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 concluded 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 treating 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 indispensable 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
