Conservation of Energy 21 



or that some unknown form was present 

 which he had not allowed for : thereby 

 getting a clue which, if followed up, he 

 would hope might result in a discovery. 



But the term " energy " itself, as used 

 in definite sense by the physicist, rather 

 involves a modern idea and is itself a 

 generalisation. Things as distinct from 

 each other as light, heat, sound, rotation, 

 vibration, elastic strain, gravitative separa- 

 tion, electric currents, and chemical affinity, 

 have all to be generalised under the 

 same heading, in order to make the law 

 true. Until " heat " was included in the 

 list of energies, the statement could not be 

 made ; and, a short time ago, it was some- 

 times discussed whether " life " should or 

 should not be included in the category of 

 energy. I should give the answer decidedly 

 No, but some might be inclined to say Yes ; 

 and this is sufficient as an example to show 

 that the categories of energy are not neces- 

 sarily exhausted ; that new forms may be 

 discovered ; and that if new forms exist, 



