262 Prof. J. G. MacGresor on the 



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of complete transformation, in the following words : — " If one 

 body exert on another a certain force through a certain dis- 

 tance, the same work is done on it whether the former body 

 lose kinetic or potential energy in doing the work : while the 

 effect produced in the latter body will in general be a change 

 both in its motion and its state of strain, i. <?., both in its 

 kinetic and its potential energy. Thus, whether the former 

 lose kinetic or potential energy, the latter will in general gain 

 both, or transference of energy will, in general, involve partial 

 but not complete transformation." Prof. Lodge thinks, how- 

 ever, the difficulty may be met. For he observes in con- 

 clusion : — " But I venture to say that on any view of the 

 identity of energy the bit of kinetic which it first attains is a 

 bit of energy that has been transmitted through the elastic 

 stress of the spring, and that just as the second half of the 

 energy must admittedly exist in the spring before it can reach 

 the mass, so the first half has already passed through the 

 spring and has reached the mass only after transmutation, 

 although the transformation is disguised while the trans- 

 ference is obvious." The difficulty therefore is overcome only 

 by assuming bodies to consist of massive molecules connected 

 by a massless but elastic medium. Compare with this the 

 following from my paper (p. 141) : — " If we assume the 

 particles [of bodies] to be rigid they can of course have kinetic 

 energy only. If the medium be assumed to have no inertia, 

 its elements can have potential energy only. Hence if both 

 assumptions be made, transference of energy between the 

 particles and the medium must involve complete transforma- 

 tion, while transference from element to element of the medium 

 must occur without transformation. If, however, both the 

 particles and the medium be assumed to have both inertia 

 and elasticity, the transference of energy will, in general, in- 

 volve only partial transformation, whether it occur between 

 the elements of the medium or between the particles and the 

 medium." 



It will thus be obvious that the necessity, which Prof. Lodge 

 here acknowl edges, of making definite assumptions as to 

 acting mechanism, wdien he endeavours to follow his contact- 

 action energy and to make its transformations agree with 

 those prescribed by his thesis, is in entire agreement with what 

 I have pointed out above, viz., that the assumption of contact- 

 action alone is not sufficient for the purpose of completely 

 localizing energy. Without being conscious of the fact, he 

 assumes, not only contact-action, but also that bodies consist 

 of massive and rigid particles, and that the medium through 



