EXAMPLES OF THE EXPLANATION OF LAWS. 569 



There are two oxides of iron, a protoxide and a 

 peroxide. In the arterial blood the iron is in the 

 form of peroxide : in the venous blood we have no 

 direct evidence which of the oxides is present, but 

 the considerations to be presently stated will prove 

 that it is the protoxide. As arterial and venous 

 blood are in a perpetual state of alternate conver- 

 sion into one another, the question arises, under 

 what circumstances the protoxide of iron is capable of 

 being converted into the peroxide, and vice versd. 

 Now the protoxide readily combines with oxygen in 

 the presence of water, forming the hydrated peroxide : 

 these conditions it finds in passing through the lungs ; 

 it derives oxygen from the air, and finds water in the 

 blood itself. This would already explain one portion 

 of the phenomena of respiration. But the arterial 

 blood, in quitting the lungs, is charged with hydrated 

 peroxide : in what manner is the peroxide brought 

 back to its former state ? 



The chemical conditions for the reduction of the 

 hydrated peroxide into the state of protoxide, are 

 precisely those which the blood meets with in circu- 

 lating through the body ; namely, contact with organic 

 compounds. 



Hydrated peroxide of iron, when treated with 

 organic compounds (where no sulphur is present) gives 

 forth oxygen and water, which oxygen, attracting the 

 carbon from the organic substance, becomes carbonic 

 acid ; while the peroxide, being reduced to the state 

 of protoxide, combines with the carbonic acid, and 

 becomes a carbonate. Now this carbonate needs only 

 come again into contact with oxygen and water to be 

 decomposed ; the carbonic acid being given off, and 

 the protoxide, by the absorption of oxygen and water, 

 becoming again the hydrated peroxide. 



