produced by the Action of Thorium Compounds. 



175 



Assuming that the intensity falls off as e~ Xx , the values of 



\ for the types of radiation are given below. 



" Excited " radiation 

 Uranium 



Thorium 



Value of X. 

 . -42 

 . 1-6 

 . -69 



The order of absorption in air of the above three types of 

 radiation is the same as for aluminium and paper. 



The " excited " radiation is of a more penetrating kind 

 than the easily absorbed type (the a. radiation) * given oat 

 by uranium, but much less than the ft type. The radiations 

 from radium and polonium are also more readily absorbed in 

 air than the excited radiation is. 



Fiar. 6. 



100 

 90 

 80 

 70 

 60 

 50 



40 

 30 

 10 















































\ -\ 



&> 

 \-^. 













X 5 ? 



& 













Ic 



V33 



\° 



\^r 











\o 



N 



^ 







































D/STANCE 



IN CM. F/f 



om Plate. 









1-0 



1*5 



Z-Q 



2-5 



3-0 



Duration of the Radioactivity. 



If a plate or wire which has been made radio-active is 

 removed from the action of the thorium, the intensity of the 

 radiation diminishes according to a very simple law. 



A large number of experiments have been made on the 

 duration of the induced radioactivity in various substances 

 under varying conditions. A typical table of the results 

 obtained is given below for a rod of brass which has been 



* Rutherford, Phil. Mag. Jan. 1899, p. 116. 



