12 FKAGMENTS OF SCIENCE. 



To produce motion by gravity space must also inter- 

 vene 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 

 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 to- 

 gether, the motion possessed by the masses, prior to im- 

 pact, 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 it& destruction refuse to be united in most philo- 

 sophic 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 destruc- 

 tion of force which was supposed to occur when un- 

 elastic 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 col- 

 lision. They would retain their motion afterwards, 

 though its direction might be changed; and it is only 

 when sensible motion is wholly or partly destroyed, 



