AND THE NATURE OF FORCE. 505 



sense. Thus, in the illustration given in case one of the falling weight, 

 it is possible to call its distance from the earth's surface that due to 

 potential energy, simply because the influence of all other bodies in 

 the universe, on the motion of the falling body, is inappreciable ; but 

 strictly speaking, they influence its motion, and it is falling towards 

 the same point of the earth's surface at no two successive instants of 

 time. 



We see thus that this term potential energy, which can only be 

 used under conditions which do not obtain generally in nature, and 

 which obtain with only an approach to strictness in a very few cases, 

 has no right to be included in the expression of a law which professes 

 to represent not an approximate, but an absolute truth of nature. 



We have also seen, by the foregoing example of six bodies whose 

 motions are governed by a law which actually holds in nature, that 

 the chances are almost infinitely great against the conservation of 

 actual energy, even in the sense of periodic restorations. 



What must our conclusion hel Is it not that the principle of 

 conservation cannot be predicated of the material universe if the 

 observed laws of force are laws of action at a distance 1 On the other 

 hand, the conservation of dynamic or actual energy is a necessary 

 result of the action by contact theory, if we admit the rigidity of the 

 atom. 



It may be asked. How are the phenomena of latent heat, of lique- 

 faction and evaporation, to be explained on these principles. We 

 have not to go far for an answer. Liquidity and gaseity are modes 

 of motion of matter as well as heat. We do not expect the sensible 

 vibrations of an elastic ball to afiect the thermometer, neither should 

 we expect the motions of liquidity and gaseity to do so. In a liquid 

 the molecules are gliding around each other in all directions. In a 

 gas the molecules are flying about in straight lines in all directions. 

 To these motions is due the pressure in both cases. Since the pressure 

 is equal in all directions at a given point, the inference is that the 

 molecules are moving in all directions. 



Whenever a motion appears in a heated body which itself is not 

 such a motion as would affect the thermometer, a corresponding 

 amount of heat necessarily disappears. Just as in a steam engine, 

 heat disappears as the motions of the parts of the engine and the 

 attached machinery increase. If the machinery be applied to doing 

 work, that is, to setting other bodies in motion, a corresponding 

 amount of heat disappears. 



