=Exchange of Gases in the Lungs.=—In the lungs each red corpuscle takes from the air a charge of oxygen which it carries to the tissues. In the tissues the plasma of the blood receives carbonic acid, which escapes from it when it reaches the lungs. Water dissolves oxygen and carbonic acid. Towards animals and plants which live in it, water plays the same rôle as the atmosphere towards dwellers on land. The quantity of a gas which will dissolve in water is proportional to the pressure to which it is subjected. If water were the circulating fluid, some oxygen would enter it in the lungs; some carbonic acid would be taken up in the tissues and liberated in the lungs. But it is clear that the small quantity of fluid which the vascular system will hold would be incapable of serving as an efficient medium of exchange between the tissues and the lungs. When a given quantity of venous blood is agitated with air, five times as much oxygen is taken up as the blood could carry if the gas were simply dissolved. Both oxygen and carbonic acid are held by the blood in chemical combination. The condition in which oxygen is carried was discovered in 1864 (_cf._ p. 68). From all time it had been noticed that the blood which flows from a vein is darker and of a more purple tint than the blood which spurts out of a cut artery. Shortly before the date mentioned above, the spectroscope had begun to be used to distinguish more accurately than the eye can do the groups of rays which a coloured solution transmits. The colour of a ray of light depends upon its wave-length. The light of the sun, when its rays are sorted by a prism, according to their wave-lengths, shows all colours from the long waves of red to the short rays of violet, with certain gaps. At intervals where rays are missing, the spectrum exhibits dark bands—Fraunhofer’s lines. The colour of a solution is measured by placing a flat-sided vessel containing it in the course of a beam of the sun’s light, on its way to a prism. When the rays are spread out, it is observed that certain groups have been absorbed by the coloured fluid. The colour of the solution is due to the rays which it transmits. It had been pointed out in 1862 that blood diluted with water absorbs parts of each end of the spectrum, and also two groups of rays lying between the fixed bands of Fraunhofer which spectroscopists had labelled D and E. Stokes observed that this is true only of arterial blood. Venous blood absorbs a broad band in this part of the spectrum in place of the two narrow bands. He showed that, “like indigo, it is capable of existing in two states of oxidation, distinguishable by a difference of colour and a fundamental difference in the action on the spectrum. It may be made to pass from the more to the less oxidized condition by the action of suitable reducing agents, and recovers its oxygen by absorption from the air.” The reducing agents of which Stokes made use were alkaline