16 FRAGMENTS OF SCIENCE 



inal 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 

 becomes radiant heat. 



In this sense, and in this sense only, is chemical affin- 

 ity converted into heat. There is, first of all, the attrac- 

 tion between the atoms; there is, secondly, space between 

 them. Across this space the attraction urges them. They 

 collide, they recoil, they oscillate. There is here 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 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 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 mo- 

 tion possessed by the masses, prior to impact, was more 

 or less annihilated. They believed in an absolute destruc- 

 tion 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 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 



