406 



Sir W. Crookes. 



[Mar. 17, 



of light. No two flashes succeed one another on the same spot, but 

 are scattered over the surface, coming and going instantaneously, no 

 movement of translation being seen. 



The scintillations are somewhat better seen with a pocket lens 

 magnifying about 20 diameters. They are less visible on the barium 

 platinocyanide than on the zinc sulphide screen. 



A powerful electro-magnet has no apparent effect on the scintillations, 

 which appear quite unaffected when the current is made or broken, 

 the screen being close to the poles and arranged axially or equatorially. 



A solid piece of radium nitrate is slowly brought near the screen. 

 The general phosphorescence of the screen as visible to the naked eye 

 varies according to the distance of the radium from it. On now 

 examining the surface with the pocket lens, the radium being far off 

 and the screen faintly luminous, the scintillating spots are sparsely 

 scattered over the surface. On bringing the radium nearer the screen 

 the scintillations become more numerous and brighter, until when 

 close together the flashes follow each other so quickly that the 

 surface looks like a turbulent luminous sea. When the scintillating 

 points are few there is no residual phosphorescence to be seen, and the 

 sparks succeeding each other appear like stars on a black sky. When, 

 however, the bombardment exceeds a certain intensity, the residual 

 phosphorescent glow spreads over the screen, without, however, inter- 

 fering with the scintillations. 



If the end of a platinum wire which has been dipped in a solution 

 of radium nitrate and dried is brought near the screen, the scintilla- 

 tions become very numerous and energetic, and cease immediately 

 the wire is removed. If, however, the end of the wire touches the 

 screen, a luminous spot is produced, which then becomes a centre of 

 activity, and the screen remains alive with scintillations in the neigh- 

 bourhood of the spot for many weeks afterwards. 



" Polonium " basic nitrate produces a similar effect on the screen, 

 but the scintillations are not so numerous. 



Microscopic glass, very thin aluminium foil, and thin mica do not 

 stop the general luminosity of the screen from the X-rays, but arrest 

 the scintillations. 



I could detect no variation in the scintillations when a rapid blast 

 of air was blown between the screen and the radium salt. 



A beam of X-rays from an active tube was passed through a hole 

 in a lead plate on to a blende screen. A luminous spot was pro- 

 duced on the screen, but I could detect no scintillations, only a smooth 

 uniform phosphorescence. A piece of radium salt brought near gave 

 the scintillations as usual, superposed on the fainter phosphorescence 

 caused by the X-rays, and they were not interfered with in any degree 

 by the presence of X-rays falling on the same spot. 



During these experiments the fingers soon become soiled with radium, 



