and the Conservation 0} Energy. 533 



being at the same time transferred from one body to another 

 by an act of work. 



The fact is, that the conservation of energy has no real 

 physical meaning if potential energy is to be regarded solely as 

 a mathematically convenient abstraction, or " force-function." 

 It is useful enough in that capacity ; but when one comes to 

 think of things physically, one perceives that it must be a 

 good deal more. 



Dr. Schuster first remarked to me that my statement of the 

 conservation of energy, though possibly correct, was not iden- 

 tical with that currently accepted and expressed in familiar 

 equations, but was a slight (very slight) extension of it. I 

 think this is probably so ; but I believe it to be fully as axio- 

 matic as the other ; indeed I believe it to express the real 

 physical facts which give to those equations the whole of their 

 validity. 



The essential points on which I wish to lay stress are 

 these : — 



1st. That it is impossible to have a force without a body 

 which is exerting that force, and also without another body 

 on which the force is exerted, and which is exerting an equal 

 counter force. 



2nd. That it is not the force which does work, but the body 

 which is exerting the force that does it. 



3rd. That the thing which does the work must possess the 

 energy, and, hence, that energy, if existent at all, must be 

 possessed by a body. 



4th. That work is always done by one body upon another, 

 the second body gaining the energy which the first loses ; 

 whence, since they exert on each other equal opposite forces, 

 they must both move together over the same distance, i. e. 

 they must touch during the action. 



5th. That this latter argument may be worked with greater 

 ease conversely, and instead of proving that universal contact- 

 action holds wherever energy is conserved, one may prove 

 the conservation of energy by assuming universal contact- 

 action. 



[This is what I did in the paper in Oct. 1879. The argu- 

 ment is simpler in this form, merely because one thus arrives 

 at the conservation of energy in its (as I believe) true and 

 complete form, while the data assumed are simple and defi- 

 nite ; whereas the converse argument, though equally 

 conclusive in itself, may be eluded by denying the universal 

 applicability of the conservation statement.] 



6th. That energy can only be transferred from one body 

 to another by work being done by the first body upon the 

 second. 



