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Mr. F. P. Slater on the Rise of 



from radium B are less penetrating than those from radium 

 C, " K " depends on the thickness of matter through which 

 the radiations pass before entering the ionization chamber. 

 Rise curves have been experimentally determined for 

 different thicknesses of absorption material, both lead and 

 aluminium being used. The values of K for various thick- 

 nesses have been deduced by trial, and are shown in fig. 1. 



Fig. 1, 



0-30 



V 



f - 



;\ i i I 



> Mms. cf /ead. 



A comparison of the experimental and calculated rise curves 

 of the 7~ray activity through 12'0 mm. of lead is given 

 in fig. 2, After six minutes from the introduction of pure 

 emanation, the calculated and experimental curves agree 

 very closely. 



From these curves the absorption coefficient of the 

 radium B-7 rays can be deduced, and the values found are 

 given in Table I. along with comparative determinations 

 by Makower and Moseley (loc. cit.) and Rutherford and 

 Richardson *. 



The values of the absorption coefficients for the thick- 

 nesses of aluminium are somewhat doubtful, since the 

 supposition of homogeneity of the radium-C 7 rays is not 

 justifiable through such small thicknesses. The increasing 



* Rutherford and Richardson, Phil. Mag. vol. xxv. p. 722 (1913). 



