1902.] Electrical Conduction in Gases, and Luminosity. 105 



with these conclusions. An intensity of cathode rays that gave a 

 brilliant light with "pure" thoria and with a mixture of 99 per cent, 

 thoria with 1 per cent, ceria was found by him to give practically no 

 light with "pure" ceria and with a mixture of equal parts of thoria and 

 ceria. It is improbable that Mr. Swinton dealt with pure thoria, as the 

 methods of purification practised at that time did not suffice to give a 

 pure product. That thoria alone gave more light under the influence 

 of cathode rays than in the Bunsen flame may have been due to the 

 fact that it was more intensely heated in the former case, the effect 

 produced by minute proportions of the exciting oxide being much 

 dependent on temperature, and greater the higher the temperature. 



The need of an explanation such as is here given of the luminosity 

 of oxides at high temperatures has, I know, occurred to others. My 

 colleague, Professor Ayrton, has long held such a view, and he informs 

 me that, in the course of conversation with him in 1897, Professor 

 Elihu Thomson gave it as his opinion that the brilliant incandescence 

 of lime in the oxy-hydrogen flame was not a mere high temperature 

 effect.* 



The phenomena of phosphorescence under the influence of the 

 electric discharge exhibited by the oxides and basic sulphates of the 

 rare earths, with which. Sir ¥m. Crookes's classical researches have 

 made us familiar, may be included in the same category with those 

 above discussed : if not due to recurrent oxidation, they may be 

 cases of recurrent polymerisation. Apparently the same oxides act 

 as excitants whether a flame or cathode rays be used. Sir Wm. 

 Crookes, it is well known, holds the view that yttria, lanthana, &c, 

 are characterised by definite phosphorescent spectra; on the other 

 hand, Lecoq de Boisbaudran has contended that the yttria and 

 gadolinite earths are not self-luminous. This latter view has recently 

 been confirmed by Baur and Marc.f According to these observers, 

 whilst the colourless oxides and salts of yttrium, gadolinium, and 

 lanthanum are not specifically luminescent, if mixed with minute 

 proportions of the earths which have coloured salts, viz., erbium, 

 neo- or praseodymium, they afford spectra such as have hitherto been 

 regarded as characteristic of themselves; and such spectra may be 

 equally well developed by using lime or calcium sulphate as diluents ; 

 yttria itself, however, fails to give a spectrum when mixed in small 

 proportion with lime. 



[If it should eventually be established that the emission spectra are 

 conditioned solely by " earths " which afford coloured salts, as the 

 emission spectra appear to be closely related to the absorption spectra, 

 dark lines in the one taking the place of bright lines in the other, it 



* Comp. 'Journal of the Society of Arts,' Feb. 10, 1899, p. 256; E. Pringslieim, 

 International Physical Congress at Paris, Keport 2. 

 t 'Deut. Cheni. Oes. Ber.,' 1901, p. 2460. 



