266 THEORY OF RESPIRATION. 



blood undergo in the lungs, thus much is certain, 

 that the globules of venous blood experience a 

 change of colour, and that this change depends on 

 the action of oxygen. 



Now we observe that the globules of arterial 

 blood retain their colour in the larger vessels, and 

 lose it only during their passage through the ca- 

 pillaries. All those constituents of venous blood, 

 which are capable of combining with oxygen, take 

 up a corresponding quantity of it in the lungs. 

 Experiments made with arterial serum have shewn, 

 that when in contact with oxygen it does not dimin- 

 ish the volume of that gas. Venous blood, in con- 

 tact with oxygen, is reddened, while oxygen is ab- 

 sorbed ; and a corresponding quantity of carbonic 

 acid is formed. 



It is evident that the change of colour 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 se- 

 rum ; 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 absorbs from half its volume to an equal 

 volume of carbonic acid, and, at ordinary tempera- 

 tures, is not saturated with that gas. (See the ar- 

 ticle " Blut " in the " Handworterbuch der Chemie 

 von PoggendorfF, Wohler, and Liebig, p. 877.) 



