528 Theory of the Nutrition [BOOK in.

the discussion, to which it gave rise especially among chemists and agriculturists, turn rather on the question of the source of the nitrogen in the substance of plants. The humus-theory had made the nitrogen like the carbon enter the plant in the form of organic compounds. De Saussure in his great work of 1804 had named ammonia as a compound of nitrogen which might be taken into consideration with others, but he arrived at no definite conclusion. Liebig, from different points of view and in reliance on his own investigations into the nature of nitrogen and its compounds, arrived at the result, that ammonia must ultimately be the sole source of the nitrogen in the plant, and that the ammonia in the atmosphere and in the soil is quite sufficient to supply vegetation with the requisite amount of nitrogen just as the carbon dioxide of the atmosphere is the sole source of the carbon of the plant ; and so he con- cluded that ' carbon dioxide, ammonia, and water contain in their elements the requisites for the production of all the substances that are in animals and plants during their life-time. Carbon dioxide, ammonia, and water are the ultimate products of the chemical process of their putrefaction and decay.'

Liebig was less happy, at least as regards his mode of treat- ing the subject, in his remarks on the necessity and specific importance of the constituents of the ash to the nutrition of plants. Instead of insisting on an experimental answer to the question, what constituents of the ash are absolutely indispens- able to the health of one or all plants, he lost himself in ingenious chemical theories, intended to show the operation of inorganic bases in fixing vegetable acids, the extent to which different bases can replace each other, and similar matters.

It is not requisite for our purpose to follow Liebig in his applications of his theoretical remarks to agriculture, still less to occupy ourselves with the sensation and the discussions which his work excited among practical and theoretical farmers and agricultural chemists. The scientific value of

Liebig's considerations on the nutrition of plants stood out in