254 THEORY OF RESPIRATION. 



Whatever change the other constituents of the blood 

 undergo in the lungs, thus much is certain, that the 

 globules of venous blood experience a change of color, 

 and that this change depends on the action of oxygen. 



Now we observe that the globules of arterial blood 

 retain their color in the larger vessels, and lose it only 

 during their passage through the capillaries. All those 

 constituents of venous blood, which are capable of 

 combining with oxygen, take up a corresponding quan- 

 tity of it in the lungs. Experiments made with arterial 

 serum have shown, that when in contact with oxygen it 

 does not diminish the volume of that gas. Venous 

 blood, in contact with oxygen, is reddened, while oxy- 

 gen is absorbed ; and a corresponding quantity of car- 

 bonic acid is formed. 



It is evident that the change of color in the venous 

 globules depends on the combination of some one of 

 their elements with oxygen ; and that this absorption of 

 oxygen is attended with the separation of a certain 

 quantity of carbonic acid gas. 



This carbonic acid is not separated from the serum ; 

 for the serum does not possess the property, when in 

 contact with oxygen, of giving off carbonic acid. On 

 the contrary, when separated from the globules, it ab- 

 sorbs from half its volume to an equal volume of car- 

 bonic acid, and, at ordinary temperatures, is not satu- 

 rated with that gas. (See the article " Blut " in the 

 u Handworterbuch der Chemie von Poggendorff, Woh- 

 ler, und Liebig," p. 877.) 



