year 1804 witnessed the publication of by far the most important
contribution made to the science up till this time. This was
'Recherches Chimique sur la Vegetation,' by Theodore de Saussure, one of
the most illustrious agricultural chemists of the century. De Saussure
was the first to draw attention to the mineral or ash constituents of
the plant; and thus anticipate, to a certain extent, the subsequent
famous "mineral" theory of the great Liebig. The French chemist
maintained that these ash ingredients were essential; and that without
them plant-life was impossible. He also adduced fresh experiments of his
own in support of the theory, based on the experiments of Bonnet,
Priestley, Ingenhousz, and Senebier, that plants obtain their carbon
from the carbonic acid gas in the air, under the influence of the
sunlight. He was of opinion that the _hydrogen_ and _oxygen_ of the
plant were, probably, chiefly derived from water. He showed that by far
the largest portion of the plant's substance was derived from the air
and from water, and that the ash portion was alone derived from the
soil. To Saussure we owe the first definite statement on the different
sources of the plant's food. It may be said that the lapse of nearly a
century has shown his views to be, in the main, correct.

_Source of Plant-nitrogen._

There was one question, which, even at that remote period in the history
of the subject, engaged the attention of agricultural chemists--viz.,
the question of the source of the plant's _nitrogen_--a question which
may be fitly described at the present hour as still the burning
question of agricultural chemistry.[8]

As soon as it was discovered that nitrogen was a constituent of the
plant's substance; speculations as to its source were indulged in. The
fact that the air furnished an unlimited storehouse of this valuable
element, and the analogy of the absorption of carbon (from the same
source by plant-leaves), naturally suggested to the minds of early
inquirers that the free nitrogen of the air was the source of the
plant's nitrogen. As, however, no direct experiments could be adduced to
prove this theory, and as, moreover, nitrogen was found in the soil, and
seemed to be a necessary ingredient of all fertile soils, the opinion
that the soil was the only source gradually supplanted the older theory.
Little value, however, must be attached to these early theories, as they
can scarcely be said to have been based on ex