The office of the air is to support animal life: no animal can live without air: even fishes require air. The water in which they swim contains air mixed with it, and this water washing the gills, which are their lungs, serves to them as the air directly acts on us. If we boil water until the air is expelled from it, and let it cool in a close vessel, we may drown a fish by putting it into such water, as easily as a land animal; it could not breathe. It is thus that in the lakes on the tops of very high mountains there are no fish. The heights are deserted by land and by water animals, in consequence of the air being too thin to support life. The way in which the air acts upon the body is very interesting. The most abundant element of our food is what the chemists term carbon, of which, in a gross manner, charcoal may serve as an example. Now, we eat much more of this than we require for the supply of our bodies, and it must be got rid of. This is done by its uniting in the body with a substance termed oxygen, and forming carbonic acid, the sort of air which boils up in soda water and ginger beer. This dissolves in the blood, colouring it a deep purple, and escapes from it when by the action of the heart the black blood is exposed to the action of the air on the surface of the lungs. Now, the office of the air is to supply this oxygen which removes the carbon from the blood. But the air is not pure oxygen. If it were, it would act too violently. An animal which breathes pure oxygen, becomes flushed, pants violently, and, if not choked, dies of inflammation of the lungs, produced by the intense action. In the air we breathe, the oxygen gas is diluted to the proper degree by another gas, termed nitrogen, which is totally destitute of power; it does of itself no good and no harm; it is the only substance that could be mixed in the air we breathe, without interfering in any way. When thus the blood loses, by exposure to the air in the lungs, its carbonic acid, it takes oxygen in its place; from dark purple it becomes bright red, and is then proper to take up a fresh quantity of carbon, and to sustain the body in health by its removal. When any thing burns in the air, it is the oxygen which is active. The nitrogen dilutes here also the oxygen, and keeps its activity down to the degree most suitable to our wants. If the air were pure oxygen, all our domestic fires would be violent conflagrations; our iron pokers and tongs, if heated red hot, would take fire and burn like squibs; no comfort, no safety for society could exist. But in burning, this oxygen is destroyed. If a candle be placed lighted under a glass bell, it will, after a little, go out. The air will become unfit to support combustion. Here also, as well as in the burning of coals, coke, gas, oil, charcoal, &c. the oxygen is changed into carbonic acid, and precisely as a fresh supply of oxygen is necessary for the continuance of life, so is it for combustion.