nnecessary, provided that a thorough system of tillage was carried out.
Manures also, according to him, might be entirely dispensed with under
his system of cultivation, for the true function of all manures is to
aid in the pulverisation of the soil by fermentation.

The first really valuable scientific facts contributed to the science
were made by Priestley, Bonnet, Ingenhousz, and Senebier.

_Discovery of the Source of Plants' Carbon._

To Charles Bonnet (1720-1793), a Swiss naturalist, is due the credit of
having made the first contribution to a discovery of very great
importance--viz., the true source of the _carbon_, which we now know
forms so large a portion of the plant-substance. Bonnet, who had devoted
himself to the question of the function of leaves, noticed that when
these were immersed in water bubbles were seen, after a time, to collect
on their surface. De la Hire, it ought to be pointed out, had noticed
this same fact about sixty years earlier. It was left to Priestley,
however, to identify these bubbles with the gas he had a short time
previously discovered--viz., oxygen. Priestley had observed, about this
time, the interesting fact that plants possessed the power of purifying
air vitiated by the presence of animal life.[4] The next step in this
highly interesting and important discovery was taken by John Ingenhousz
(1730-1799), an eminent physician and natural philosopher. In 1779,
Ingenhousz published a work in London entitled 'Experiments on
Vegetables.' In it he gives the results of some important experiments he
had made on the question already investigated by Bonnet and Priestley.
These experiments proved that plant-leaves only gave up their oxygen in
the presence of sunlight. In 1782 he published another work on 'The
Influence of the Vegetable Kingdom on the Animal Creation.'[5]

The source of the gas, which Bonnet had first noticed to be given off
from plant-leaves, Priestley had identified as oxygen, and Ingenhousz
had proved to be only given off under the influence of the sun's rays,
was finally shown by a Swiss naturalist, Jean Senebier[6] (1742-1809),
to be the _carbonic acid gas_ in the air, which the plant absorbed and
decomposed, giving out the oxygen and assimilating the carbon.

_Publication of First English Treatise on Agricultural Chemistry._

In 1795, a book dealing with the relations between chemistry and
agriculture was published. This work was written by a Scottish nobleman