orrelation of physical forces.] My own experiments have
proved that it is the light of the sun, in contradistinction to the
heat, which occasions the decomposition of carbonic acid, furnishing
carbon to plants and oxygen to the atmosphere. But such is the relation
of the so-called imponderable principles of chemistry to each other, and
their mutual convertibility, that that which has disappeared in
performing its function as light may reappear as heat or electricity, or
in the production of some mechanical effect.

[Sidenote: The nature of food.] Food is used by all animals for the sake
of the force it thus contains, the remark applying to the carnivora as
well as the herbivora. In both cases the source of supply is the
vegetable kingdom, indirectly or directly. The plant is thus
indispensable to the animal. It is the collector and preserver of that
force the expenditure of which constitutes the special display of animal
life.

From this point of view, animals must therefore be considered as
machines, in which force obtained as has been described, is utilized.
The food they take, or the tissue that has been formed from it, is acted
upon by the air they breathe, and undergoes partial or total oxydation,
and now emerges again, in part as heat in part as nerve-force, in some
few instances in part as light or electricity, the force that originally
came from the sun.

[Sidenote: Cycle through which matter and force pass.] There is,
therefore, a cycle or revolution through which material particles
suitable for organization incessantly run. At one moment they exist as
inorganic combinations in the air or the soil, then as portions of
plants, then as portions of animals, then they return to the air or soil
again to renew their cycle of movement. The metamorphoses feigned by the
poets of antiquity have hence a foundation in fact, and the vegetable
and animal, the organic and inorganic worlds are indissolubly bound
together. Plants are reducing, animals oxydizing, machines. Plants form,
animals destroy.

Thus, by the light of the sun, the carbonic acid of the atmosphere is
decomposed--its oxygen is set free, its carbon furnished to plants. The
products obtained serve for the food of animals, and in their systems
the carbon is re-oxydized by the air they respire, and, resuming the
condition of carbonic acid, is thrown back into the atmosphere in the
breath, ready to be decomposed by the sunlight once more, and run
through the