ON THE O-RIGIN OF FORCE. 469 



sion from dynamical laws, is no other than the well- 

 known dynamical theorem of the conservation of vis viva 

 (or of " energy," as some prefer to call it) supplemented to 

 save the truth of its verbal enunciation, by the introduc- 

 tion of what is called " potential energy," a phrase which 

 I cannot help regarding as unfortunate, inasmuch as it 

 goes to substitute a truism for the announcement of a 

 great dynamical fact. No such conservation, in the 

 sense of an identity of total amount of vis viva at all 

 times, and in all circumstances, in fact, exists. So far 

 as a system is maintained by the mutual actions and re- 

 actions of its constituent elements at a distance (i.e., by 

 force), vis viva may temporarily disappear, and be sub- 

 sequently reproduced between certain limits. Collision, 

 indeed, betwen its ultimate atoms, regarded as absolutely 

 rigid, and therefore inelastic (for that which cannot change 

 its figure can have no resilience), cannot take place without 

 producing a 'permanent destruction of it, which there 

 exists no means of repairing. And here we may remark 

 that, this being the case, to ascribe to such atoms any 

 magnitude becomes not only superfluous, but embarrass- 

 ing. The system of Boscovich has to be accepted in 

 its integrity, if absolute permanence is to be one of the 

 conditions insisted on ; and they come to be considered 

 as mere localizations of inertia and such other attributes, 

 including the centralization of force if any other than 

 this there be which belong to our notion of material 

 substance. The conservation of energy, then, is in effect 

 no conservation at all in any strict sense of the term, 

 unless so supplemented. It is a fact dynamically demon- 



