260 Prof. J. G. MacGregor on the 



energy, or potential energy, the other body will gain, or lose, 

 potential, or kinetic, respectively, i. e., that in these cases 

 there will be some transformation. In order to prove com- 

 plete transformation, it would be necessary to show that the 

 potential or kinetic energy lost or gained by the one body 

 was equal to the kinetic or potential gained or lost respectively 

 by the other. Thus even if we admit the validity of the 

 argument without criticism, all that it proves is that, except 

 in certain specified cases in which transference occurs with- 

 out transformation, transference always involves some trans- 

 formation, which is equivalent to the affirmative part of my 

 conclusion that "transference of energy will in general involve 

 partial but not complete transformation." 



Besides this demonstration Prof. Lodge gives a discussion 

 of two examples. The first, the loaded air-gun with its 

 muzzle plugged, is an example of the transformation of 

 potential energy during transference ; the second, the impact 

 of a couple of equal elastic rods moving end-on, exemplifies 

 the transformation of kinetic energy. 



With regard to the former, Prof, Lodge says : — " The 

 compressed air has potential energy; on its release its energy 

 is transferred to the moving wad, which instantaneously 

 hands it on to the air near the muzzle, compressing it, and 

 thus retransforming itself into the potential form/'' This 

 seems to me a very inadequate account of what occurs. It 

 assumes the wad to be a rigid body. As it is not, however, 

 the first effect of the expansion of the compressed air must be 

 to compress the adjacent end of the wad. But compression 

 involves the relative motion of its parts. Hence during the 

 first small expansion of the compressed air the wad must 

 simultaneously gain both potential and kinetic energy ; and 

 therefore the potential energy lost by the compressed air has 

 not been completely transformed in transference. If the wad- 

 be " dematerialized " and the example treated from the point 

 of view of the kinetic theory of gases, the energy of the com- 

 pressed air is practically wholly kinetic ; and if, as Prof. 

 Lodge says, the collision of two particles is most conveniently 

 thought of in terms of elasticity, the question which has to 

 be settled is Avhat transformation occurs during the collision 

 of two elastic particles. Treated in this way, the example thus 

 resolves itself into the second example * — the impact of two 

 elastic rods. 



* I need not make further reference to Dr. Lodge's discussion of this 

 example from the point of view of the kinetic theory of gases, partly for the 

 reason mentioned and partly because a portion of it is said to be " true 

 in one sense, but not a final or complete statement." 



