12 Mr Norman Campbell, The Radioactivity of Rubidium. 



3. The method of calculation explained on pp. 563, 565 of 

 the last paper give directly /q, the activity of an infinitely thick 



layer of the area (o-) of the tray, and the product — , where \ is 



the absorption coefficient of the material for the rays it emits, 

 and p the density of the material. If a is the ionising power of 

 the total radiation from 1 gramme of the material, when all that 

 radiation is absorbed in air, then 



"^ P 

 For rubidium metal we obtain 



a = 14-47 ± 0-365, - = 53-2 ±2-1. 

 P 

 For potassium metal, using the figures given in the last paper, we 

 obtain 



a = 2-003 ±-0376, - = 8-23+0-1. 

 P 



It appears then that, though, for layers infinitely thick and 

 containing the same proportion of the active metal, the activity 

 is slightly greater for potassium than for rubidium, the intrinsic 

 activity of the latter metal is at least seven times as great as that 

 of the former. 



It will be noted that the statement that the penetrating 

 power of the rubidium rays is considerably less than that of the 

 potassium rays is confirmed. The difference is so notable that it 

 may be discovered readily by covering the active layer with a sheet 

 of stout paper. Such paper absorbs half of the rubidium rays, but 

 not more than 5 °/^ of the potassium rays. It is clear, therefore, 

 that the activities of the two elements cannot be attributed to a 

 common constituent. 



Prof McLennan and Mr Kennedy* have stated that they have 

 been unable to detect any activity in rubidium salts. But, since 

 they have revised some of their statements concerning the activity 

 of the alkali metals, it is not impossible that agreement will be 

 obtained in this matter also. 



* Phil. Mag. Sept. 1908, p. 377. 



