y~Rays of Thorium and Actinium, 139 



useful in calculating the rise in intensity of the 7-radiation 

 of a mesothorium preparation with time, due to the generation 

 of radiothorium and thorium D, provided that the fraction of 

 the radiation due to radium is known. 



Section II. — Variations in the Values of the Absorption 

 Coefficients of y-r 'ays *. 



Effect of distance oj the preparation from the electroscope. — 

 In all this work a lead electroscope with detachable base, 

 similar to that previously described (II. p. 752, fig. 17), has 

 been used. It has been shown to exclude external secondary 

 radiations effectually. Moreover it gives of all metals easily 

 the maximum ionization for a given intensity of ^-radiation 

 (I. p. 642j, which Bragg has since explained on the view 

 that the /3-n\y, produced by the 7- ray, possesses really a 

 greater penetrating power in lead than in equal weights of 

 other metals, though its trajectory is more entangled. 



As a result of experiments on the effect of the distance of 

 the preparation from the electroscope, it was found that at a 

 distance above 14 cm. the values of A, became constant, the 

 beam being now practically parallel (II. p. 736). In most 

 experiments the distance employed was therefore 14 cm. 

 The following table gives the value of A at various distances 

 for radium 7-rays, a thick lead base being used with the 

 electroscope, the absorbing lead being laid directly on the 

 preparation. 



Distance (cm.)... 5'0 



A (cm.)- 1 0-574 



6-4 



0-553 



8-5 



13 



0-517 



14 



25 



0-500 0500 0-498 0-498 



115 



Similarly, for zinc, the value of A, at 13 cm. was 0278, 

 and at 115 cm. 0*274, which agree within the experimental 

 error. 



The u Hardening " of y-rays by passage through Lead. — ■ 

 Some further experiments were done on this effect. By 

 passage through lead the value of A for lighter metals is 

 reduced, though other bodies possess the same power as 

 lead in lesser degree. In no case have we observed any 

 " softening." Hardening is more pronounced when the rays 

 pass first through the lead and then through the lighter metal, 

 than vice versa. Thus, with a brass base to the electroscope. 



* This section may be regarded as a continuation of Section III. of the 

 previous paper (II, p. 744). 



