378 



Dr. T. Godlewski on the Absorption of 



density d. For comparison, the values of X and -=- for 



a 



uranium given by Rutherford % and for radium given by 



Strutt f, are added. 



We see that although the value of -= is not constant, its 



variations are considerably smaller than in the case of the 

 other radio-elements. Thus the absorption law of density 

 holds better for the j3 rays of actinium than for those of 

 radium and uranium. 



7 Rays of Acti 



mum. 



The absorption of the 7 rays was also investigated in a 

 similar way. The ratio of the ionization produced by the 

 /3 rays in the electroscope employed to that produced by the 

 7 rays of actinium was found to be about 300. The 

 absorption of the 7 rays could not be investigated over so 

 wide a range as that of the j3 rays, owing to the smallness of 

 the effect produced. 



The results are graphically represented in fig. 2, where the 

 ordinates give the logarithms of the 7 activity, expressed in 

 per cent, of the total 7 activity, and the abscissae the thickness 

 of absorbing matter. The initial value of the 7 activity was 

 obtained by graphical extrapolation of the curves. 



Fig. 2, 



^55 73" 



THICKNESS IN MMIO' 



It is seen that the activity measured by the 7 rays decreases 

 according to an exponential law with the thickness traversed. 

 This law is applicable over the range examined, viz., to 



* Eutlierford, ' Radioactivity/ p. 114. 

 t Strutt, Nature, 1900, p. 439. 



