You see now one great way of spoiling the air. How are we to get rid of this bad air, and obtain fresh air, without being too cold?
In summer time this is quite simple, but in winter it is more difficult; because it is a very bad thing to be cold, and a thin, cold draught of air is especially bad.
The bad air loaded with carbonic acid gas, when we first breathe it out, is warm. Warm gases are much lighter than cold ones, therefore the bad air at first goes up to the ceiling.
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.