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thus apart and then liberate them. They clash together,
but, by virtue of their elasticity, they quickly recoil from
each other, and a sharp vibratory rattle succeeds their col-
lision. This experiment will enable you to figure to your,
mind a pair of clashing atoms. We have, in the first place,
a motion of the one atom toward the other—a motion of
translation, as it is usually called. But when the atoms
come sufficiently near each other, elastic repulsion sets in,
the motion of translation is stopped, and converted into a
motion of vibration. To this vibratory motion we give the
name of heat. Thus, three things are to be kept before
the mind—first, the atoms themselves ; secondly, the force
with which they attract each other; and thirdly, the mo-
tion consequent upon the exertion of that force. This mo-
tion must be figured first as a motion of translation, and
then as a motion of vibration ; and it is not until the mo-
tion reaches the vibratory stage that we give it“ the name
of heat. It is this motion imparted to the nerves thatpro-
duces the sensation of heat.

It would be useless to attempt a more detailed descrip-
tion of this molecular motion. After the atoms have
been thrown into this state of agitation, very complicated
motions must ensue from their incessant collision. There
must be a wild whirling about among the molecules. For
some time after the act of combination this action is so
violent as to prevent the molecules from coming together.
The water is maintained for a time in a state of vapor.
But as the vapor cools, or in other words loses its mo-
tion, the water molecules coalesce to form a liquid. And
now we are approaching a new and wonderful display of
force. N 0 one who had only seen water in its vaporous or
liquid form could imagine the existence of the forces now
to be referred to; for as long as the substance remains in
a liquid or vaporous condition, the play of these forces is
altogether masked and hidden. But let the heat be gradu-

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