cially the leaves, decompose carbonic acid,
one of the ingredients of the atmospheric air. This substance is
composed of two elements, carbon and oxygen; the former is appropriated
by the plant, and enters into the composition of elaborated or
descending sap, from which forthwith organic products, such as starch,
sugar, wood fibre, acids, and bases are made. The other element, the
oxygen, is for the most part refused by the plant, and returns to the
air. As the process of decomposition goes on, new portions of carbonic
acid are presented through mechanical movements, the trembling of the
leaf, breezes, and currents rising from the foliage warmed by the solar
beams giving place to other cool currents that set in below.

The action of a plant upon the air is therefore the separation of
combustible material from that medium. Carbon is thus obtained from
carbonic acid; from water, hydrogen. Plant life is chemically an
operation of reduction, for in like manner ammonia is decomposed into
its constituents, which are nitrogen and hydrogen; and sulphuric and
phosphoric acids, which like ammonia, may have been brought into the
plant through its roots in the form of salt bodies, are made to yield up
the oxygen with which they had been combined, and their sulphur and
phosphorus, combustible elements, are appropriated.

[Sidenote: Composition and resolution of matter and force.] Every plant,
from the humblest moss to the oak of a thousand years, is thus formed by
the sun from material obtained from the air--combustible material once
united with oxygen, but now separated from that body. It is of especial
importance to remark that in this act of decomposition, force, under the
form of light, has disappeared, and become incorporated with the
combustible, the organizing material. This force is surrendered again,
or reappears whenever the converse operation, combination with oxygen,
occurs.

Vegetable products thus constitute a magazine in which force is stored
up and preserved for any assignable time. Hence they are adapted for
animal food and for the procuring of warmth. The heat evolved in the
combustion of coal in domestic economy was originally light from the sun
appropriated by plants in the Secondary geological times, and locked up
for untold ages. The sun is also the source from which was derived the
light obtained in all our artificial operations of burning gas, oil,
fat, wax, for the purposes of illumination.

[Sidenote: C