; others remind us to open our windows wide.

All the things I have been saying to you about pure air, apply still more to sick people than to healthy ones.

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Directions for Reading.—­Read the following sentences carefully, and avoid running the words together.