this is written “10 volumes per cent.” To ascertain experimentally the tension of a particular gas in a particular fluid when dissolved to the amount of 10 volumes per cent. at the ordinary pressure of the atmosphere and at the temperature of the body, it would be necessary to place it in an open vessel in air containing a sufficient admixture of the gas to prevent its escape from the fluid. Suppose that it were found that, when the fluid containing the dissolved gas was placed in air mixed with the same gas to the extent of one-tenth of its volume, the fluid neither gave up gas nor absorbed more gas, the tension of the gas would be equal to one-tenth of an atmosphere. Since the pressure of the atmosphere equals 760 millimetres of mercury, the tension of the dissolved gas would be 76 millimetres. If more gas were added to the air, more would dissolve in the fluid; if some of the gas were removed from the air, gas would escape from the fluid. Gas passes from the medium in which its tension is high to the medium in which its tension is low. The tension of carbonic acid in tissues, particularly in muscles and glands, is higher than in lymph; in lymph higher than in blood; in blood higher than in air. Hence it passes by these several stages from the tissues in which it is formed to the air in the lungs. Much ingenuity has been devoted to perfecting methods for the determination of the tension of carbonic acid in lymph and in venous blood. Frequently results have been obtained which seemed opposed to the doctrine that carbonic acid progresses from one medium to another in accordance with the law of pressures; but such perplexing results were probably due either to imperfections in method or to the establishment of abnormal physiological conditions during the course of the observations. When, for example, it was found that the tension in lymph was less than the tension in blood, the specimen of lymph examined was probably not in the same condition as the lymph in the tissue-spaces where the exchange occurs. The experimenter in such a case was in error in supposing that the specimen of lymph which he examined contained as much carbonic acid as did the lymph in the tissue-spaces from which the blood which he compared with it received its supply of this gas. We have already given the figures for the composition of the air in the air-chambers of the lungs. The figures commonly accepted as correct for the percentages of the several gases in the blood are, at 0° C. and 760 millimetres of mercury pressure: Carbonic Oxygen. anhydride. Nitrogen. In 100 vol. of arterial blood 20 39 1-2 In 100 vol. of venous blood 8-12 46 1-2