"That golden chain,
Whose strong embrace holds heaven and earth and main."

Co-incidentally with the conversion of the mineral constituents of the
food of plants into organised structures--albumen, fibre, and such
like substances--the light, and the heat, and the various other forces
likewise suffer a change. Although the precise nature of the new force
into which they are converted is still a mystery--one, too, which may
never be revealed to us--still we know sufficient of it to satisfy
us that it can only exist in connection with organic or organised
structures. It is owing to its presence that the elements of these
structures (the natural state of which is mineral) are bound together
in what may be aptly designated a constrained state; or, as Liebig
aptly expresses it, like the matter in a bent spring. So long as the
organic structure retains its form, it will be a reservoir of latent
force--which will manifest itself in some form during the recoil of the
atoms of the matter forming the structure to their original mineral, or
statical condition: so the bent spring, when the pressure is removed,
returns to its original straight form.

_Animal Life._--The chief manifestation of the life of a plant is the
accumulation of force; very different are the functions of animal life.
It is only by the continuous _expenditure_ of force that the vitality of
animals is preserved; the heat of a man's body, his power of locomotion,
the performance of his daily toil, even his very faculty of thought, are
all dependent upon, and to a great extent proportionate to, the amount
of organised matter disorganised in his body. It is by the conversion
of this organised matter into its original mineral state of water,
carbonic acid, and ammonia, that the force originally expended in
arranging, through the agency of plants, its atoms, is again restored,
chiefly in the form of heat and animal motive power.

Animals, as a class, are completely dependent upon vegetables for
their existence. There is every reason to believe that the most lowly
organised beings in the scale of animal life, even those of so
simple a structure as to have been long regarded as vegetables or as
plant-animals, are incapable of organising mineral matter. The so-called
vegetative life of animals--for I believe the term to be exceedingly
inexact--is applied to their growth, that is, to the increase in their
weight. This increase takes place by their