s of thought. If by it be
meant that a particle of matter can never be deprived of its weight,
the assertion is correct; but the law which affirms the convertibility
of natural forces was never intended, in the minds of those who
understood it, to affirm that such a conversion as that here implied
occurs in any case whatever. As regards convertibility into heat,
gravity and chemical affinity stand on precisely the same footing.
The attraction in the one case is as indestructible as in the other.
Nobody affirms that when a stone rests upon the surface of the earth,
the mutual attraction of the earth and stone is abolished; nobody
means to affirm that the mutual attraction of oxygen for hydrogen
ceases, after the atoms have combined to form water. What is meant,
in the case of chemical affinity, is, that the pull of that affinity,
acting through a certain space, imparts a motion of translation of the
one atom towards the other. This motion is not heat, nor is the force
that produces it heat. But when the atoms strike and recoil, the
motion of translation is converted into a motion of vibration, which
is heat. The vibration, however, so far from causing the extinction
of the original attraction, is in part carried on by that attraction.
The atoms recoil, in virtue of the elastic force which opposes actual
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 pendulum, they oscillate, until
their motion is imparted to the surrounding aether; or, in other
words, until their heat becomes 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 here 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 by
gravity space must also intervene between the attracting bodies. When
they strike together 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, when communicated to
the proper nerves, announces itself as heat.

It was for