of the Bare Earths when heated in Vacuo by Cathode Rays. 117 



shaped, and slightly hollow, area on each side of the mantle was 

 subjected to the rays, and could be brought to high incandescence. 



A preliminary experiment was made with a mantle of asbestos, 

 powdered over in patches with pure thorium oxide. With this it was 

 found that at a suitable degree of exhaustion, the patches of thoria 

 became brilliantly incandescent, with an intensity of cathode rays that 

 made the asbestos barely red hot. 



Experiments were next made with mantles consisting entirely of 

 thoria and ceria, both separately, and mixed in different proportions. 

 These mantles were prepared in a similar manner to the Welsbach 

 incandescent gas mantles, by saturating a carefully purified cotton 

 fabric with ammonium nitrate of thorium and cerium, and then 

 binning out the cotton. Very thick and closely woven cotton lamp 

 wick, freed from foreign matter by treatment with caustic soda, 

 hydrochloric acid, and ammonia, was employed in place of the thin 

 fabric usually used, so that the resulting mantle after burning out the 

 cotton, was very close in texture, and fully 0*2 inch thick. This was 

 found necessary, as otherwise some of the cathode rays passed through 

 the mantle and melted the opposite aluminium electrode. 



In order to obtain accurate comparisons between pure oxides and 

 different mixtures, the mantles were made in patchwork, each complete 

 mantle being made up of two or four sections, separately impregnated 

 with different solutions, and then sewn together with impregnated 

 cotton before being burnt. 



The mantles were so mounted in the vacuum tube that the cathode 

 rays impinged equally upon the portions that consisted of different 

 mixtures, so that an equal amount of energy was imparted to each 

 sample. 



With a compound mantle prepared in this way, composed one-half 

 of pure thorium oxide, and the other half of a mixture of 99 per cent, 

 thorium oxide plus 1 per cent, of cerium oxide, it was found after 

 exhaustion that on starting the cathode discharge the thoria plus ceria 

 heated up to incandescence more rapidly, and, on stopping the dis- 

 charge, cooled more rapidly than the pure thoria. Further, when at 

 full incandescence and observed through a dark glass, the thoria plus 

 ceria was slightly more luminous than the pure thoria, though the 

 difference was very small, probably not more than 5 per cent. Owing 

 to the difficulty of maintaining a constant vacuum, accurate photo- 

 metrical measurements were not possible, but the amount of light 

 under favourable conditions was roughly estimated at, at least, 150- 

 candle power per square inch of incandescent surface, this being 

 obtained with an expenditure of electrical energy in the secondary 

 circuit at about 8,000 volts pressure of approximately 1 watt per 

 candle. The amount of exhaustion suited to give the best results 

 varied with the dimensions of the tube and the conditions mentioned 



