the Absorption of X-Rays. 193 



Thus we obtain : 



(£.)'■ ■•■.••..« 



"*'<* 



Moore * confirms this equation, but instead of the atomic 

 number he puts the atomic weight (M) ; consequently, 



K a/Cu 



(£) ™ 



Yet Moore admits that the determinations hitherto per- 

 formed are not sufficiently exact to allow of asserting 

 whether atomic weight or atomic number should properly 

 appear in the formula. 



Glocker f recently has derived the following formulae for 

 the mass absorption coefficient : 



-£= 0-0004 Z^X 2 ' 8 for X>X A , . . . (11) 

 r 



£ = 0-0195 Z 2 ' 58 X 28 for \<X A . . . . (12) 

 P _ 



where \a indicates the wave-length of the absorption-zone 

 limit for K-radiation. Now, as in all examined elements 

 with Z<47\ m <A,A, it follows that /c a /Q u for these elements 

 might be calculated after the formula 

 /Z \ 2 '* 8 M 



*^ = \zj ' M^ ( 13 ) 



In Table XI. are arranged the values of K a , Cxl which have 

 been obtained from observations, mean amounts from 

 Table VIII., with values of the same constant that have 

 been calculated by the aid of the formulae (7), (9), (10), 

 and (13). In the formula (7) the values of K seen in the 

 following table have been used. 



Table X.— Values of K in Formula (7). 



Electrons 



Group. Z. of the K. 

 inner regions. 



1 1-7 005 



II 8-11 4 0-26 



HI 12-15 8 0-98 



IV 16-19 12 2-55 



V 20-23 16 642 



VI 24-28 18 900 



VII 29-46? 24 19-8 



* Moore, Phys. Soc. of London, xxvii. p. 432 (1918) 

 t Glocker, Phijs. Z. xix. p. 71 (1918). 



