toring energy by chemical means.* Apparatus for decomposing
water with electricity.

Energy is stored by chemical means by causing it to do work in opposition
to the force of chemism, or chemical affinity. Instead of changing the
form of bodies or moving them against gravity, it overcomes the force that
causes atoms to unite and to hold together after they have united. Since
in most cases the atoms on separating from any given combination unite at
once to form other combinations, we may say that _energy is stored when
strong chemical combinations are broken up and weak ones formed_. Energy
stored by this means becomes active when the atoms of weak combinations
unite to form combinations that are strong.(70)

*How Plants store the Sun's Energy.*--The earth's supply of energy comes
from the sun. While much of this, after warming and lighting the earth's
surface, is lost by radiation, a portion of it is stored up and retained.
The sun's energy is stored both through the force of gravity(71) and by
chemical means, the latter being the more important of the two methods.
Plants supply the means for storing it chemically (Fig. 83). Attention has
already been called to the fact (page 112) that growing plants are
continually taking carbon dioxide into their leaves from the air. This
they decompose, adding the carbon to compounds in their tissues and
returning the oxygen to the air. It is found, however, that this process
does not occur unless the plants are exposed to sunlight. The sunlight
supplies the energy for overcoming the attraction between the atoms of
oxygen and the atoms of carbon, while the plant itself serves as the
instrument through which the sunlight acts. The energy for decomposing the
carbon dioxide then comes from the sun, and through the decomposition of
the carbon dioxide the sun's energy is stored--becomes potential. It
remains stored until the carbon of the plant again unites with the oxygen
of the air, as in combustion.

[Fig. 83]

Fig. 83--*Nature's device* for storing energy from the sun. See text.

*The Sun's Energy in Food and Oxygen.*--Food is derived directly or
indirectly from plants and sustains the same relation to the oxygen of the
air as do the plants themselves. (The elements in the food have an
attraction for the oxygen, but are separated chemically from it.) On
account of this relation they have potential energy--the energy