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THE CONSTITUTION OF NATURE. 31

tremors of heat to raise a weight, as is done through the
intermediation of an elastic fluid in the steam-engine, a
certain definite pOrtion of the molecular motion is de-
stroyed in raising the weight. In this sense, and this sense
only, can the heat be said to be converted into gravity, or,
more correctly, into potential energy of gravity. It is not
that the destruction of the heat has created any new
attraction, but simply that the old attraction has now a
power conferred upon it, of exerting a certain definite pull
in the interval between the starting-point of the falling
weight and its collision with the earth.

When, therefOre, writers on the conservation of energy
speak of tensions being “consumed” and “ generated,”
they do not mean thereby that old attractions have been an-
nihilated, and new ones brought into existence, but that,
in the one case, the power of the attraction to produce
motion has been diminished by the shortening of the dis-
tance between the attracting bodies, and that in the other
case the power of producing motion has been augmented
by the increase of the distance. These remarks apply to
all bodies, whether they be sensible masses or molecules.

Of the inner quality that enables matter to attract
matter we know nothing; and the law of conservation
makes no statement regarding that quality. It takes the
facts of attraction as they stand, and affirms only the con-
stancy of working-power. That power may exist in the
form of MOTION; or it may exist in the form of FORCE, with
distance to act through. The former is dynamic energy,
the latter is potential energy, the constancy of the sum of
both being affirmed by the law of conservation. The con-
vertibility of natural forces consists solely in transforma-
tions of dynamic into potential, and of potential into dy-
namic energy, which are incessantly going on. In' no other
sense has the convertibility of force, at present, any scien-
tific meaning.

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