106 



THE NATURALIST. 



is carried back to the heart, it has acquired a scarlet tint, and is distin- 

 guished as arterial blood. In spite of the number of researches made on 

 the subject, great doubt has always existed as to the precise nature of the 

 change which is effected in the lungs. This much is certainly known, that in 

 some way or other oxygen is absorbed by the blood, that this oxygen combines 

 with carbon and hydrogen, and that this combination produces carbonic acid 

 and water, both of which are thrown into the atmosphere by the act of expi- 

 ration. Moreover, it is certain that the heat of the body is entirely due to 

 this constant oxidation, which is therefore exactly analogous to the combus- 

 tion of a lamp or candle. But how is the oxidation effected ] Does the 

 oxygen combine directly with the carbon and hydrogen as soon as it comes in 

 contact with them, so that in fact, the whole combustion is performed in the 

 lungs j or is the oxygen first dissolved by the blood, which it is constantly 

 oxidizing in all parts of its course % The first view has been abandoned for 

 years, in consequence of one fundamental objection to it. If the whole of 

 the combustion took place in the lungs, it is evident that the lungs and heart 

 ought to be materially higher in temperature than any other part of the body. 

 But this is not found to be the case. There is but a trifling difference in 

 temperature between the heart and the most remote vessels of the vascular 

 system, so that the probability seems to favour the idea of a continuous 

 oxidation in all parts of the body. 



Our readers will now be in a position to understand the exact bearing 

 of Professor Stoke's discovery. They will perceive that the gradual altera 

 tion of colour from bright scarlet to dark purple which blood undergoes as i 

 flows through the vessels is attended by a constant loss of oxygen, whic 

 oxygen is employed in the formation of carbonic acid and water. Thi 

 appeared to indicate the existence of two varieties of cruorine, a scarlet an 

 a purple kind, the latter containing less oxygen than the former. The fol 

 lowing experiment demonstrated the truth of this theory. A little clea 

 solution of scarlet blood was placed in a tube, and the two lines in its spec 

 trum observed. A liquor had been previously prepared by adding tartari 

 acid and caustic potash to a solution of protosulphate of iron (green vitriol. 

 Such a liquid has a pale sea-green tint, has no perceptible effect on the spec' 

 trum, and, above all, has a most powerful aflinity for oxygen, which it wi 

 absorb rapidly from the air, if exposed to it. A little of this solution wa 

 now added to the blood, and the result was that its scarlet colour disappeare 

 almost immediately, and a purple tint just like that of venous blood succeeded 

 it. It was self-evident that the scarlet cruorine had given up oxygen to the 



