THE CONSTITUTION OF NATURE. 17 



contact, and in the recoil they are driven too far back. The 

 original attraction then triumphs over the force of recoil, 

 and urges the atoms once more together. Thus, like a pen- 

 dulum, they oscillate, until their motion is imparted to the 

 surrounding ether ; or, in other words, until their heat be- 

 comes radiant heat. 



In this sense, and in this sense only, is chemical affinity 

 converted into heat. There is, first of all, the attraction 

 between the atoms ; there is, secondly, space between them. 

 Across this space the attraction urges them. They collide, 

 they recoil, they oscillate. There is a change in the form 

 of the motion, but there is no real loss. It is so with the 

 attraction of gravity. To produce motion here, space must 

 also intervene between the attracting bodies : when they 

 strike 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 

 announces itself to the nerves as heat. 



It was formerly universally supposed that by the colli- 

 sion of unelastic bodies force was destroyed. Men saw, for 

 example, when two spheres of clay, or painter's putty, or 

 lead, were urged together, that 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 ex- 

 perienced 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 com- 

 plete 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. 



