the Radiations from Radioactive Substances. 1$ 



due to radium which is very similar to the curve for radium (e) 

 fig. 7. 



Fig. 8. 



40 



10 20 SO 40 50 60 7Q 80 90 100 HO l£0 130 140 



The decay-curves shown all exhibit to a greater or less 

 degree the following peculiarities : — 



(1) An initial rapid rate of decay for about 10 minutes 

 after removal. 



(2) A very slow decrease for the next 20 minutes* Then 

 follows 



(3) A more rapid decrease till radioactivity disappears. 

 The rate of diminution of intensity thus passes through a 



minimum about 20 to 25 minutes after removal of the excited 

 plate from the presence of the emanation. 



An explanation of these changes can be given in the light 

 of some recent experiments by one of us on thorium-excited 

 radioactivity. 



It has been found that the excited radiation from a plate 

 exposed for a short interval in the presence of thorium emana- 

 tion increases to three or four times its initial value in the 

 course of a few hours after removal. A preliminary account of 

 these experiments is given in Physikalische Zeitschrift, No. 11, 

 1902. g 



No increase is observed when a plate has been exposed for 

 many hours in the presence of thoria. This is to be expected, 



C2 



