The atmosphere is composed of two gases, oxygen and nitrogen, a small quantity of watery vapour, and a fraction of carbonic acid gas, or fixed air. Oxygen is the agent on which its more active properties depend. The other component, viz. nitrogen, serving to dilute it. They are united in the proportion of about seventy-seven of nitrogen by volume, and twenty-one of oxygen, the rest being made up of watery vapour, and carbonic acid gas. The atmosphere supports combustion,—oxygen gas being the essential agent. During combustion, it is consumed, and at the end of the process, it will be found wanting,—the other gas being undiminished. This may be seen, at least the diminution in the volume of air, by burning a candle in a large wide mouthed bottle, inverted over coloured water. As it continues to burn, the water ascends in the bottle, and occupies the place of the oxygen consumed. Atmospheric air supports respiration, a process essential to the continuance of life. Oxygen gas here, too, is the agent on which it depends. Air, which has been once respired, is found to be deprived of part of its oxygen, from ten to twelve per cent. Air, deprived of oxygen, or even deprived of a small portion of it, is unfit for respiration. A mouse, put into a vessel containing air, which has been robbed of that fluid, dies immediately. Put into one containing pure air, it breathes well at first, but, as the oxygen gets less, its breathing becomes laboured, it is convulsed, and shortly dies. The air is concerned, besides, in a thousand operations, constantly going on at the surface of the earth. It gives up a portion of its component parts in an immense number, and in a considerable proportion receives bodies foreign to its constitution. By one set of operations, it is deprived of its oxygen, and becomes vitiated by the admixture of deleterious principles. By others, again, its oxygen is restored, and the impurities removed; so that between two opposite forces, it is in general kept in a wholesome condition. An immense number of bodies on the surface of the earth, are constantly attracting to themselves the oxygen of the air; some become what is called oxydized, as the metals, the dull incrustation which is found upon them after long exposure to the air, being an oxide, or a combination of the metal, and the oxygen of the air. Some bodies become acids, as the various vegetable juices which form their respective acids, by combination with the oxygen of the atmosphere.