Rays emitted by Substances exposed to y Rays. 629 



ratio of the number of electrons ejected per second per 

 atom. 



The quantity M x is of theoretical interest and importance, 

 and it will therefore be profitable to see whether we can 

 obtain values of M 2 for some of the substances. It 

 will be seen that to calculate M x for a particular substance, 

 we must know the values of the quantities //,, k, and K l5 

 relating to the substances. But the quantities fi and tc for the 

 electrons ejected by y rays in a substance have not yet been 

 experimentally determined. It appears very probable, how- 

 ever, from the experiments of Allen mentioned, that these 

 quantities have approximately the same values as found by 

 McClelland and Hacket* for the secondary rays of the 

 (3 rays from radium. Table II. contains the values of K : 



Table II. 



Substance. 

 Aluminium 



fi. 



K. 



K r 



M r 



K, 



M 2 . 



23 



•625 



100 



10-0 



10-0 



10-0 



42-8 

 394 

 497 



Nickel 



127 

 307 

 103 



•755 



•780 

 •785 



62-5 

 52-4 

 50-4 



40-0 

 37-2 

 437 



66-2 

 57-7 

 55-9 



Copper 



Zinc 



1 



Silver 



203 

 160 

 133 



•825 

 •830 

 •835 



108-0 

 86-2 

 73-3 



107 

 107 

 113 



116-0 

 91-8 

 76-7 



115 

 115 

 119 



Tin 





Platinum 



479 

 268 



•870 

 •885 



333 

 182 



288 

 315 



289 

 164 



235 



286 



Lead 



and Mj for a number of substances calculated by means of 

 the last two equations, using the values of /ul and k found by 

 McClelland and Hacket, and the values of K x obtained from 

 Table I. by making aluminium equal to 330. The values of 

 M x and K x for aluminium have both been put equal to 10 in 

 the table. 



It will be seen that the values K T and M x fall into groups 

 corresponding to the periodic classification of the elements. 

 The values of K x for the substances belonging to the same 

 group differ considerably from one another, and increase 

 with decrease of atomic weight. The values of M t for the 

 same group differ little from one another, but there is a 

 considerable change in the values from group to group. It 

 would be interesting to calculate the values of K ± and M L 

 for all the elements, if the necessary data were known. 



* Trans. Eoy. Soc. Dublin, vol. ix. p. 27 (1906), and p. 37 (1907). 



