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 derived
through the plant from the sun. When a person eats the food and breathes
the oxygen, this energy becomes the possession of the body. It is then
converted into kinetic energy as the needs of the body require.