DR. STEVENS ON THE THEORY OF RESPIRATION. 349 



water. In the eighth experiment the pump was used immediately, and before the air 

 had time to act on the blood, or the blood on the air. In the last, one hour was al- 

 lowed for the action of these agents upon each other ; during which the blood on 

 the surface changed from venous to arterial, and the air over the blood received the 

 addition of carbonic acid. 



Those who maintain that carbonic acid informed in the lungs will say, that in the 

 last experiment the carbon of the blood attracted the oxygen of the air, and that the 

 carbonic acid so formed was then evolved. But there is one circumstance which is 

 I think fatal to such an explanation, for all the acids blacken the blood, and carbonic 

 acid possesses this blackening property in a remarkable degree. When we agitate 

 a small quantity of carbonic acid gas with arterial blood, the colour immediately 

 changes to venous, and when we add carbonic acid to venous blood it becomes 

 almost black. Now, if the carbon of the blood attracted the oxygen of the air, and 

 if the carbonic acid were thus formed in the blood itself, it is evident that the Jirst 

 effect of the air on the blood would be to make this fluid blacker than it had been 

 before ; but the opposite of this is the fact, for the Jirst effect of the air is, not to 

 blacken, but to brighten the blood ; consequently, from this alone, we may infer, that 

 the acid is not formed during the experiment, but that it exists ready formed in the 

 blood, and that it is only removed, and not produced or formed, by the atmospheric 

 air. 



I have already observed that one class of experimenters have obtained results by 

 means of the air-pump which are in direct opposition to those obtained by others. 

 May we not now, from the above experiments, explain this difference by supposing 

 that those who could not obtain carbonic acid made their experiments before the air 

 had had time to act on the blood, whilst the others had allowed some time to elapse ? 



From the preceding statement we may, I think, conclude, 



1st, That venous blood contains carbonic acid; 



2nd, That the mere effect of diminished pressure upon the surface of the blood is 

 not necessarily followed by the escape of its carbonic acid*. 



We have seen in some of the above experiments that atmospheric air possesses a 

 property of removing carbonic acid from venous blood ; it becomes therefore a 

 question how this effect is produced. I have ascertained that nitrogen is ineffective; 

 we may therefore infer that the oxygen is the principal agent ; and that such is the 

 fact is proved by the following experiments. 



1 0. A piece of moist bladder was tied firmly over the mouth of a tumbler contain- 

 ing pure oxygen gas. This was introduced into a large bell glass filled with carbonic 

 acid. In a short period the membrane which had been tied over the glass became 

 convex, and so tense that it appeared to be on the point of bursting. On examining 

 the air contained in the tumbler, it was found that the oxygen had drawn in a large 



* In performing the above experiments I was assisted by Mr. Squire, to whom I feel under great obligation 

 for the zealous and able manner in which he aided me in the whole of the present investigation. 



