THEORY OF RESPIRATION. 257 



Not only the oxides of iron, but also the cyanides 

 of that metal, exhibit similar properties. Prussian blue 

 contains iron in combination with all the organic ele- 

 ments of the body ; hydrogen and oxygen (water), 

 carbon and nitrogen (cyanogen). 



When it is exposed to light, cyanogen is given off, 

 and it becomes white ; in the dark it attracts oxygen, 

 and recovers its blue color. 



All these observations, taken together, lead to the 

 opinion that the globules of arterial blood contain a 

 compound of iron saturated with oxygen, which, in the 

 living blood, loses its oxygen during its passage through 

 the capillaries. The same thing occurs when it is 

 separated from the body, and begins to undergo de- 

 composition (to putrefy). The compound, rich in 

 oxygen, passes, therefore, by the loss of oxygen (re- 

 duction), into one far less charged with that element. 

 One of the products of oxidation formed in this pro- 

 cess is carbonic acid. The compound of iron in the 

 venous blood possesses the property of combining with 

 carbonic acid ; and it is obvious, that the globules of 

 the arterial blood, after losing a part of their oxygen, 

 will, if they meet with carbonic acid, combine with 

 that substance. 



When the globules reach the lungs, they will again 

 take up the oxygen they have lost ; for every volume of 

 oxygen absorbed, a corresponding volume of carbonic 

 acid will be separated ; they will return to their former 



22* 



