, and then by virtue of
their resiliency, they recoil and quiver. To this quivering motion we
give the name of heat. This atomic vibration is merely the
redistribution of the motion produced by the chemical affinity; and
this is the only sense in which chemical affinity can be said to be
converted into heat. We must not imagine the chemical attraction
destroyed, or converted into anything else. For the atoms, when
mutually clasped to form a molecule of water, are held together by the
very attraction which first drew them towards each other. That which
has really been expended is the _pull_ exerted through the space by
which the distance between the atoms has been diminished.

If this be understood, it will be at once seen that gravity, as before
insisted on, may, in this sense, be said to be convertible into heat;
that it is in reality no more an outstanding and inconvertible agent,
as it is sometimes stated to be, than is chemical affinity. By the
exertion of a certain pull through a certain space, a body is caused
to clash with a certain definite velocity against the earth. Heat is
thereby developed, and this is the only sense in which gravity can be
said to be converted into heat. In no case is the _force_, which
produces the motion annihilated or changed into anything else. The
mutual attraction of the earth and weight exists when they are in
contact, as when they were separate but the ability of that attraction
to employ itself in the production of motion does not exist.

The transformation, in this case, is easily followed by the mind's
eye. First, the weight as a whole is set in motion by the attraction
of gravity. This motion of the mass is arrested by collision with the
earth, being broken up into molecular tremors, to which we give the
name of heat.

And when we reverse the process, and employ those tremors of heat to
raise a weight--which is done through the intermediation of an elastic
fluid in the steam-engine--a certain definite portion of the molecular
motion is consumed. In this sense, and in this sense only, can the
heat be said to be converted into gravity; or, more correctly, into
potential energy of gravity. Here the destruction of the heat has
created no new attraction; but the old attraction has conferred upon
it a power of exerting a certain definite pull, between the
starting-point of the falling weight and the earth.

When, therefore, writers on the conservation of energy speak