1903.] 



The Emanations of Radium. 



405 



The Emanations of Radium." By Sir William Ckookes, F.R.S. 

 Received March 17 —Bead March 19, 1903. 



A solution of almost pure radium nitrate which had heen used for 

 spectrographic work, was evaporated to dryness in a dish, and the 

 crystalline residue examined in a dark room. It was feehly luminous. 



A screen of platinocyanicle of barium brought near the residue 

 glowed with a green light, the intensity varying with the distance 

 separating them. The phosphorescence disappeared as soon as the 

 screen was removed from the influence of the radium. 



A screen of Sidot's hexagonal blende (zinc sulphide), said to be 

 useful for detecting polonium radiations, was almost as luminous as 

 the platinocyanide screen in presence of radium, but there was more 

 residual phosphorescence, lasting from a few minutes to half an hour 

 •or more according to the strength and duration of the initial excite- 

 ment. 



The persistence of radio-activity on glass vessels which have con- 

 tained radium is remarkable. Filters, beakers, and dishes used in the 

 laboratory for operations with radium, after having been washed in the 

 usual way, remain radio-active ; a piece of blende screen held inside 

 the beaker or other vessel immediately glowing with the presence of 

 radium. 



The blende screen is sensitive to mechanical shocks. A tap with 

 the tip of a penknife will produce a sudden spark of light, and 

 & scratch with the blade will show itself as an evanescent luminous 

 line. 



A diamond crystal brought near the radium nitrate glowed with a 

 pale bluish-green light, as it would in a " Eadiant Matter " tube under 

 the influence of cathoclic bombardment. On removing the diamond 

 from the radium it ceased to glow, but, when laid on the sensitive 

 screen, it produced phosphorescence beneath, which lasted some 

 minutes. 



During these manipulations the diamond accidentally touched the 

 radium nitrate in the dish, and thus a few imperceptible grains of the 

 radium salt got on to the zinc sulphide screen. The surface was 

 immediately dotted about Avith brilliant specks of green light, some 

 being a millimetre or more across, although the inducing particles 

 were too small to be detected on the white screen when examined by 

 daylight. 



In a dark room, under a microscope with a §-inch objective, 

 each luminous spot is seen to have a dull centre surrounded by a 

 luminous halo extending for some distance around. The dark centre 

 itself appears to shoot out light at intervals in different directions. 

 Outside the halo, the dark surface of the screen scintillates with sparks 



