THE CONSTITUTION OF NATURE 17 



perfect the elasticity the more complete being the restitu- 

 tion. 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 princi- 

 ple of conservation is vindicated. 



But ambiguity in the use of the term "force" makes 

 itself more and more felt as we proceed. We have called 

 the attraction of gravity a force, without any reference to 

 motion. A body resting on a shelf is as much pulled by 

 gravity as when, after having been pushed off the shelf, 

 it falls toward the earth. We applied the term force also 

 to that molecular attraction which we called chemical affin- 

 ity. When, however, we spoke of the conservation of 

 force, in the case of elastic collision, we meant neither a 

 pull nor a push, which, as just indicated, might be exerted 

 upon inert matter, but we meant force invested in motion 

 the vis viva, as it is called, of the colliding masses. 



Force in this form has a definite mechanical measure, 



