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In what sense, then, is the sun to be regarded as the
origin of the energy derivable from plants and animals ?
Let us try to give an intelligible answer to this question.
Water maybe raised from the sea-level to a high elevation,
and then permitted to descend. In descending 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 mill-stones, 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 ma-
chinery; 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 distri-
bution 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 qualita-
tive 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

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