CHAPTEE VI. 



THE PERSISTENCE OF FOKCE.* 



60. IN the foregoing two chapters, manifestations of 

 force of two fundamentally-different classes have been dealt 

 with the force by which matter demonstrates itself to us 

 as existing, and the force by which it demonstrates itself 

 to us as acting. 



Body is distinguishable from space by its power of 

 affecting our senses, and, in the last resort, by its opposition 

 to our efforts. We can conceive of body only by joining in 

 thought extension and resistance: take away resistance, 

 and there remains only space. In what way this force 

 which produces space-occupancy is conditioned we do not 



* Some explanation of this title seems needful. In the text itself are 

 given the reasons for using the word " force " instead of the word " energy ; " 

 and here I must say why I think " persistence " preferable to " conserva- 

 tion." Some two years ago (this was written in 1861) I expressed to my 

 friend Prof. Huxley, my dissatisfaction with the (then) current expression 

 " Conservation of Force : " assigning as reasons, first, that the word " con- 

 servation " implies a conserver and an act of conserving ; and, second, that 

 it does not imply the existence of the force before the particular manifesta- 

 tion of it which is contemplated. And I may now add, as a further fault, 

 the tacit assumption that, without some act of conservation, force would 

 disappear. All these implications are at variance with the conception to be 

 conveyed. In place of " conservation " Prof. Huxley suggested persistence. 

 This meets most of the objections ; and though it may be urged against it 

 that it does not directly imply pre existence of the force at any time mani- 

 fested, yet no other word less faulty in this respect can be found. In the ab- 

 sence of a word specially coined for the purpose, it seems the beat ; and as 

 such I adopt it. 



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