require a special condition of things for the releasing of them. Thus
they form a reserve of force, which can be held in restraint until the
conditions required for their release are realized.

The process can be illustrated more particularly by a single case. Water,
one of the substances absorbed by plants, is composed of oxygen and
hydrogen, which are united by an affinity of prodigious force. It is the
same with carbon and oxygen, in a compound called carbonic acid, which is
also one of the principal substances absorbed by plants from the air. Now
the heat and other emanations from the sun, acting upon these substances in
the leaves, forces the hydrogen and the carbon away from their strong bond
of union with oxygen, and sets the oxygen free, and then combines the
carbon and hydrogen into a sort of unwilling union with each other--a union
from which they are always ready and eager to break away, that they may
return to their union with the object of their former and much stronger
attachment--namely, oxygen; though they are so locked, by some mysterious
means, that they can not break away except when certain conditions
necessary to their release are realized.

_Hydrocarbons_.

The substances thus formed by a weak union of carbon with hydrogen are
called hydrocarbons. They comprise nearly all the highly inflammable
vegetable substances. Their being combustible means simply that they have
a great disposition to resume their union with oxygen--combustion being
nothing other than a more or less violent return of a substance to a union
with oxygen or some other such substance, usually one from which it had
formerly been separated by force--giving out again by its return, in the
form of heat, the force by which the original separation had been effected.

A compound formed thus of substances united by very weak affinities, so
that they are always ready to separate from each other and form new unions
under the influence of stronger affinities, is said to be in a state of
_unstable equilibrium._ It is the function of vegetable life to create
these unstable combinations by means of the force derived from the sun; and
the combinations, when formed, of course hold the force which formed them
in reserve, ready to make itself manifest whenever it is released. Animals
receive these substances into their systems in their food. A portion of
them they retain, re-arranging the components in some cases so as to form
new compounds, but