When, for example, two atoms
of hydrogen unite with one of oxygen, to form water the atoms are
first drawn towards each other--they move, they clash, and then by
virtue of their resiliency, they recoil and quiver. To this
quivering motion we give the name of heat. Now this quivering
motion 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 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 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.
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