﻿418 Dr. G. Johnstone Stoney on the "Electron" 



not really reach this point, I have not entered into that 

 question, nor do I desire to do so. 



It does not appear, therefore, that the mathematical in- 

 vestigation prevents us in any way from taking a general 

 view of the matter under the light of the positive teachings 

 of geology. We know that a cooling body must contract, 

 and that it will not contract equally throughout ; we know 

 that the surface-features demonstrate a great amount of con- 

 traction, and whether other causes of contraction have operated 

 or not, we are at liberty to look on the cooling as one of 

 the most potent ; and geological observations must teach us 

 how far and in what manner such contractions have been 

 effected. 



XLIX. Of the "Electron," or Atom of Electricity. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, 



PROFESSOR EBERT, in his paper on the Heat of 

 Dissociation in last month's (September) Phil. Mag.. 

 says on p. 332 : — " Yon Helmholtz, on the basis of Faraday's 

 Law of Electrolysis, was the first to show in the case of 

 electrolytes that each valency must be considered charged 

 with a minimum quantity of electricity, the ' valency-charge/ 

 which like an electrical atom is no longer divisible." 



Now I had already twice pointed out this remarkable fact : 

 first, at the Belfast meeting of the British Association in 

 August 1874, in a paper " On the Physical Units of Mature," 

 in which I called attention to this minimum quantity of 

 electricity as one of three * physical units, the absolute 

 amounts of which are furnished to us by Mature, and which 

 may be made the basis of a complete body of systematic 

 units in which there shall be nothing arbitrary. This same 

 paper was again read before the Royal Dublin Society on 

 the 16th of February, 1881, and is printed both in the Pro- 

 ceedings of that meeting and in the Phil. Mag. of the 

 following May. 



* The two other units being (1) the " Maxwell* of velocity, that 

 velocity which connects electrostatic with electromagnetic units in a 

 medium of which the inductive capacity is unity, and which, under the 

 electromagnetic theory of light, is also the maximum velocity of light ; 

 and (2) the " Newton " of gravitation, that coefficient of universal gravi- 

 tation of the amount of which Professor Boys has lately made so accurate 

 a determination. 



