the (3 and 7 Rays of Actinium. 



379 



30 per cent, of the initial value. In order to show whether 

 the 7 rays of actinium are entirely homogeneous, it would be 

 necessary to repeat the experiment, using a more active 

 preparation of this substance. 



The 7 rays of actinium differ also very considerably from 

 the 7 rays emitted by other radioactive elements in having a 

 very small penetrating power. The following table gives the 



constants of absorption, and the values of -j for the metals 



investigated ; for comparison the corresponding values for 

 radium * are also given. 



Table III. 





Actinium. 







Radium. 



Metal. 



Thickness of 

 metal in mms. 

 to absorb half 



of the rays. 



X cm. . 



X 



2" 



Less pen 



etrating. 



More penetrating. 



X. 



X 

 d' 



X. 



X 



d' 



Iron . . . 

 Zinc . . . 

 Lead ... 



5"70 mms. 

 5-60 „ 

 192 „ 



1-23 

 1-24 

 4-54 



0-16 

 0-17 

 0-40 







0-28 

 064 



0039 

 0-056 



0-24 

 0-44 



0-033 

 0037 



We see from this table that the 7 rays of actinium have a 

 penetrating power of only about one-tenth of that of more 

 penetrating rays of radium. This unusually small penetrating- 

 power of the 7 rays of actinium may perhaps be the cause of 

 the great divergence from the density law for these rays f. 

 Further experiments are in progress to examine the behaviour 

 of the j3 and 7 rays of actinium in a magnetic field. An 

 investigation of this character may perhaps throw some light 

 on the peculiarities of these rays. 



In conclusion the writer has pleasure in acknowledging 

 the kind advice received from Prof. Eutherford. 



McGill University, Montreal. 

 April 7, 1905. 



* See McClelland, Phil. Mag. July 1904, p. 67. 



t See J. J. Thomson, ' Conduction of Electricity through Gases,' p. 312 ; 

 also McClelland, Phil. Mag. July 1904, p. 67. 



