It has parted with about one-sixth of its oxygen, and taken up an equivalent of carbonic acid. And were the same air to be breathed six times successively, it would have parted with all its oxygen, and could no longer sustain life.
"A prudent man forseeth the evil, and hideth himself; but the simple pass on, and are punished."—Proverbs xxvii.
14. Is the impure air sent out of the lungs lighter or heavier than common air?
At first, being rarefied by warmth, it is lighter. But, if undisturbed, it would become heavier as it cooled, and would descend.
15. Why is it proper to have beds raised about two feet from the ground?
Because at night, the bed-room being closed, the breath of the sleeper impregnates the air of the room with carbonic acid gas, which, descending, lies in its greatest density near to the floor.
16. What are the chief sources of carbonic acid gas?
The vegetable kingdom (as will be hereafter explained), the combustion of substances composed chiefly of carbon, the breathing of animals, and the decomposition of carbonic compounds.