EXAMPLES OF THE EXPLANATION OF LAWS. 281 



globules, then>f)re, arc concluded to bo tlio portion of the blood wliich 

 is operative in respiration. These globules contain a certain quantity 

 of iron, which (rom chemical tests is inferred to be in the state of 

 oxide. 



Dr. Liebig recognized, in tlic known chemical properties of the 

 oxides of iix)n, laws which, if followed out deductively, wtoiild lead to 

 the prediction of the precise series of phencmena which respiration 

 exhibits. 



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 vtmous blood 

 we have no direct evidence which of riie oxides is ]>resent, but the 

 considerations to be presently stated will prove that it is the prottjxide. 

 As arterial and venous blood are in a perpetual state of alternate con- 

 version into one another, the question arises, under what circumstances 

 the protoxide of iron is capable of being converted into the i)croxide, 

 and vice versd. Now the protoxide readily combines with (jxygen iii 

 the presence of water, forming the hydrated peroxide : these condi- 

 tions 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 phenojnena of respiration. But the arterial blood, 

 in (juitting the luntrs, 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 circulating through the body ; namely, contact with organic 

 compounds. 



Hydrated peroxide of iron, when treated with organic compounds 

 (whei"e no sulphur is present) gives forth oxygen and wsiter, which 

 oxyggn, attracting the carbon from the organic substance, becomes 

 carbonic acid ; while the peroxide, being reduced to the state of prot- 

 oxide, 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 add being given off, and 

 the protoxide, by the absorption of oxygen and water, becoming again 

 the h^-drated peroxide. 



The mysterious chemical phenomena connected with respiration 

 can now, by a beautiful deductive process, be completely explained. 

 The arterial blood, containing iron in the form of hydrated peroxide, 

 passes into the capillaries, where it meets with the decaying tissues, 

 receiving also in its course certain non-azotized but highly carbonized 

 animal products, in particular the bile. In these it finds the precis^ 

 conditions required for decomposing the peroxide into oxyge\i and the 

 protoxide. The oxygen combines with the carbon of the decaying 

 tissues, and forms carbonic acid, which, although insufficient in amount 

 to neutralize the whole of the protoxide, combines with a pcntion (<me- 

 fourth) of it, and returns in the form of a carl)wiiat<', along with the 

 other three-fourths of the i)rotoxide, through the venous system into 

 the lungs. There it again meets with «jxygen and water : the free 

 protoxide becomes hydrated peroxide; the carbcmate of protoxide 

 parts with its carbonic acid, and by absorbing oxygen and water, enters 

 also into the state of hydrated peroxid*-. The heat evolvvd in the 

 transition from protoxide to p(!roxide, as well as in the previous oxidq.- 

 tion of the carbon contained in the tissues, is considered by Liebig as 

 . Nn ■ 



