the present hour--viz., the relation of the "free" nitrogen
in the atmosphere to the plant. Of the very highest value also have been
the elaborate researches of Mr R. Warington, F.R.S., on the important
question of _Nitrification_, which have been in course in the Rothamsted
Laboratory for the last fifteen years, and to which full reference will
be made in the chapter on Nitrification.

To the Rothamsted experiments also we owe the refutation of Liebig's
mineral theory. In fact it may safely be said that no experimenters in
the field of agricultural chemistry have made more numerous or valuable
contributions to the science than these illustrious investigators.

_Review of our present Knowledge of Agricultural Chemistry._

Some attempt may now be made to indicate briefly our present knowledge
of the more important facts regarding plant physiology, agronomy, and
manuring.

_Proximate Composition of the Plant._

The great advance made in the direction of the improvement of the
accuracy of old analytical processes and the discovery of numerous new
ones have furnished us with elaborate analyses of the composition of
plants. We now know that the plant-substance is made up of a large
number of complex organic substances, formed out of carbon, hydrogen,
oxygen, and nitrogen,[20] and that these substances form, on an average,
about 95 per cent of the dry vegetable matter; the other 5 per cent
being made up of mineral substances. As to the source of these different
substances, our knowledge is, on the whole, pretty complete. With regard
to the carbon of green-leaved plants, which amounts to from 40 to 50 per
cent, subsequent research has confirmed Senebier and de Saussure's
conclusions, that its source is the carbonic acid gas of the air. The
decomposition of the carbonic acid gas is effected by the leaves under
the influence of sunlight. That a certain quantity of carbon may be
obtained from the carbonic acid absorbed by plant-roots, is indeed
probable. Especially during the early stages of plant-growth this source
of carbon may be of considerable importance. Generally speaking,
however, it may be said of all green-leaved plants, that the chief
source of their carbon is the carbonic acid gas in the atmosphere.

_Carbon Fixation by Plants._

The exact way in which this decomposition of carbonic acid gas is
effected by the leaves is not yet clear. It seems to be directly
dependent, in some way or other, on the chloro