Air, as you will remember (p.

132), is a mixture of oxygen and nitrogen, and its value in the body is that it gives off part of its oxygen to combine with the body wastes and burn them to carbon dioxid.

Oddly enough, even pure outdoor air contains tiny traces of carbon dioxid; but the amount is so very small as to be of no practical importance, in spite of the fact that every kind of animal that lives and moves upon the earth is pouring it out from his lungs every second.

The rapidity with which it disappears is due in part to the rapidity with which it rises and spreads, or is blown, in every direction; and in part to the wonderful arrangement by which, while animals throw off this poisonous gas as waste, plants eagerly suck it in through the pores in their leaves and eat it, turning it into the carbohydrates, starch and sugar, which, in turn, become valuable foods for the animals.

So perfect is this system of escape, or blowing away, of carbon dioxid, combined with its being eaten up by plants, that even the air over our great cities and manufacturing towns contains only the merest trifle more of carbon dioxid than that over the open country.