274 . Prof. W. McF. On- on 



attraction between the electrons. We may finally suppose 

 the key made in two parts which can slide or telescope over 

 each other, and the mobility of the electrons in the direction 

 of the line joining them has free scope. 



At the same time these electron models are totally devoid of 

 mobility in any other direction, and exert no mechanical 

 force on any electrons but each other. 



Thus in this instance the same mechanism that provides for 

 " free mobility " transmits the mechanical force. The trans- 

 mission, nay the existence, of electrostatic force and the 

 existence of " free mobility " are, it seems evident, intimately 

 connected, and are explained or unexplained together. 



7. " Free mobility " of electrons is, as stated by the author 

 of the theory, one of its fundamental postulates. While 

 willing to admit that from one point of view he is justified in 

 endowing the aether and the electrons with any properties, 

 limited in number and consistent, necessary for the explanation 

 of observed phenomena, and to grant the possibility of such a 

 mobility as a thing unexplained, I venture to think that it is a 

 most mysterious property, immaterial, and, in that it possibly 

 demands something else than sether to perform the constructive 

 process which is required, almost superaefhereal ; that it 

 involves a far larger concession than does a rotational Iy 

 elastic aether ; that no model illustrating the properties of the 

 primordial medium can be complete as regards electrical 

 phenomena, unless it explains the nature of the constitution 

 of an electron, the origin of its "free mobility," and the 

 transmission of mechanical attraction or repulsion ; and that 

 in respect of this last the theory of electrons, in its present 

 stage of development, might be held to be little more satis- 

 factory than that of direct action at a distance. 



8. It is pointed out by Larmor * that if the range of mole- 

 cular action were comparable with the size of the element of 

 mass that is sensible to our powers of observation, the potential- 

 energy function of a strained material medium would involve 

 second as well as first differential coefficients of the displace- 

 ments ; in which case disturbances would still be transmitted 

 by the medium but not by the agency of simple elastic stress 

 definable in terms of surface tractions alone. In such a case, 

 our notions of force, and of elastic stress as well, would be 

 different from and wider than what they are j but the state 

 of things as regards the theory of elasticity of such bodies 

 would, I think, resemble the actual state in that we should 

 still succeed in describing the phenomena of motion as well 

 as of rest of the various parts of a material body as due to 



* ' Mthev and Matter,' p. 331. 



