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502 tained even till after 1830 respecting the necessity of the constituents of the ash to vegetation.

It was known in de Saussure's time that nitrogen entered into the substance of living plants; the question was, whence it was obtained. As it was known that four-fifths of the atmosphere consists of nitrogen, it was natural to suppose that it is this which the plant makes use of for forming its nitrogenous substance. De Saussure endeavoured to settle the question by the volumetric method, which, as was afterwards discovered, was not in this case to be trusted. Nevertheless he arrived at the right conclusion, that plants do not assimilate the nitrogen of the atmosphere; this gas must therefore be taken up by the roots in some form of chemical combination. He made no experiments on growing plants to decide what that form was, but contented himself with the conjecture that vegetable and animal matter in the soil and ammoniacal exhalations from it supply the nitrogen in plants. This question, first ventilated certainly by de Saussure, and afterwards the subject of protracted discussion, was finally settled fifty years later by the experiments of Boussingault.

In connection with his researches into the importance of the constituents of the ash, de Saussure proposed the question whether roots take up the solutions of salts and other substances exactly in the form in which they offer themselves. He found first of all that very various and even poisonous matters are absorbed by them, and that there is therefore no such power of choice, as Jung had once supposed; on the other hand, it appeared that the solutions do not enter unchanged into the roots, for in his experiments in every case the proportion of water to the salt absorbed was greater than the proportion between them in the solution, and that some salts enter the plant in larger, some in smaller quantities, under circumstances in other respects the same. But at this time, and for a long time after, it was not possible to understand and rightly explain these facts; the theory of diffusions was not yet known,