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 liver fulfils the 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 of plants, and particularly in the green parts. Their habitual presence does not, however, authorize the conclusion that this metal is necessary to the support and development of vegetable life. Some substances, evidently indifferent, foreign, and even injurious, if they exist abundantly in a soil, may be drawn into roots through the movement of the sap, and fix themselves in various organs. This occurs with copper in certain exceptional circumstances when the soil is saturated with its compounds, and if such a condition should be found to be repeated over a large extent of country, we might be led, by analysis alone of its vegetable productions, to the false conclusion that copper was an essential or even necessary constituent of them. But the value of the part performed by an element can not be determined by analysis alone. Direct proofs are necessary for that, methodical and comparative experiments in cultivation in mediums artificially deprived or furnished with the element the importance of which we wish to estimate. This has been done for combinations of iron, and the utility of that metal, especially to the higher plants, has been made thereby to appear.