2. Carbonic acid in respired air is always increased; but the quantity exhaled in a given time is subject to change from various circumstances. From every volume of air inspired about 4 1/2 per cent of oxygen is abstracted; while rather a smaller quantity of carbonic acid is added in its place. Under ordinary circumstances, the quantity of carbonic acid exhaled into the air breathed by a healthy adult man amounts to 1,346 inches, or about 636 grains, per hour. It is estimated that the weight of carbon excreted from the lungs is about 173 grains per hour, or rather more than 8 ounces in 24 hours. Of course the influence of age, sex, respiratory movements, external temperature, season of the year, purity of the respired air, hygrometric state of the atmosphere, period of day, food and drink, exercise and sleep, have to be taken in consideration. The oxygen of respired air is always less than in the same air before respiration, and its diminution is generally proportionate to the increase of the carbonic acid. It has been shown that for every volume of carbonic acid exhaled into the air 1.17421 volumes of oxygen are absorbed from it; and that when the average quantity of carbonic acid, i.e., 1,346 cubic inches, or 636 grains, is exhaled in the hour, the quantity of oxygen absorbed in the same time is 1,584 cubic inches, or 542 grains. The nitrogen in the atmosphere, in relation to the respiratory process is supposed to serve only mechanically, by diluting the oxygen, and moderating the action upon the system. The most obvious change which the blood undergoes in its passage through the lungs is that of color, the dark venous blood being exchanged for the bright scarlet arterial blood. It gains oxygen, loses carbonic acid, becomes 1° to 2° F. warmer; it coagulates sooner and more firmly, and contains more fibrine.