﻿380 



Mr. J. A. Crowther on the Coefficient of 



If the stopping-power of the corpuscle were the same for all 

 substances, and independent of the mode of grouping of the 

 corpuscles in the atom, we should have the ratio of the co- 

 efficient of absorption to the density of the absorbing medium 

 a constant for all substances. 



This is the " density law " of absorption, and was first 

 propounded by Lenard * as the result of some experiments 

 on the absorption of the cathode rays. Although these experi- 

 ments included substances ranging in density from 3'6 x 10 -7 

 for hydrogen at 3 mm. pressure, to 19*3 for gold, and gave 

 values of \ varying between '00149 for the former and 

 55,600 for the latter, the extreme limits for the ratio \/p were 

 only 2070 and 5610; thus showing that although the ratio 

 might not be absolutely constant, it was at any rate a relation 

 of fundamental importance. 



Similar experiments were performed by Struttf using the 

 j3 rays from radium, with similar results, the values obtained 

 for \/p varying from 3*84 for card, to 7*34 for platinum. 

 The radiation of radium, however, is complex, and consists 

 of rays travelling with very different velocities. As each 

 component ray has its own coefficient of absorption, the 

 absorption of the bundle as a whole is not represented by any 

 simple law, and the values obtained were therefore somewhat 

 indeterminate. 



Further experiments were therefore performed by Ruther- 

 ford $ using uranium as the source of radiation. The /3 rays 

 from this substance are practically homogeneous, and are 

 therefore not open to the same objection as the radium rays. 

 The results were as follows : — 



Substance. 



X/p. 



Glass 



Mica 



Ebonite ... 



Wood 



Cardboard 



Iron 



Aluminium 

 Copper . . . 



Silver 



Lead 



Tin 



5-7 

 51 

 5-7 

 5-4 

 53 

 5-6 

 5-4 

 7-0 

 7-1 

 10-8 

 132 



* Lenard, Wied. Ann. lvi. p. 255 Q895). 



+ Strutt, 'Nature,' lxi. p. 539 (1900). 



X Rutherford, ' Radioactivity/ p. 114 (1904). 



