still unstable. These they use in constructing the
tissues of the animal system, and some they reserve for future use. As
fast as they require the heat and the force which are stored in them they
expend, them, thus recovering the force which was absorbed in the formation
of them, and which now, on being released, re-appears in the three forms of
_animal heat, muscular motion_, and _cerebral_ or _nervous energy_.

There are other modes besides the processes of animal life by which the
reserved force laid up by the vegetable process in these unstable compounds
may be released. In many cases it releases itself under ordinary exposures
to the oxygen of the atmosphere. A log of wood--which is composed chiefly
of carbon and hydrogen in an unstable union--lying upon the ground will
gradually _decay_, as we term it--that is, its elements will separate from
each other, and form new unions with the elements of the surrounding air,
thus returning to their normal condition. They give out, in so doing, a low
degree of heat, which, being protracted through a course of years, makes
up, in the end, the precise equivalent of that expended by the sun in
forming the wood--that is, the power expended in the formation of the wood
is all released in the dissolution of it.

This process may be greatly accelerated by heat. If a portion of the wood
is raised in temperature to a certain point, the elements begin to combine
with the oxygen near, with so much violence as to release the reserved
power with great rapidity. And as this force re-appears in the form of
heat, the next portions of the wood are at once raised to the right
temperature to allow the process of reoxidation to go on rapidly with them.
This is the process of combustion. Observations and experiments on decaying
wood have been made, showing that the amount of heat developed by the
combustion of a mass of wood, though much more intense for a time, is
the same in _amount_ as that which is set free by the slower process of
re-oxidation by gradual decay; both being the equivalent of the amount
absorbed by the leaves from the sun, in the process of deoxidizing the
carbon and hydrogen when the wood was formed.

The force imprisoned in these unstable compounds may be held in reserve for
an unlimited period, so long as all opportunity is denied them of returning
the elements that compose them to their original combinations. Such a case
occurs when large beds of vegetable substances are b