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 solutions of ferrous sulphate or of stannous chloride containing some citric or tartaric acid. These sub-salts of iron and tin very rapidly absorb oxygen from the air or from any chemical substance which parts with it readily. With these solutions Stokes replaced the tissues. He abstracted the oxygen from the oxyhæmoglobin; then, shaking the solution of reduced hæmoglobin with air, he reproduced the action which occurs in the lungs.