g PR A GMENTS OF SCIENCE. 
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 formerly universally supposed that by the colli- 
sion 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 de- 
struction 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 destruc- 
tion refuse to be united in most philosophic minds. In 
the collision of elastic bodies, on the contrary, it was ob- 
served 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 ot 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 afterward, 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 extin- 
guished. This heat virtually declares that the property of 
elasticity, dejaied to the masses, exists among their atoms; 
by the recoifand oscillation of which the principle of con- 
servation is vindicated. 
But ambiguity in the use of the term " force " makes 
itself more and more felt as we proceed. "We have called 
