630 Mr. A. van den Broek on 



If we may assume the rule, and combine it with the conclusion 

 reached above, we have the very general law 



Atomic absorption coefficient = CN 4 X 5/2 , 



where N is the atomic number of the absorber and X the 

 wave-length of the X ray. The quantity C is constant over 

 considerable ranges but changes suddenly at critical points. 

 For instance, the results given above are covered bv putting 

 C equal to 1*79 X 10~ 6 for all values of N from 13 (Al) to 

 46 (Fd), and to 0'235 x 10" 6 for all greater values. 



It can be seen on examination that the results already 

 given by Barkla are in general agreement with the formula 

 over a wider range than that we have considered here. But 

 we hope to make a fuller examination when more results are 

 obtained from the spectrometer methods. 



LXV. Ordinals or Atomic Numbers ? 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



N the Philosophical Magazine for July 1914, Prof. Rydberg 

 suggests that all the high-frequency spectral lines de- 

 termined by Moseley in his well-known experiments * might 

 be expressed by a general formula 



10'l\-=v = v a 2 (N-Cy, 



where a is a constant, N are " ordinals " two units higher (for 

 all elements, hydrogen excepted) than the atomic numbers 

 in the present periodic system, and C a multiple of 3 for 

 the a, of 3*5 for the b series. 



That C should have these values only seems to be disproved 

 by the formula found by the writer for the Y-rays' spectrum 

 of radium B, as given by Rutherford and Andrade t : 



v = (K±n/2) 2 c 2 Vo, 



where M is the atomic number, n an integer, and c a 

 constant ~(2'5~ 2 — 5 -2 )*. But Moseley's series, especially 

 L a , showing systematic deviation on both sides, suggest 

 likewise other values for C (Tables I. and II.). Of course, 

 if C can be any integer or any integer plus a half (between 

 certain limits), ordinals cannot be supported by the special 

 values of C Prof. Rydberg suggests. 



* Phil. Mag. xxvi. p. 1024 (1913) and xxvii. p. 703 (1914). 

 t Phil. Mag-, xxvii. p. 854 (1914). 



