434. When the same portion of atmospheric air is repeatedly respired by an animal, the oxygen contained in it gradually disappears, the gas lessening with every successive respiration, until at last so small a quantity remains that it is no longer capable of sustaining the life of an animal of that class. When respiration has deprived the air of its oxygen to such an extent, that it can no longer support animal life, the air is said to be consumed; but, correctly speaking, it is merely changed in composition, in the proportions in which its constituents are combined; consequently the effect of respiration is to alter the chemical composition of the air. 435. The essential change that takes place consists in the diminution of the oxygen and the increase of the carbonic acid. When inspired, atmospheric air goes to the lungs loaded with oxygen; when expired, it returns loaded with carbonic acid. That the air which returns from the lungs is loaded with carbonic acid, may be rendered manifest even to the eye. If a person breathe through a tube into water holding lime in solution, the carbonic acid contained in the expired air will unite with the lime and form a white powder analogous to chalk (carbonate of lime), which being insoluble, becomes visible. 436. On the other hand, the diminution of oxygen is demonstrated by chemical analysis. If 100 parts of atmospheric air be successively respired, until it is no longer capable of supporting life, and if it be then subjected to analysis, it is found that in place of being composed of 79 parts azote, 21 oxygen, and a variable quantity of carbonic acid, sometimes amounting to half a grain per cent., it consists of 77 parts azote, and 23 carbonic acid. The oxygen is gone, and is replaced by 23 parts of carbonic acid; at least this is the ordinary estimate; but different experimentalists differ somewhat in their account of the absolute quantity of oxygen that disappears, and of carbonic acid that is generated.