12 FKAGMENTS OF SCIENCE. 



gether 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 molecula^ 

 oscillation which, when communicated to the proper 

 nerves, announces itself as heat. 



It was formerly universally supposed that by the col- 

 lision 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 destruc- 

 tion 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 to- 

 gether 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 con- 

 servation 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. Per- 

 fectly 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 gene- 

 rated. This always occurs in unelastic collision, the 

 heat developed being the exact equivalent of the sensible 



