/3 Rays from Radium by Solutions and Liquids. 615 



water for a layer of thickness h, then the absorption of the 

 layer of water in the solution is 



=A„ x «g x D, 



since the absorption is proportional to the mass of matter 

 traversed. If A is the observed absorption of a given layer 

 of the same thickness, the absorption for the salt itself is 



Ag xM M xD 

 A 100 ' 



or per unit mass : 



AxlOO-A^xM^x D 



M,xDx/i 



If the absorption of the /3 rays is an additive property of 

 matter, then the above ratio must be constant for different 

 concentrations, i. e., 



A x 100 -A w xM w xB _ K m 



M.xDxh ■ • • jUJ 



For pure substances ^1^ = and M s = 100, obviously the above 

 equation becomes : 



D^= K ^ 



If the absorption in terms of aluminium is multiplied by 

 the density of aluminium, 2*70, then the two equations give 

 the ratio between the mass of aluminium and the mass of 

 any substance which absorbs the same quantity of the 

 /3 particles. This ratio may be called the relative absorption 

 of the substance compared with aluminium. 



The values of K calculated in this way from equations (1) 

 and (2) are given in Table IV. (p. 616) for a number of 

 solutions. The relative absorption per unit weight compared 

 with aluminium is also given in the third column. 



By the method of comparison of absorption in terms of 

 aluminium the results are quite independent of the actual 

 coefficient of absorption "\" of the rays employed, i.e. the 

 results are independent of the penetrating power or lack of 

 homogeneity of the rays. This conclusion was confirmed by 

 a direct experiment. In the earlier observations the radium 

 was covered by a sheet of aluminium 1*60 mm. thick, and 

 the constant of absorption K for sodium chloride solutions 

 was *35. This constant was unaltered in later experiments, 

 when the thick sheet of aluminium was removed. 



