36 WONDERS OF ORGANIC LIFE. 



four per cent, less of carbon than albumen con- 

 tains. Now gelatine enters into the structure 

 of every part of the animal frame, and especially 

 of the skin ; the skin, indeed, consists of little 

 else besides gelatine. It is most probable, 

 therefore, that a large portion of the carbonic 

 acid of venous blood is formed in the skin and 

 in the analogous textures. Indeed, we know 

 that the skin of many animals gives off carbonic 

 acid, and absorbs oxygen ; or, in other words, 

 performs all the offices of the lungs ; a function 

 of the skin perfectly intelligible, on the suppo- 

 sition that near the surface of the body the 

 albuminous portions of the blood are always 

 converted into gelatine." It is the presence of 

 carbonic acid that gives darkness of colour to 

 the venous blood, and on the removal of this it 

 resumes its hue of scarlet. Carbonic acid is 

 composed of pure carbon and oxygen, but the 

 0X3 T gen is not in a maximum proportion ; and 

 it has been demonstrated that between oxygen 

 and carbonic acid a very powerful attraction 

 exists. When, therefore, the atmospheric air 

 is admitted into the lungs, the oxygen of that 

 air unites with the carbonic acid of the blood, 

 and, as experiments prove, is expired in the 

 form of a gas, which will render lime-water 

 turbid, and cause a precipitate of carbonate of 

 lime. Thus freed from carbon, the blood be- 

 comes scarlet or vermilion, a colour due to the 

 action of the salts it contains on the blood 

 discs. It would appear, however, that the 

 blood retains in itself a portion of pure oxygen, 



