the Absorption of X-liays. 171 



compounds in which absorption is dependent as much as 



possible on the element examined. "When we calculate 



absorption of substances in solutions we have to correct the 



observed values in respect to the influence of the solvent. 



All the values below are corrected in this manner. 



In calculating k . ,„ I have made use of the mean values 

 » A/Cu 



of k c rcr o obtained when examining Cu in metal form and 

 in water" solutions of OuCl 2 , CuS0 4 , CuN0 8 (Table VII.). 

 By the aid of the values exhibited in Barkla and White's 

 paper on the mass absorption coefficient of copper and water 

 in case of different wave-lengths * Cu , H can De calculated, 

 and by interpolation one can readily find the wave-lengths 

 corresponding to the values found by the writer for k c , s q . 

 In the Table below I have given the values obtained for 

 the mean wave-lengths {\ m ) in the four cases with different 

 compositions of radiation. Likewise I have indicated the 



values of the mass-absorption coefficients of Ou ( - Cu J 



corresponding to these wave-lengths, which values have 

 been obtained by interpolation between the values given 

 by Barkla and White. 



Table I. 



Filter. 



Thickness 

 Notation. in mm. 



K "Cu/H 2 0* 



K- 



fi 



p Cu. 



1 1-12 



78-9 



0-38 



7-10 



II 2-50 



67-8 



0-36 



592 



III o-00 



576 



0-34 



489 



IV 1000 



48-4 



0-30 



3-73 



In the table, as well as in the following demonstration, 

 the wave-lengths are denoted by the Angstrom units 

 (1 A.E. = 10- 8 cm.). 



By filtering, most of the waves of greater wave-lengths in 

 proportion are done away with, by which means radiation 

 for an increasing thickness of filter is more and more apt to 

 be homogeneous. Of course the range in which the wave- 

 lengths fall has had an appreciable extension on both sides 

 of the above-mentioned mean wave-lengths. Among the 

 substances examined, certainly the characteristic wave- 

 lengths of K-radiation for Sn, I, and Ba have fallen 

 within the range mentioned. This is evident from the 



