RESPIRATION 185 



carbonic acid are held by the blood in chemical combina- 

 tion. 



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, accord- 

 ing 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, dis- 

 tinguishable by a difference of colour and a fundamental dif- 

 ference 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 absorp- 

 tion 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 thes esolutions Stokes replaced the tissues. He abstracted 

 the oxygen from the oxyhaemoglobin ; then, shaking the 

 solution of reduced haemoglobin with air, he reproduced the 

 action which occurs in the lungs. 



