354 DR. STEVENS ON THE THEORY OF RESPIRATION. 



Oxygen, it is true, changes the colour from venous to arterial ; this, however, is ef- 

 fected not by any specific action, but by the removal of the carbonic acid, which 

 had been the cause of the dark colour in the venous circulation. 



Many objections have been made to the above theory, some of which are frivolous; 

 but there are two which are worthy of notice. The first was made by Mr. Prater 

 of Edinburgh, who stated, that according to this theory the blood ought to become 

 arterial under the exhausted receiver of the air-pump. This objection is removed by 

 the foregoing experiments, which prove that the mere removal of the air's pressure 

 is insuflicient to overcome the attraction that subsists between the carbonic acid and 

 the blood. The second objection was made by Dr. Gregory and Mr. Irvine of Edin- 

 burgh. These gentlemen admit that if the blood were a stronger saline fluid than 

 it is, the salts would be capable of producing all the effects described ; but they con- 

 ceive that the blood is not sufficiently impregnated with saline matter to account for 

 the whole of the phenomena. This objection, even if proved, would only require a 

 modification of the theory ; but that there was a fallacy in their experiments which 

 neutralized their conclusion, has been proved by a paper in the Medical Gazette of 

 the 12th of April, 1834. 



