256 Prof. J. G. MacGreo-or on ih 



O 



would then be obvious that the potential energy must be con- 

 sidered to be resident in the medium. But until some such 

 hypothesis becomes axiomatic, no localization of potential 

 energy is possible. 



It should be noted here that the adoption of the hypothesis 

 of contact-action alone does not enable us to localize potential 

 energy. As an axiom of acting mechanism it is incomplete ; 

 and it involves only the residence of energy in some body or 

 other *. Tf we are to know in what body the energy resides, 

 the axiom of acting mechanism must be made sufficiently 

 complete. Thus, as just stated, if we assume, in addition to 

 mere contact-action, that material bodies consist of rigid 

 particles, and that the medium is elastic, the potential energy 

 in the case of the raised stone must be considered to be 

 resident in the medium. If, however, we assume bodies to 

 consist of elastic particles, then the potential energy must be 

 regarded as possibly resident partly in the medium and partly 

 in the particles of the earth and stone. Prof. Lodge does not 

 seem to realize this; for though he has proposed no hypothesis 

 as to acting mechanism beyond that of contact-action, he has 

 no hesitation in saying dogmatically (for he makes no attempt 

 to justify the assertion) that in the case of the raised stone 

 and in similar cases f the potential energy resides in the 

 medium J. 



Prof. Lodge claims that the law of the conservation of 

 contact-action energy is more precise and definite than the 

 ordinary law " because it is the law not only of conservation, 

 but of identity." As to what is meant by its being a law of 

 identity, he gives us two statements, of which we may con- 

 sider the later first. " My proposition," he says, in his 



* I use the word body here in Prof. Lodge's general sense as applicable 

 to a portion of the medium as well as to a material body. When, accord- 

 ing to the contact-action conception, potential energy is regarded as 

 resident in a body, the body must be considered to be an elastic body in a 

 state of strain (p. 20). Since it is thus considered to consist of parts 

 capable of relative motion, it is a system of relatively movable parts. 

 Thus, according to Dr. Lodge's conception, potential energy is resident in 

 systems just as truly as it is according to the ordinary conception, only the 

 systems in the one case are small, while in the other they may be large. 

 See Mr. E. T. Dixon's letter to 'Nature,' vol. xlviii. p. 102, and Dr. Lodge's 

 reply, p. 126. 



t Prof. Lodge is not so precise in his localization in all cases. In the 

 case of the bent bow, even the ordinary conception of potential energy 

 would admit of our localizing it " in the bow," and in the case of the 

 gunpowder that conception would give us a more definite localization 

 than "in the powder." 



\ Probably my meaning in the statement quoted by Prof. Lodge on 

 p. 16, footnote, will now be clear. 



