232 Force, Energy, and Will 



of different kinds. The conception of the same or different 

 bodies being successively affected, and acting successively in 

 different manners, with a quantitative equivalence between the 

 modes of their affection and activity, seems a sufficient con- 

 ception to apply to the mechanism and action of a moving 

 locomotive steam-engine, etc. — one as consonant with facts 

 as is the conception of a force which is transformed from 

 heat into motion. 



On the other hand, to speak of force persisting and being 

 transformed, favours the conception of force as some objec- 

 tively existing thing which really passes out of one body into 

 another, and has a positive substantial existence. Thus it is 

 sometimes said that a coal-bed contains the heat and light 

 of the sun of bygone ages shut up within it, like enchanted 

 knights, and once more to be set free upon that coal's com- 

 bustion. But does it really contain them ? Surely neither 

 that light nor that heat is in the coal, nor are they in the 

 oxygen with which that coal may one day combine; they 

 are activities resulting from the rapid combustion of those 

 bodies. 



It may perhaps be replied that there has in fact been no 

 intention of really inculcating the substantial existence of 

 force, and that the language used has been employed simph 

 as a convenient way of speaking. Now, I most willingh' 

 concede the reasonableness of making use of the conception 

 of such an entity as ' force ' as a working hypothesis, pro- 

 vided care be taken that its real nature be not misunder- 

 stood ; but if by that term not a real existence, but an ideal 

 abstraction be, in fact, what is meant, then it would surely 

 be better not to speak of its ' persistence,' and a fortiori of 

 its 'transformation,' since nothing can be 'transformed' 

 which does not really exist. 



It will perhaps be replied that if we ought not to speak 

 without qualification of force, we ought not so to speak even 



