112 



Mr. W. Crookes. 



[Feb. 17, 



second with uranium glass and sulphate of quinine. In my examina- 

 tions of the phosphorescent earths glowing under the excitement of the 

 induction discharge in vacuo, I have found very great differences in 

 the duration of the residual glow. Some earths continue to phos- 

 phoresce for an hour or more after the current is turned off, while 

 others cease to give out the light the moment the current stops. 

 Having succeeded in splitting up yttria into several simpler forms of 

 matter differing in basic power,* and always seeking for further 

 evidence of the separate identity of these bodies, I noticed occasionally 

 that the residual glow was of a somewhat different colour to that it 

 exhibited while the current was passing, and also that the spectrum 

 of this residual glow seemed to show, as far as the faint light enabled 

 me to make out, that some of the lines were missing. This pointed 

 to another difference between the yttrium components, and with a 

 view to examine the question more closely I devised an instrument 

 similar to Becquerel's phosphoroscope, but acting electrically instead 

 of by means of direct light. 



The instrument, shown in fig. 1, A and B, consists of an opaque 

 disk, a b c, 20 inches in diameter, and pierced with twelve openings 

 near the edge as shown. By means of a multiplying wheel, d, and 



Fiq. 1, A. 



* ' Roy. Soc. Proc.,' vol. 40, pp. 50'i— 509 (June 10, ISSfJ). 



