IRON IN THE LIVING BODY. 809 



if we regarded only the uses we make of it. It exists as an oxide in 

 Nature; and the metallurgy of it has no other object than to re- 

 vivify burned iron, remove the oxygen from it, and extract the metal. 

 Of the two oxides of iron, the ferrous, or lower one, is an energetic 

 base, readily combining with even the weakest acids, and forming 

 with them ferrous or protosalts. Ferric oxide, on the other hand, is 

 a feeble base, which combines only slowly with even strong acids 

 to form ferric salts or persalts, and not at all with weak acids like 

 carbonic acid and those of the tissues of living beings. It is these 

 last, more highly oxidized ferric compounds that provide organic 

 substances with the oxygen that consumes them, when, as a result of 

 the operation, they themselves return to the ferrous state. 



Facts of this sort are too nearly universal not to have been ob- 

 served very long ago, but they were not fully understood till about 

 the middle of this century. The chemists of the time — Liebig, 

 Dumas, and especially Schonbein, Wohler, Stenhouse, and many 

 others — established the fact that ferric oxide provokes at ordinary 

 temperatures a rapid action of combustion on a large number of sub- 

 stances: grass, sawdust, peat, charcoal, humus, arable land, and ani- 

 mal matter. A very common example is the destruction of linen by 

 rust spots; the substance of the fiber is slowly burned up by the 

 oxygen yielded by the oxide. About the same time, Claude Ber- 

 nard inquired whether the process took place within the tissues, in 

 contact with living matter in the same way as we have just seen it 

 did with dead matter — the remains of organisms that had long since 

 submitted to the action of physical laws — and received an affirmative 

 answer. Injecting a ferric salt into the jugular vein of an animal, he 

 found it excreted, deprived of a part of its oxygen, as a ferrous salt. 



This slow combustion of organic matter, living or dead, accom- 

 plished in the cold by iron, represents only one of the aspects of its 

 biological function. A counterpart to it is necessary in order to 

 complete the picture. It is easy to perceive that the phenomenon 

 would have no bearing or consequence if it was limited to this first 

 action. "With the small provision of oxygen in the iron salt used up, 

 and, if reduced to the minimum of oxidation, the source of oxygen be- 

 ing exhausted, the combustion of organic matter would stop. The 

 oxidation obtained would be insignificant, while the oxidation should 

 be indefinite and unlimited, and it is really so. 



There is a counterpart. The iron salt, which has gone back to 

 the minimum of oxidation and become a ferrous salt, can not remain 

 long in that state in contact with the air and with other sources of the 

 gas to which it is exposed. It has always been known that ferrous 

 compounds absorb oxygen from the air and pass into the ferric state ; 

 we might say that we have seen it done, for the transformation is 



