not till 1864 that Hoppe-Seylerin obtaining it pure and crystallized. But Liebig had already perceived its essential properties, and was able to point out approximately its functions as early as 1845; yet the single fact that there was no assimilation possible between this substance and the salts of iron, cut this question off into a kind of negative suspense. Different from these compounds, it could not behave like them, and accomplish slow combustions of the same type. It is a remarkable fact, and one that illustrates well how iron preserves through all its vicissitudes some trace of its fundamental property of favoring the action of oxygen on substances, that this composition, so special and so different from the salts of iron, behaves nearly as they do. While it is not of itself an energetic combustible, it is, according to Liebig's expression, "a transporter of oxygen"—a luminous view, which the future was destined to confirm. Although the transportation is not produced by the mechanism supposed by Liebig, but by another, the general result is very much the same from the point of view of the physiology of the blood. The coloring matter of the blood conveyed by the globules fixes oxygen in contact with the pulmonary air, and distributes it as it passes through the capillaries upon the tissues. The globule of blood brings nothing else and distributes nothing else, contrary to the opinion that had been held before. The theory of slow combustion effected through iron, while not absolutely contradicted in principle, was not entirely confirmed in detail, so far as concerned iron, or the more prominently ferruginous tissue.
No search was made for other tissues or organs presenting more favorable conditions, for no others were known that had iron in themselves. The liver and the spleen were supposed to receive it from the blood under the complicated form in which it exists there, or under some equivalent form. It was not, therefore, supposed till within a very few years that the two conditions were realized in any organ that were required to secure a slow combustion by iron—that is, combinations resembling ferrous and ferric salts with a weak acid and a source of oxygen. The doubt has been resolved by recent studies. The liverthe requirement. It contains iron existing under forms precisely comparable to the ferrous and ferric compounds, and is washed by the blood which carries oxygen in a state of simple solution in its plasma and of loose combination in its globules. Thus all the conditions necessary for the production of slow combustion are gathered here, and we can not doubt that it takes place. A new function is therefore assigned to the liver, and it becomes one of the great furnaces of the organism.
Compounds of iron are so abundant in the ground and the water that we need not be surprised when we find them in various parts