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CHAP. II.] breaks up into gradually smaller and smaller tubes until the little blood-vessels are reduced to a size smaller than any hair, when they are called capillaries. These capillaries are fine tubes, with marvellously thin walls, and they are twisted round the tiny air-sacs, multitudes of which form the body of the lungs. Now, in the venous or dark blood in these hair-like vessels there is a quantity of carbonic acid and water, waste matters from the tissues, and in the air-sacs is the air that is taken in when we draw a breath, and the blood is only separated from the air by an exceedingly thin membrane. Hence an exchange easily takes place between the gaseous contents of the blood and those of the air by a process called osmosis, in which light and heavy gases change places. Air is composed of about twenty-one per cent, of oxygen, and the remainder of nitrogen with traces of other gases. Osmosis takes place, and the oxygen passes into the blood, while the carbonic acid passes into the air-sacs—the expired air losing about five per cent, of oxygen, the place of which is taken by about five per cent, of carbonic acid. The nitrogen is unchanged in the lungs; it serves to dilute the oxygen, for pure oxygen cannot be breathed with impunity. At the same time moisture also passes from the blood into the air-sacs, and, however dry the outside air may be, what we expire is always quite, or nearly, saturated with watery vapour; it contains, moreover, a certain amount of highly decomposable animal matter. The quantity of carbon which passes from the lungs