viii

the degradation of which they were formed, or as aqueous forma-
tions of different periods and ages, appearing in distant regions
throughout the surface of our globe, to which they were conveyed
by livers and streams or by mighty floods, as they occurred in for-
mer ages.

The soil (the upper portion of these masses) consists, according
to the above, of Quartz-Sand; of smaller or larger fragments of
rocks which still remain undecomposed; and of Clay, impregnated
with Silicates of Potash and Soda, Lime, Magnesia, Oxyds of
Iron, &c. A fertile soil, however, requires besides these ingredi-
ents, comparatively small quantities of some other substances, viz:
such as contain Phosphoric Acid, Sulphuric Acid and Chlorine, the
origin of each of which is also easily explained. For Phosphoric
Acid is the constituent of Apatite, (Phosphate of Lime)—a mineral
which, though in small quantities, is diffused through almost all
rocks of all ages, and consequently carried along with the products
of their degradation. Sulphuric Acid originates from Sulphurets,
combinations of metals with sulphur, which also are more generally
diffused through rocks, and which under the influence of the at-
mospheric Oxygen, are converted into the sulphates of the corre-
sponding metals as already explained. Chlorine in its combination
with Soda, (common Salt,) we meet in abundance in sea-water; in
the waters of fountains and also in small quantities in rain-water.

Another substance which is never absent in cultivated foils, is
Humus. It has already been mentioned, that after death, the con-
stituents of organic bodies (the organs) become exposed to the de-
stroying influences of the atmosphere, which finally converts them
into Water, Carbonic Acid and Ammonia. In the meantime, be-
tween death and total conversion into these three constituents of
the atmosphere, the remaining organic parts run through many
phases of decomposition, and in this period form the so called Hu-
mus; a substance which, therefore, in every moment changes its
composition. With every new crop we take from our soils, we
also supply them with new quantities of Humus, which originates
from those parts of the plants that remain upon the soil. For this
reason, a soil cannot become poorer in Humus though its quantity
is constantly diminished by decomposition.

The Constituents op Plants, and the Forms in which
they are assimilated.

A plant, or any part of it, when heated to a proper degree of
heat in the open air, burns, and produces volatile substances, which
in gaseous form are diffused through the atmosphere, whilst only
a comparatively small part of the original plant remains as an in
combustible, earthy substance, called the ash.