been made public.

The year 1844 was also distinguished by the foundation of the
Agricultural Chemistry Association of Scotland, an event of no small
importance in the history of scientific agriculture. That association
was instituted through the exertions of a small number of practical
farmers, for the purpose of pursuing investigations in agricultural
chemistry, and affording to its members assistance in all matters
connected with the cultivation of the soil, and has formed the model of
similar establishments in London, Dublin, and Belfast, as well as in
Germany; and it is peculiarly creditable to the intelligence and energy
of the practical farmers of Scotland, that with them commenced a
movement, which has already found imitators in so many quarters, and
conferred such great benefits on agriculture. Within the last ten or
twelve years, and mainly owing to the establishment of agricultural
laboratories, great progress has been made in accumulating facts on
which to found an accurate knowledge of the principles of agricultural
chemistry, and the number of chemists who have devoted themselves to
this subject has considerably increased, though still greatly less than
its exigencies require.

Notwithstanding all that has recently been done, it must not be
forgotten that we have scarcely advanced beyond the threshold, and that
it is only by numerous and frequently repeated experiments that it is
possible to arrive at satisfactory results. Agricultural inquiries are
liable to peculiar fallacies due to the perturbing influence of climate,
season, and many other causes, the individual effects of which can only
be eliminated with difficulty, and much error has been introduced, by
hastily generalising from single experiments, in place of awaiting the
results of repeated trials. Hence it is that the progress of scientific
agriculture must necessarily be slow and gradual, and is not likely to
be marked by any great or startling discoveries. Now that the relations
of science to practice are better understood, the extravagant
expectations at one time entertained have been abandoned, and, as a
necessary consequence, the interest in agricultural chemistry has again
increased, and the conviction daily gains ground that no one who wishes
to farm with success, can afford to be without some knowledge of the
scientific principles of his art.

CHAPTER I.

THE ORGANIC CONSTITUENTS OF PLANTS.

When the water naturally existin