LUCRETIUS AND THE ATOMIC THEORY 187 



the very answer given by Leibnitz to the above objection, urged 

 as fatal to the doctrine of vis viva which he had enounced. 



It may be seen that our two bodies need not continue to 

 move in straight lines after striking ; they may glance off, so 

 as to spin round. The vis viva, or energy, will be perfectly 

 represented by the velocity of the rotating masses, and the 

 centre of gravity may remain undisturbed. When two actual 

 bodies strike and come to rest, it is probable that their atoms 

 do acquire some periodic motion, such as spinning, which mo- 

 tion produces the appearance of heat, but is on so small a scale 

 as to be otherwise invisible to our senses. When we consider 

 the collision of a multitude of bodies, innumerable changes 

 may take place in their relative velocities without violating the 

 two principles, that the motion of the centre of gravity and the 

 energy of the system shall both remain unchanged. Among 

 these combinations some will admit of one or more parts of the 

 system coming wholly to rest, contrary to Lucretius's views, 

 but the following consideration shows that it is difficult to see 

 how this would be brought about if we adhered strictly to his 

 assumption, that the motion of a hard mass is the sole form of 

 energy. He almost unconsciously, and certainly without any 

 express statement, assumes elasticity as a property of his atoms, 

 which he describes as rebounding one from another ; but, revert- 

 ing to our two hard bodies, if they do strike and rebound they 

 must gradually slacken speed, stop for an inconceivably short 

 time, and 'then gradually resume their pace in an opposite 

 direction, so that, if they rebound, they must stop and pass 

 through all speeds intermediate between zero and their original 

 velocity ; so that if we admit no form of energy but a hard mass 

 in motion, we must conclude that no two bodies ever could 

 strike one another, and yet, as neither we nor Lucretius have 

 assumed anything to keep them apart, we find ourselves in a 

 droll dilemma, which seems to prove the impossibility of the 

 existence of a universe containing simple hard atoms in motion. 

 We moderns jump out of the difficulty at once by saying that 

 the hard bodies are elastic, and elasticity is a form of energy, so 

 that the energy or vis viva which at one time was represented 

 by the body in motion, is at another time represented by the 

 potential energy of elasticity. Lucretius would have shaken. 



