exposed to the action of the atmosphere and moisture. The
washing completely removes all the soluble part of the disintegrated
rock; the insoluble part, moreover, cannot undergo any further
change while it is covered with water, and so excluded from the
influence of the atmosphere at the bottom of the pond. But being
exposed at once to the air and moisture, a powerful chemical action
takes place in the whole mass, which becomes indicated by an
efflorescence of salts covering the whole surface of the heaps in
considerable quantity. After being exposed for two or three years,
the mud is again subjected to the same process of washing, and a
considerable quantity of gold is obtained, this having been
separated by the chemical process of decomposition in the mass. The
exposure and washing of the same mud is repeated six or seven times,
and at every washing it furnishes a new quantity of gold, although
its amount diminishes every time.

Precisely similar is the chemical action which takes place in the
soil of our fields; and we accelerate and increase it by the
mechanical operations of our agriculture. By these we sever and
extend the surface, and endeavour to make every atom of the soil
accessible to the action of the carbonic acid and oxygen of the
atmosphere. We thus produce a stock of soluble mineral substances,
which serves as nourishment to a new generation of plants, materials
which are indispensable to their growth and prosperity.

LETTER XIII

My dear Sir,

Having in my last letter spoken of the general principles upon which
the science and art of agriculture must be based, let me now direct
your attention to some of those particulars between chemistry and
agriculture, and demonstrate the impossibility of perfecting the
important art of rearing food for man and animals, without a
profound knowledge of our science.

All plants cultivated as food require for their healthy sustenance
the alkalies and alkaline earths, each in a certain proportion; and
in addition to these, the cerealia do not succeed in a soil
destitute of silica in a soluble condition. The combinations of this
substance found as natural productions, namely, the silicates,
differ greatly in the degree of facility with which they undergo
decomposition, in consequence of the unequal resistance opposed by
their integral parts to the dissolving power of the atmospheric
agencies. Thus the granite of Corsica degenerates into a powder in a
time