Prof. Stokes on the Colouring Matter of the Blood. 399 



pany the change of conditions under which the turbid mixture is 

 seen, as I have elsewhere more fully explained*. 



No doubt the form of the corpuscles is changed by the action 

 of the reagents introduced ; but to attribute the change of colour to 

 this is, I apprehend, to mistake a concomitant for a cause, and to 

 attribute, moreover, the change of colour to a cause inadequate to 

 produce it. 



19. Very different is the effect of carbonic acid. In this case 

 the existence of a fundamental change in the mode of absorption 

 cannot be questioned, especially when the fluid is squeezed thin 

 between two glasses and viewed by transmitted light. I took two 

 portions of defibrinated blood ; to one I added a little of the redu- 

 cing iron solution, and passed carbonic acid into the other, and then 

 compared them. They were as nearly as possible alike. We must 

 not attribute these apparently identical changes to two totally differ- 

 ent causes if one will suffice. Now in the case of the iron salt, the 

 change of colour is plainly due to a deoxidation of the cruorine. On 

 the other hand, Magnus removed as much as 10 or 12 per cent, 

 by volume of oxygen from arterialized blood by shaking the blood 

 with carbonic acid. If, as we have reason to believe, this oxygen was 

 for the most part chemically combined, it follows that carbonic acid 

 acts as if it were a reducing agent. We are led to regard the change 

 of colour not as a direct effect of the presence of carbonic acid, but 

 a consequence of the removal of oxygen. There is this difference 

 between carbonic acid and the real reducing agents, that the former 

 no longer acts on a dilute and comparatively pure solution of scarlet 

 cruorine, while the latter act just as before. 



If even in the case of blood exposed to an atmosphere of carbonic 

 acid we are not to attribute the change of colour to the direct pre- 

 sence of the gas, much less should we attempt to account for the 

 darker colour of venous than arterial blood by the small additional 

 percentage of carbonic acid which the former contains. The ascer- 

 tained properties of cruorine furnish us with a ready explanation, 

 namely that it is due to a partial reduction of scarlet cruorine in 

 supplying the wants of the system. 



20. I am indebted to Dr. Akin for calling my attention to a very 

 interesting pamphlet by A. Schmidt on the existence of ozone in the 

 blood \. The author uses throughout the language of the ozone 

 theory. If by ozone be meant the substance, be it allotropic oxygen 

 or teroxide of hydrogen, which is formed by electric discharges in 

 air, there is absolutely nothing to prove its existence in blood; for 

 all attempts to obtain an oxidizing gas from blood failed. But if by 

 ozone be merely meant oxygen in any such state, of combination or 

 otherwise, as to be capable of producing certain oxidizing effects, 

 such as turning guaiacum blue, the experiments of Schmidt have 

 completely established its existence, and have connected it, too, with 

 the colouring matter. Now in cruorine we have a substance admit- 

 ting of easy oxidation and reduction ; and connecting this with 

 Schmidt's results, we may infer that scarlet cruorine is not merely a 



* Philosophical Transactions, 1852, p. 527. 

 f Ueher Ozon im Blute. Dorpat, 1862. 



