d 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 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 n