278 Prof. J. J. .Thomson on the Genesis of the Jons 



you do not obtain ponderomotive forces, but only sethereal 

 forces. (For the distinction between sethereal forces and 

 electric forces, cf. 'iEther and Matter,' p. 97.) You will, in 

 reply, ask for a model of an electron which will enable you 

 to introduce the virtual displacements of the different elements 

 of the surface of its nucleus, on which you can locate the 

 action of the mechanical force. And you will in fact anti- 

 cipate that it cannot be given : that all that one can definitely 

 picture is the displacement of the electron as an aggregate. 

 The electron has to be postulated as able to traverse the 

 medium like a shadow without displacing it. It seems to be 

 an undue limitation of possibilities when you assume that the 

 nucleus must have a definite surface to which mechanical 

 traction can be applied : a vortex-ring circulating round a 

 vacuous core, in facta vortex atom, is a structure which forms, 

 or may be held to form, a case in point. This, as you say, 

 amounts to postulating " free mobility " without explaining 

 it : but the permanence and mobility of the strain-configu- 

 ration as a whole, which really constitutes the electron, are, 

 I think, intelligible in the model except in the immediate 

 neighbourhood of the singular point at its centre, so that on pos- 

 tulating mobility for the unknown core all else is explicable. 

 As regards the remark towards the end of § 9, and in the 

 footnote to § 2, that the electrification might be spread conti- 

 nuously through the medium instead of being concentrated in 

 electrons, that sort of generality of course destroys the physical 

 simplicity of the scheme, which makes an electron an isolated 

 structural singularity in an unmodified aether, and it is not 

 called for by any facts. I admit that the representation which 

 you discuss in § 6 is, as it stands, so imperfect as to be mis- 

 leading rather than helpful. In 'iEther and Matter' §37 

 should be deleted : see § 73.] 



XXV. The Genesis of the Ions in the Discharge of Electricity 

 through Gases. By J. J. Thomson, 31. A., F.R.S.* 



IN many cases of discharge through gases, the ions have not 

 to be supplied by the action of the electric field itself, 

 but are produced by external agents such as Eontgen rays, 

 radiation from uranium and other radio-active substances, the 

 action of ultra-violet light on metals, incandescent filaments, 

 &c. In addition, however, to such cases as these we have the 

 very important series of phenomena which includes the spark- 

 discharge and the ordinary discharge through vacuum-tubes, 



* Communicated by the Author, having been read before the Cam- 

 bridge Philosophical Society, Feb. 5, 1900. 



