732 Mr. G. Stead on the Separation of 



all, elements always go to the pole that is of opposite sign to 

 the charge which chemical considerations indicate they carry. 

 Were Kayser's view of the matter correct, all these curious 

 facts, so far from being explained, must be supposed to be 

 merely the result of chance. Moreover, there would appear 

 to be a fatal objection to the view that the differences in the 

 spectra at the two electrodes are to be ascribed solely to 

 temperature differences. Take the case of carbon disulphide. 

 With the hammer-break no sulphur bands are seen at all ; 

 and this is true also for the Wehnelt interrupter so long as 

 the current is kept small. But as the temperature is 

 gradually raised by increasing the current, the band spectrum 

 of sulphur appears at the anode. Now if this be a pure tem- 

 perature effect, why do not the sulphur bands show first at 

 the cathode, since the latter is always hotter than the anode ? 

 This objection (if valid) also disposes of Kayser's explanation 

 of the lingering of spectra when the coil is reversed (see 

 previous paper), viz. that as the temperature at the electrodes 

 cannot change suddenly, the spectra cannot do so either. A 

 similar argument is afforded by hydrogen sulphide. Hence 

 the hypothesis (a) can hardly be regarded as otherwise than 

 inadequate ; in other words, some further explanation is 

 necessary. For this, I believe that (b) in the main suffices. 

 It is not asserted that there are no difficulties connected 

 with (b); it is claimed that (a) is insufficient, and that (b) is 

 in some way involved. 



To give a perfectly satisfactory and complete explanation 

 is a matter of extreme difficulty, and indeed is probably an 

 impossibility in the present state of our knowledge. It is 

 not known with any certainty what are the processes involved 

 in the production of spectra, and there are many questions 

 to be answered in connexion with the structure of a vacuum- 

 tube discharge. Why is there a definite positive column ? 

 What is the meaning of the negative glow ? Why do certain 

 parts of the tube give out little or no light (Faraday's and 

 Crookes's dark spaces) . So long as there is any doubt about 

 these points we cannot account in detail for the phenomena 

 described in this paper. 



In framing any explanatory hypothesis certain points must 

 be borne in mind :— 



(1) There is a kind of electrolysis or pseudo-electrolysis 

 taking place. 



(2) The intensity of the discharge is of fundamental im- 

 portance, probably because high intensity is associated with 



