a distance from the earth's surface is a source of working-power;
because the body can be moved by the attraction, and in falling to
the earth can perform work. When it rests upon the earth's surface
it is not a source of power or energy, because it can fall no
further. But though it has ceased to be a source of energy, the
attraction of gravity still acts as a force, which holds the earth
and weight together.

The same remarks apply to attracting atoms and molecules. As long
as distance separates them, they can move across it in obedience to
the attraction, and the motion thus produced may, by proper appliances,
be caused to perform mechanical work. 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