268 THEORY OF RESPIRATION. 



ably necessary to animal life ; and, since physiology 

 has proved, that the globules take no share in the 

 process of nutrition, it cannot be doubted that they 

 play a part in the process of respiration. 



The compound of iron in the globules has the 

 characters of an oxidised compound ; for it is de- 

 composed by sulphuretted hydrogen, exactly in the 

 same way as the oxides or other analogous com- 

 pounds of iron. By means of diluted mineral acids, 

 peroxide (sesquioxide) of iron may be extracted, at 

 the ordinary temperature, from the fresh or dried 

 red colouring matter of the blood. 



The characters of the compounds of iron may, 

 perhaps, assist us to explain the share which that 

 metal takes in the respiratory process. No other 

 metal can be compared with iron, for the remark- 

 able properties of its compounds. 

 - The compounds of protoxide of iron possess the 

 property of depriving other oxidised compounds of 

 oxygen ; while the compounds of peroxide of iron, 

 under other circumstances, give up oxygen with the 

 utmost facility. 



Hydrated peroxide of iron, in contact with organic 

 matters destitute of sulphur, is converted into car- 

 bonate of the protoxide. 



Carbonate of protoxide of iron, in contact with 

 water and oxygen, is decomposed ; all the carbonic 

 acid is given off, and, by absorption of oxygen, it 

 passes into the hydrated peroxide, which may again 

 be converted into a compound of the protoxide. 



