s themselves. As long as the rocks which
compose them can fall to a lower level, they possess potential energy,
which is converted into actual when the frost ruptures their cohesion
and hands them over to the action of gravity. The stone avalanches of
the Matterhorn and Weisshorn are illustrations in point. The hammer
of the great bell of Westminster, when raised before striking,
possesses potential energy; when it falls, the energy becomes dynamic;
and after the stroke, we have the rhythmic play of potential and
dynamic in the vibrations of the bell. The same holds good for the
molecular oscillations of a heated body. An atom is driven against
its neighbour, and recoils. The ultimate amplitude of the recoil
being attained, the motion of the atom in that direction is checked,
and for an instant its energy is all potential. It is then drawn
towards its neighbour with accelerated speed; thus, by attraction,
converting its potential into dynamic energy. Its motion in this
direction is also finally checked, and again, for an instant, its
energy is all potential. It once more 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 arrangement always
occurs, and to produce this change heat is consumed. Hence, a portion
only of the heat communicated 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 molecular dynamic energy,
developed at the moment of collision, is not the exact equivalent of
the sensible dynamic energy destroyed. 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 afterwards, 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
atmosp