_

The report above referred to was made by Liebig at the request of the
Chemical Section of the British Association. It was read to a meeting of
the Association held in Glasgow in 1840, and was subsequently published
in book form, under the title of 'Chemistry in its Application to
Agriculture and Physiology,' Liebig's position, past training and
experience were such as to peculiarly fit him for the part of pioneer in
the new science. As Sir J. H. Gilbert has remarked,[14] "In the
treatment of his subject he not only called to his aid the previously
existing knowledge directly bearing upon his subject, but he also turned
to good account the more recent triumphs of organic chemistry, many of
which had been won in his own laboratory."

In his dedication to the British Association at the beginning of the
book, Liebig says: "Perfect agriculture is the true foundation of all
trade and industry--it is the foundation of the riches of States. But a
rational system of agriculture cannot be formed without the application
of scientific principles; for such a system must be based on an exact
acquaintance with the means of nutrition of vegetables, and with the
influence of soils and actions of manure upon them. This knowledge we
must seek from chemistry, which teaches the mode of investigating the
composition and of studying the characters of the different substances
from which plants derive their nourishment."

_His criticism of the "Humus" Theory._

The first subject which Liebig discusses is the scientific basis of the
so-called "humus" theory. The humus theory seems to have been first
promulgated by Einhof and Thaer towards the close of last century. Thaer
held that humus was the source of plant-food. He stated in his published
writings that the fertility of a soil depended really upon its humus;
for this substance, with the exception of water, is the only source of
plant-food. De Saussure, however, by his experiments--the results of
which he had published in 1804--had shown the fallacy of this humus
theory; and his statements had been further developed and substantiated
by the investigations of the French chemist Braconnot and the German
chemist Sprengel. Despite, however, the experiments of Saussure,
Braconnot, and Sprengel, the belief that plants derived the
carbonaceous portion of their substance from humus still seemed to be
commonly held in 1840.

While Liebig, therefore, can scarcely be said to have been the first