1 86 SPECULATIVE SCIENCE 



blow ; but nevertheless it is quite impossible that the relative 

 motion of two perfectly hard elastic bodies, such as Lucretius 

 imagined, can ever be altered by knocking one against the other. 

 Motion is essentially relative ; we only know that a body moves 

 by observing that it changes its position relatively to another. 

 When, therefore, treating of two isolated bodies only, we need 

 only speak of their relative velocity. The motion of the centre 

 of gravity of any system of bodies remains quite unaffected by 

 their collision one with another, and, in considering our two 

 isolated atoms, we may as well, for simplicity's sake, assume 

 the motion of their joint centre of gravity to be nilj though this 

 is not necessary to our argument. Moreover, it is found that 

 a certain quantity, sometimes called vis viva, sometimes the 

 kinetic energy of the system, is also constant after and before 

 any collision. This quantity is proportional for each body to 

 the mass of the body, and to the square of its velocity. It 

 must be remembered that we are now speaking of two simple 

 bodies which have only the properties of hardness and elasticity, 

 not being compressible, hot, or susceptible of vibration, so that 

 the transformation of energy due to motion into other forms of 

 energy such as heat is excluded by hypothesis. 



Now, in the case of two such bodies striking one another, 

 since their mass will not change, it is impossible that this quan- 

 tity should remain constant unless each body kept its own 

 velocity. The one cannot hand over a part of its velocity to 

 the other, for in that case the centre of gravity of the system 

 would acquire motion. The velocity of the two cannot in- 

 crease or, decrease simultaneously, or the vis viva of the system 

 would alter, so the bodies have no choice but to bound back or 

 to glance aside with their original velocity. In the latter case a 

 spinning motion might represent the vis viva, but this would not 

 be rest. If it be asked how it is that we do see the relative 

 motion of bodies alter after striking one another, we answer that 

 heat and other forms of energy have been found equivalent to 

 vis viva, which may therefore pass into these forms, and so 

 allow a change in the relative velocities of bodies. Had 

 Lucretius known this he would have answered, that heat can 

 only be equivalent to vis viva inasmuch as it substitutes the 

 motion of small parts for the motion of the whole ; this being 



