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THE CONSTITUTION OF NATURE. 17

contact, and in the recoil they are driven too far back. The
original attraction then triumphs over the force of recoil,
and urges the atoms once more together. Thus, like a pen-
dulum, they oscillate, until their motion is imparted to the
surrounding ether; or, in other words, until their heat be-
comes radiant heat.

In this sense, and in this sense only, is chemical affinity
converted into heat. There is, first of all, the attraction
between the atoms ; there is, secondly, space between them.
Across this space the attraction urges them. They collide,
they recoil, they oscillate. There is a change in the form
of the motion, but there is no real loss. It is so with the
attraction of gravity. To produce motion here, space must
also intervene between the attracting bodies: when they
strike motion is apparently destroyed, but in reality there
is no destruction. Their atoms are suddenly urged together
by the shock; by their own perfect elasticity these atoms
recoil; and thus is set up the molecular oscillation which
announces itself to the nerves as heat.

It was formerly universally supposed that by the colli-
sion of unelastic bodies force was destroyed. Men saw, for
example, when two spheres of clay, or painter’s putty, or
lead, were urged together, that the motion possessed by
the masses prior to impact was more or less annihilated.
They believed in an absolute destruction of the force of
impact. Until recent times, indeed, no difficulty was ex-
perienced in believing this, whereas, at present, the ideas
of force and its destruction refuse to be united in most
philosophic minds. . In the collision of elastic bodies, on the
contrary, it was observed that the motion with which they
clashed together was in great part restored by the resiliency
of the masses, the more perfect the elasticity the more com-
plete being the restitution. This led to the idea of perfectly
elastic bodies—bodies competent to restore by their recoil
the whole of the motion which they possessed before impact.

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