*Storing of Energy through Chemical Means.*—A good example of storing
energy by chemical means is that of decomposing water with electricity. If
a current of electricity is passed through acidulated water in a suitable
apparatus (Fig. 82), the water separates into its component gases, oxygen
and hydrogen. These gases now have power (energy) which they did not
possess before they were separated. The hydrogen will burn in the oxygen,
giving heat; and if the two gases are mixed in the right proportions and
then ignited, they explode with violence. This energy was derived from the
electricity. It was stored by _decomposing_ the water.

[Fig. 82]


Fig. 82—*Storing 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.