of the Bare Earths when heated in Vacuo by Cathode Bays. 119 



and the thoria plus ceria with a yellowish light. The fluorescence in 

 each case was much less bright when the oxides were white than when 

 they had become discoloured by previous bombardment. With very 

 higlj exhaustions the thoria plus ceria fluoresced the more brightly ; at 

 lower exhaustions the pure thoria gave the brighter fluorescence. 



On the suggestion of Mr. W. Mackean, the tube was pumped up to 

 a very high vacuum and oxygen admitted. A similar experiment was 

 made with hydrogen, the tube being completely filled with the gas, 

 and then pumped to the proper degree of exhaustion. Though at low 

 exhaustions these gases gave distinctive appearances to the discharge 

 in the tube, no difference in the behaviour of the mantles with them 

 and with air . could be detected when once the vacuum reached the 

 degree required for producing incandescence of the mantle. 



Further experiments were made with a similar tube containing a 

 compound mantle made up of four sections, composed as follows : — 

 (1) pure ceria, (2) pure thoria, (3) 50 per cent, thoria 50 per cent, ceria, 

 (4) 99 per cent.. thoria 1 per cent, ceria. 



With an intensity of cathode rays that gave a brilliant light with 

 Nos. 2 and 4, Nos. 1 and 3 were found to give practically no light, 

 becoming barely red hot ; while, as before, No. 4 was found to give 

 slightly more light than No. 2, and to heat up more rapidly and cool 

 more rapidly than the latter. 



These experiments show that thoria and ceria, both alone and mixed, 

 behave quite differently when heated by cathode ray bombardment 

 than when heated in a Bunsen flame. In the latter, 99 per cent, 

 thoria plus 1 per cent, ceria gives many times as much light as pure 

 thoria alone, while, when incandesced by cathode rays of equal inten- 

 sity, the difference, though in a similar direction, is exceedingly small. 

 Again, in the flame pure ceria gives just about the same amount of 

 light as pure thoria, while with a given intensity of cathode ray bom- 

 bardment thoria gives a brilliant light, while ceria gives practically 

 none. 



In arriving at any finally satisfactory theory of the luminescent pro- 

 perties of the rare earths, these results with cathode rays, which differ 

 materially from those obtained by other methods of heating, will 

 require to be taken into account. 



I am indebted to the courtesy of the Welsbach Incandescent Gas 

 Light Company for the samples of the rare earths with which the above 

 investigations were made ; also to the assistance of Mr. J. C. M. 

 Stanton and Mr. H. Tyson Wolff in carrying out the experiments. 



