bon on a meadow, or an equal surface of forest
land, is independent of a supply of carbonaceous manure, but it
depends upon the presence of certain elements of the soil which in
themselves contain no carbon, together with the existence of
conditions under which their assimilation by plants can be effected.
We increase the produce of our cultivated fields, in carbon, by a
supply of lime, ashes, and marl, substances which cannot furnish
carbon to the plants, and yet it is indisputable,--being founded
upon abundant experience,--that in these substances we furnish to
the fields elements which greatly increase the bulk of their
produce, and consequently the amount of carbon.

If we admit these facts to be established, we can no longer doubt
that a deficient produce of carbon, or in other words, the
barrenness of a field does not depend upon carbonic acid, because we
are able to increase the produce, to a certain degree, by a supply
of substances which do not contain any carbon. The same source
whence the meadow and the forest are furnished with carbon, is also
open to our cultivated plants. The great object of agriculture,
therefore, is to discover the means best adapted to enable these
plants to assimilate the carbon of the atmosphere which exists in it
as carbonic acid. In furnishing plants, therefore, with mineral
elements, we give them the power to appropriate carbon from a source
which is inexhaustible; whilst in the absence of these elements the
most abundant supply of carbonic acid, or of decaying vegetable
matter, would not increase the produce of a field.

With an adequate and equal supply of these essential mineral
constituents in the soil, the amount of carbonic acid absorbed by a
plant from the atmosphere in a given time is limited by the quantity
which is brought into contact with its organs of absorption.

The withdrawal of carbonic acid from the atmosphere by the vegetable
organism takes place chiefly through its leaves; this absorption
requires the contact of the carbonic acid with their surface, or
with the part of the plant by which it is absorbed.

The quantity of carbonic acid absorbed in a given time is in direct
proportion to the surface of the leaves and the amount of carbonic
acid contained in the air; that is, two plants of the same kind and
the same extent of surface of absorption, in equal times and under
equal conditions, absorb one and the same amount of carbon.

In an atmosphere containing