398 Royal Society : — 



substances, can be better judged of by the tint than by the use of 

 the prism. With the prism the extreme sharpness of the bands of 

 scarlet cruorine is apt to mislead, and to induce the observer greatly 

 to exaggerate the relative proportion of that substance. 



Seeing then that the change of colour from arterial to venous 

 blood as fa?' as it goes is in the direction of the change from scarlet 

 to purple cruorine, that scarlet cruorine is capable of reduction even 

 in the cold by substances present in the blood (§ 9), and that the 

 action of reducing agents upon it is greatly assisted by warmth 

 (§ 7), we have every reason to believe that a portion of the cruorine 

 present in venous blood exists in the state of purple cruorine, and 

 is reoxidized in passing through the lungs. 



1 7. That it is only a rather small proportion of the cruorine pre- 

 sent in venous blood which exists in the state of purple cruorine 

 under normal conditions of life and health, may be inferred, not only 

 from the colour, but directly from the results of the most recent 

 experiments # . Were it otherwise, any extensive haemorrhage could 

 hardly fail to be fatal, if, as there is reason to believe, cruorine be 

 the substance on which the function of respiration mainly depends ; 

 nor could chlorotic persons exhale as much carbonic acid as healthy 

 subjects, as is found to be the case. 



But after death there is every reason to think that the process of 

 reduction still goes on, especially in the case of warm-blooded animals, 

 while the body is still warm. Hence the blood found in the veins of 

 an animal some time after death can hardly be taken as a fair spe- 

 cimen as to colour of the venous blood in the living animal. More- 

 over the blood of an animal which has been subjected to abnormal 

 conditions before death is of course liable to be altered thereby. The 

 terms in which Lehmann has described the colour of the blood of 

 frogs which had been slowly asphyxiated by being made to breathe 

 a mixture of air and carbonic acid seem unmistakeably to point to 

 purple cruorine*. 



18. The effect of various indifferent reagents in changing the 

 colour of defibrinated blood has been much studied, but not always 

 with due regard to optical principles. The brightening of the colour, 

 as seen by reflexion, produced by the first action of neutral salts, and 

 the darkening caused by the addition of a little water, are, I con- 

 ceive, easily explained ; but I have not seen stated what I feel satis- 

 fied is the true explanation. In the former case the corpuscles lose 

 water by exosmose, and become thereby more highly refractive, in 

 consequence of which a more copious reflexion takes place at the 

 common surface of the corpuscles and the surrounding fluid. In the 

 latter case they gain water by endosmose, which makes their refrac- 

 tive power more nearly equal to that of the fluid in which they are 

 contained, and the reflexion is consequently diminished. There is 

 nothing in these cases to indicate any change in the mode in which 

 light is absorbed by the colouring matter, although a change of tint 

 to a certain extent, and not merely a change of intensity, may accom- 



* Funk's Lehrbuch der Physiologie, 1863, vol. i. § 108. 

 t Physiological Chemistry, vol. ii. p. 178. 



