merly universally supposed that by the collision of
unelastic bodies force was destroyed. Men saw, for example, that when
two spheres of clay, painter's putty, or lead for example, were urged
together, 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
experienced 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 complete 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--and this
again to the idea of the _conservation_ of force, as opposed to that
destruction of force which was supposed to occur when unelastic bodies
met in collision.

We now know that the principle of conservation holds equally good with
elastic and unelastic bodies. Perfectly elastic bodies would develop
no heat on collision. They would retain their motion afterwards,
though its direction might be changed; and it is only when sensible
motion is wholly or partly destroyed, that heat is generated. This
always occurs in unelastic collision, the heat developed being the
exact equivalent of the sensible motion extinguished. This heat
virtually declares that the property of elasticity, denied to the
masses, exists among their atoms; by the recoil and oscillation of
which the principle of conservation is vindicated.

But ambiguity in the use of the term 'force' makes itself more and
more felt as we proceed. We have called the attraction of gravity a
force, without any reference to motion. A body resting on a shelf is
as much pulled by gravity as when, after having been pushed off the
shelf, it falls towards the earth. We applied the term force also to
that molecular attraction which we called chemical affinity. When,
however, we spoke of the conservation of force, in the case of elastic
collision, we meant neither a pull nor a push, which, as just
indicated, might be exerted upon inert matter, but we meant force
invested in motion--the _vis viva_, as it is called, of the colliding
masses.