accompanied by a characteristic change of color, by a transition from the pale green tint of ferrous bases to the ochery or red color of ferric compounds.
We can understand now what should happen when the ferruginous compound is placed in contact alternately with organic matter and oxygen. In the former phase the iron will yield oxygen to the organic matter; in the second phase it will take again from the atmosphere the combustible which it has lost, and will be again where it started. The same series of operations may be continued a second time and a third time, and indefinitely, as long as the alternations of contact with organic matter and exposure to atmospheric oxygen are kept up, the iron simply performing the part of a broker. The same result will occur if atmospheric air and organic matter are constantly together; the consumption will continue indefinitely, and the iron will perform the part of an intermediary till one of the elements of the process is exhausted.
This explanation was necessary to make clear the solution of the mystery of slow or cool combustion, the existence of which has been known since Lavoisier, without its mechanism being understood. That illustrious student gave out the theory that animal heat and the energy developed by vital action originated in the chemical reactions of the organism, and that, on the other hand, the reactions that produce heat consisted of simple combustions, slow combustions, that differed only in intensity from that of the burning torch. The development of chemistry has shown that this figure was too much simplified from the reality, and that most of these phenomena, while they are in the end equivalent to a combustion, differ greatly from it in mechanism and mode of execution. By this we do not mean to say that all the combustions are of this character, and that there do not exist in the organism a large number of such as Lavoisier understood, and of such as the combustions effected by the intervention of iron furnish the type of. Lavoisier's successors, Liebig among them, tried to find reactions conformed to this type. Their attempts were unsuccessful, but they had the happy result of revealing, if not the real function of iron in the blood, at least that of the red matter in which it is fixed.
The question of the presence of iron in the coloring matter of the blood gave rise to long discussions. Vauquelin denied it. He made the mistake of looking for iron in the form of a known compound, in direct combination with the blood, while later researches have shown that it is found almost exclusively in the red matter that tinges the globules, in a complicated combination that escapes the ordinary tests; or, according to a usual method of expression, it is dissimulated. Liebig also failed to find this combination, and it was
