Absorption and Scattering of X-Rays. 283 



The relation between fi/p and A- 3 is, however, so simple and 

 convenient that we have plotted these two quantities for all 

 the substances we have experimented upon. 



The wave-lengths of short waves have been obtained from 

 Hull and Rice's relation experimentally determined between 

 wave-length and absorption in aluminium ; for longer waves 

 Siegbahn's wave-lengths (K a i lines) corresponding to Barkla 

 and Sadler's and our absorption coefficients for the K charac- 

 teristic radiations have been used. The two sets overlap 

 through only a very short range of wave-lengths, and this 

 range is a particularly critical range, but the agreement is 

 remarkably close and by the graphic method the results may 

 be made perfectly continuous *. The wave-lengths obtained 

 from these relations are given in Table I. columns 1 and 2 

 respectively. The two sets of wave-length determinations 

 are distinguished by dots and circles in figure 2, in which 

 absorptions in copper, aluminium, water, and paraffin-wax 

 are plotted against the (wave-length) 3 . 



The discontinuities in the aluminium, water, and paraffin 

 curves indicate again the positions of the J spectral lines 

 for aluminium, oxygen, and carbon. There is also a slight 

 break in the copper curve suggesting a copper J radiation. 

 But there may be some doubt as to this interpretation 

 for the change is not so pronounced as we should have 

 expected ; it appears somewhat near to the J lines for 

 aluminium ; and the fact that the two sets of values are 

 confined to the two sides of the discontinuity leads us to 

 treat the result with some caution. On the other hand, there 

 are features in the relative absorptions in copper and other 

 substances which suggest a change in the copper absorption 

 in the neighbourhood of this particular wave-length (see 

 aluminium absorption curve, figure 1) ; and we have no 

 reason, beyond those stated, to question the accuracy of the 

 result. 



We thus see that over a limited range of wave-lengths 



the equation — = C + kX z (where k is a constant for a given 



substance) fairly accurately represents the experimentally 

 observed relation — very much more accurately indeed than 

 any simple formula of the type /j,/p = kX n . It, however, does 

 not express the fact of the variation of or with X such as has 

 been described, nor does kX 3 appear to hold at all accurately 



* It is unfortunate that Hull and Rice continued their experiments 

 only up to the point where the discontinuity occurred both in aluminium 

 and in copper. 



