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28 FRAGMEN TS OF SCIENCE.

potential. It is then drawn toward its neighbor with
accelerated speed, thus, by attraction, converting its poten-
tial into dynamic energy. Its motion in this direction is
also finally checked, and, for an instant, again its energy is
all potential. It again retreats, converting, by repulsion,
its potential into dynamic energy, till the latter attains a
maximum, after which it is again changed into potential
energy. Thus, what is true of the earth, as she swings to
and fro in her yearly journey round the sun, is also true of
her minutest atom. We have wheels within Wheels, and
rhythm within rhythm.

When a body is heated, a change of molecular arrange-
ment always occurs, and to produce this change heat is
consumed. Hence, a portion only of the heat communi-
cated to the body remains as dynamic energy. Looking
back on some of the statements made at the beginning of
this article, now that our knowledge is more extensive, we
see the necessity of qualifying them. When, for example,
two bodies clash, heat is generated; but the heat, or molec-
ular dynamic energy, developed at the moment of collision,
is not the equivalent of the sensible dynamic energy de-i
stroyed. The true equivalent is this heat, plus the potential
energy conferred, upon the molecules by the placing of
greater distances between them. This molecular potential
energy is afterward, on the cooling of the body, converted
into heat.

Wherever two atoms capable of uniting together by
their mutual attractions exist separately, they form a store
of potential energy. Thus our woods, forests, and coal-
fields on the one hand, and our atmospheric oxygen on the
other, constitute a vast store of energy of this kind—vast,
but far from infinite. We have, besides our coal-fields,
bodies in the metallic condition more or less sparsely dis
tributed in the earth’s crust. These bodies can be oxidized,
and hence are, so far as they go, stores of potential energy.

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