ON THE PHYSICAL VIEW OF NATURE. 181 



which this quantity can appear : they have thus exerted 

 themselves to find such general properties as belong 

 to all the forms in which energy presents itself to us. 

 They look upon energy as a much more general con- 

 ception than motion, and they think it a mistake to 

 try to narrow the conception so that it can only mean 

 the energy of attraction and repulsion (the astronomical 

 view), that between the ultimate particles of matter 

 (the atomic view), or the energy of various forms of 

 motion (the kinetic view). 



On the purely scientific side the mechanical view has 

 much to say for itself, and can point to achievements 

 which recommend it as a fruitful method of progress and 

 research, and as even more fruitful for the purposes of 

 instruction. It can claim to give in many instances an 

 apparently easy account of the common-sense or obvious 

 properties of bodies, and it gives this account in terms 

 which lend themselves to strict definition, to measure- 

 ment, calculation, and prediction of phenomena ; it 

 destroys all vagueness, and adopts, as it also stimulates, 

 mathematical, which is the most cogent kind of reasoning. 

 The kinetic theory of gases and the vibratory theory of 

 light are notable examples. The ideas of energy and the 

 remarkable properties of the lowest form of energy 

 i.e., of heat became gradually clearer and lost their 

 strangeness as potential energy came to be defined as 

 energy of position, available (or free) energy as the 

 kinetic energy of regular or orderly, unavailable (or 

 bound) energy as that of irregular or disorderly motion, 

 and when the strange quantity termed entropy, which 

 Clausius and Rankine strove in vain to bring home to 



