g it may be made to assume various forms--to fall in
cascades, to spurt in fountains, to boil in eddies, or to flow
tranquilly along a uniform bed. It may, moreover, be caused to set
complex machinery in motion, to turn millstones, throw shuttles, work
saws and hammers, and drive piles. But every form of power here
indicated would be derived from the original power expended in raising
the water to the height from which it fell. There is no energy
_generated_ by the machinery: the work performed by the water in
descending is merely the parcelling out and distribution of the work
expended in raising it. In precisely this sense is all the energy of
plants and animals the parcelling out and distribution of a power
originally exerted by the sun. In the case of the water, the source
of the power consists in the forcible separation of a quantity of the
liquid from a low level of the earth's surface, and its elevation to a
higher position, the power thus expended being returned by the water
in its descent. In the case of vital phenomena, the source of power
consists in the forcible separation of the atoms of compound
substances by the sun. We name the force which draws the water
earthward 'gravity,' and that which draws atoms together 'chemical
affinity'; but these different names must not mislead us regarding the
qualitative identity of the two forces. They are both _attractions_;
and, to the intellect, the falling of carbon atoms against oxygen
atoms is not more difficult of conception than the falling of water to
the earth.

The building up of the vegetable, then, is effected by the sun,
through the reduction of chemical compounds. The phenomena of animal
life are more or less complicated reversals of these processes of
reduction. We eat the vegetable, and we breathe the oxygen of the
air; and in our bodies the oxygen, which had been lifted from the
carbon and hydrogen by the action of the sun, again falls towards
them, producing animal heat and developing animal forms. Through the
most complicated phenomena of vitality this law runs: the vegetable
is produced while a weight rises, the animal is produced while a
weight falls. But the question is not exhausted here. The water
employed in our first illustration generates all the motion displayed
in its descent, but the _form_ of the motion depends on the character of
the machinery interposed in the path of the water. In a similar way,
the primary action of the s