THEORY OF RESPIRATION. 269 



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 or- 

 ganic elements 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 colour. 



All these observations, taken together, lead to the 

 opinion that the globules of arterial blood contain 

 a compound of iron saturated w r ith oxygen, which, 

 in the living blood, loses its oxygen during its pas- 

 sage through the capillaries. The same thing occurs 

 when it is separated from the body, and begins to 

 undergo decomposition (to putrefy). The compound, 

 rich in oxygen, passes, therefore, by the loss of oxy- 

 gen (reduction), into one far less charged with that 

 element. One of the products of oxidation formed 

 in this process 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 car- 

 bonic acid, combine with that substance. 



When they reach the lungs, they will again take 

 up the oxygen they have lost ; for every volume of 

 oxygen absorbed, a corresponding volume of car- 

 bonic acid will be separated; they will return to 

 their former state ; that is, they will again acquire 

 the power of giving off oxygen. 



