The air contains about one part in five of oxygen, and, as has been seen, this oxygen is liable to continual destruction by the breathing of animals and the burning of fuel and of lights. An ordinary man spoils in twenty-four hours 720 cubic feet of air, that is, a mass of air 11 feet 6 inches square and 6 feet thick. The burning of three ounces of charcoal, or of a mould candle of six to the pound, produces the same effect. It is not unusual in a factory to burn ten tons of coal a-day, which spoils 3,185,760 cubic feet of air, a mass of a quarter of a mile square and six feet thick. If we multiply these numbers by the number of inhabitants, of man and of all other animals upon the earth, and also by the quantity of fuel burned all over the globe, it will be evident that without some regulating power superior to all that mere human means could devise, the air might ultimately become unfit to be the sustenance of living beings, and all the numerous tribes of animated nature which now adorn its surface, would be destroyed. By the all-wise arrangement of Providence, however, the animals, in thus converting the oxygen of the air into carbonic acid, become the means of supplying nourishment to another class of beings equally interesting and numerous. All vegetables breathe; but as animals take in too much carbon with their solid food, so do plants obtain too little from the substances that give nourishment to their roots. The animal breathes to give off carbon, the vegetable breathes to take it up. The two great divisions of living nature thus act in contrary ways upon the air; the oxygen consumed by the animal or by combustion, is given out again by the carbon of the carbonic acid becoming fixed in the plant of which it forms the woody mass; and thus the composition of the air is kept balanced at its proper point, and provision for the due nutrition of animals and vegetables is secured. The air we breathe serves, however, for other important uses. Without the air, the fresh breezes which moderate the heats of summer could not exist, and there would prevail in nature an eternal silence, for it is by means of air that we not only breathe, but hear. The variety of aspect given to the sky by the formation and rapid change of clouds, arises from the mixture of warm and of cold damp air. If there was no air, there might be dew, but there could never be a cloud.