MATTER AND FORCE, IN PHYSICAL PHENOMENA. 41 



to our own time. The terms, potential energy, actual 

 or dynanic energy, tension, vis viva, &c. (Spanne- 

 Krafte and lebendige Krafte) express conditions which 

 we must needs admit, by whatever name we desig- 

 nate them. I have already alluded to the phrase of 

 latent heat, and the difficulty it involves is the same 

 when it is applied to power or energy of any kind, 

 stored up in a quiescent state for development into future 

 action. We know from ample proof the infinite minute- 

 ness of the ultimate parts of matter, and also the enor- 

 mous energy of the forces on which their atomic actions 

 depend. But while recognising these truths as neces- 

 sary to any theory whatsoever, they give us no conception 

 in what such latent power consists ; whether in some 

 physical condition of the atoms themselves, or in some 

 unseen and nameless element of force to which matter 

 in all its forms is subjected. These questions and com- 

 plex relations, which it perplexes language to express, 

 might well be deemed inscrutable, were it not a wrong 

 to science, already advanced beyond so many seeming 

 limits, to suppose it incapable of reaching higher gene- 

 ralisations than those now attained. 



In looking to the chance of bringing the several 

 natural forces to the unity of some such higher law, 

 we cannot omit consideration of two of them which 

 stand in some sort apart from the rest, though ever 

 acting in concurrence with, or relation to, them. I 

 mean gravitation and the vital force powers widely 

 different in themselves, both unknown to us in their in- 

 trinsic nature and origin, but well-defined, respectively, 



