ultural Chemistry, like most branches of natural science, may be
said to be entirely of modern growth. While it is true we have many old
speculations on the subject, they can scarcely be said to possess much
scientific value. The great questions which had first to be solved by
the agricultural chemist were,--What is the food of plants? and,--What
is the source of that food? The second of these two questions more
easily admitted of answer than the first. The source of plant-food could
only be the atmosphere or the soil. As the composition of the
atmosphere, however, was not discovered till the close of last century,
and the chemistry of the soil is a question which is still requiring
much work ere we shall be in possession of anything like a full
knowledge of it, it will be at once obvious that the very fundamental
conditions for a solution of the question were awanting. The beginning,
then, of a true scientific agricultural chemistry may be said to date
from the brilliant discoveries associated with the names of Priestley,
Scheele, Lavoisier, Cavendish, and Black--that is, towards the close of
last century.

_Early Theories on Source of Plant-food._

While this is so, and while we must regard the early attempts made
towards solving this question as being, for the most part, of little
scientific value, it is not without interest, from the historical point
of view, to glance briefly at some of these old interesting
speculations.

The Aristotelian doctrine, regarding the possibility of dividing matter
into the so-called four primary elements, _fire_, _air_, _earth_, and
_water_, which obtained in one form or another till the birth of modern
chemistry, had naturally an important influence on these early theories.

_Van Helmont's Theory._

Among the earliest and most important attempts made to solve the problem
of plant-growth was that by Jean Baptiste Van Helmont, one of the best
known of the alchemists, who flourished about the beginning of the
seventeenth century. Van Helmont believed that he had proved by a
conclusive experiment that all the products of vegetables were capable
of being generated from water. The details of this classical experiment
were as follows:--

"He took a given weight of dry soil--200 lb.--and into this soil he
planted a willow-tree that weighed 5 lb., and he watered this carefully
from time to time with pure rain-water, taking care to prevent any dust
or dirt falling on to the earth in