the ft and y Rays of Actinium. 



377 



curves of absorption for the ft rays of radium and uranium 

 are also given. 



The experimental points obtained for the /3rays of actinium 

 lie on a straight line. The equation I = l(,e~ Xx , where x is 

 the thickness, is applicable even when I has a value smaller 

 than one per cent, of I . This indicates that the ft rays from 

 actinium are homogeneous in character, and that their pene- 

 trating power does not change with the thickness of matter 

 traversed. 



The second characteristic property of the ft rays of actinium 

 is their relatively small penetrating power. While the ft rays 

 from uranium are half absorbed in passing through the 

 thickness of 0"5 mm. of aluminium, the ft rays of actinium 

 are reduced to half value in 0'21 mm. Thus the ft rays of 

 actinium have only 40 per cent, of the penetrating power of 

 those of uranium. A still greater difference is observed when 

 we compare the penetrating power of the ft rays of actinium 

 and radium. The average penetrating power of the ft rays 

 from radium is more than three times as great as for actinium. 

 It must be pointed out, however, that the value of X for 

 radium rays changes very considerably with the thickness 

 traversed. From fig. 1 we see that the initial portion of the 

 absorption curves for radium and actinium rays are nearly 

 the same. This shows that some of the ft rays of radium are 

 of the same penetrating power as those from actinium. 



Table II. 





Act in 



IUM. 





Uranium. 



Eadium. 



Substance. 



Thickness of metal 



in mms.XlO -2 



to absorb half the 



rays. 



X cm. -1 . 



X 

 d' 



1 d 



X. 



X 



r 



. Aluminium . 

 '•• Mica 



212 

 21-0 



6-5 

 . 6-3 

 4-5 

 4*25 



32-7 

 33 



12-9 

 iv>-n 



14-0 

 17-2 



60 



122 



5-4 

 5-1 



7-0 



10-8 



116 



10-8 



49-2 



... 

 62-5 



4-30 

 394 



5-50 



5-48 



Brass 



108 131 

 139 15-9 

 154 j 15-7 

 163 14-1 



Copper 



i Tinfoil 



Lead 















A reference to the absorption constants A, given in Table II. 

 for different substances traversed by the ft rays of actinium 

 shows that the absorption is nearly proportional to the 



