8 FEA OMENTS 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 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 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, denied to the masses, exists among their atoms; 

 by the recoil and 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 



