Manchester Memoirs, Vol. xlviii. (1904), No. 8. 35 



diminishes regularly in a geometrical progression with the 

 time, falling to half value every 30 minutes. But when it 

 has fallen to about one twenty-thousandth of its initial 

 value it no longer decays but remains sensibly constant over 

 long periods of time. This phenomenon Mme. Curie terms 

 * the induced activity of radium with slow rate of dissipa- 

 tion.' The question suggests itself ' Is polonium the cause 

 of the phenomenon ? ' The disintegration series of radium 

 would then read : — 



Radium. 



\ 

 Radium Emanation. 



\ 

 Matter causing induced activity I. 



\ 

 ditto II. 



I 

 ditto III. 



^. 

 Polonium. 



\\ 



? 



One fact seems to favour the suggestion. If X^ is the 

 radio-active constant of polonium, and X^ that of the type 

 of matter which produces it, on the principles already 

 discussed, the ratio of the activity of the polonium, after 

 the change is complete, to the initial activity of the matter 



producing it, should be a number of the order of ^. Since 



this ratio as given by Mme. Curie is ■, we can deduce 



^ ^ 20,000 



X2 = 6x 10'*. X for the last stage of the matter causing 

 the induced activity is 3'8 x lo"*. 



The point seemed worthy of an experimental investiga- 

 tion, and I have been examining the phenomenon of induced 

 activity with slow dissipation. Polonium, it has been 

 mentioned, is characterised by the peculiarity that it gives 



