For these reasons we are compelled to seek for the nourishment
of plants in the contents of the atmosphere: and as Oxygen and
Nitrogen cannot add to the weight of organic bodies in their isola-
ted form, these functions must be solely fulfilled by Carbonic Acid,
Ammonia and Water. If we abstract from Humus its action on
the mechanical texture of soils, its beneficial influence on vegeta-
tion is then due to its giving rise to Carbonic Acid, Ammonia and
Water, and to no other cause.

It has already been shown that Carbonic Acid, Ammonia and
Water, are the products of the decomposition of organic substan-
ces; and we have seen that they are also assimilated by plants as
nourishment, and their elements, under the dominion of the vital
powers, arranged to form new compounds of organic character.—
The same substances which have served to produce the plant, are
thus regenerated after its death by the process of its decay, thus
forming an eternal transition from the organic state to the inorganic

The inorganic, or mineral, constituents of plants, though small in
quantity when compared with the organic part, are nevertheless
essential, and must be considered just as indispensable for their
growth as are Carbonic Acid, Ammonia and Water. For the proof
of this, it is only necessary to let experiments speak. The result
of observations made by the Prince of Salm-Horstmar in this mat-
ter, are especially interesting, and have established the following
facts: In a mixture containing all the various substances requisite
for the nourishment of plants, except. Silicic Acid, Oat plants re-
mained low, pale and dwarfish; without lime, they died after the
second leaf; without Potash and Soda, they reached only a height
of three inches; without Magnesia, they remained weak and lying
down; without Phosphoric or Sulphuric Acid, very weak and
without fruit, but upright and of normal formation; without Iron,
they remained very pale, weak and badly formed; and without
Manganese, they did not reach their full strength and showed but
a few flowers. Soda could not represent Potash in relation to the
strength of the plants; Magnesia could not represent Lime. When,
however, all of the mineral substances which vegetable life requires,
were present in their proper proportion, and in sufficient quantity,
the oat plant reaches a complete and pretty luxuriant develop-
ment, even with an entire absence of Humus, or any vegetable sub-
stance in the experimental mixture.

The Relations which exist between Plants and the Soil,
and the Manner in which these Relations influence
the greater or less fertility of the soil.

In the preceding chapter we gave a knowledge of the soil, as far
as it embraces its origin, and its consequent composition: we gave
farther, the principles upon which the nutrition of plants is based ;
all that is now left for our consideration is an inquiry into the re-

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