414 Theory oj Radiation and Structure of the Atom. 



It will be seen that these relations correspond exactly to the 

 ordinary principle of combination of spectral lines. By 

 using Moseley's measurements for K a and K^ and extra- 

 polating for the values of L a by the help of MoseleyV 

 empirical formula, Kossel showed that the first relation was 

 closely satisfied for the elements from calcium to zinc. 

 Recently T. Maimer * has measured the wave-length of K a 

 and K^ for a number of elements of higher atomic weight, 

 and it is therefore possible to test the relation over a wider 

 range and without extrapolation. The table gives Maimer's 

 values for v K — v K and Moseley's values for v L , all values 



being multiplied by 10 17 . 



N 40 42 44 46 47 50 51 57 



v K -v K .. 4=6 5-5 6-1 6-6 6-9 8-4 9-0 11-6 



a 



v L 4-93 5-53 6-17 6-84 7*19 8-29 8-67 11-21 



a 



It is seen that the agreement is close, and probably within 

 the limits of experimental error. A comparison with the 

 second relation is not possible at present, and we meet also 

 here with a difficulty arising from the fact that Moseley 

 observed a greater number of lines in the L radiation than 

 should be expected on KossePs simple scheme f . 



There is another point in connexion with the above con- 

 siderations which appears to be of interest. In a recent 

 paper W. H. Bragg J has shown that, in order to excite any 

 line of the K radiation of an element, the frequency of the 

 exciting radiation must be greater than the frequency of all 

 the lines in the K radiation. This result, which is in striking 

 contrast to the ordinary phenomena of selective absorption, 

 can be simply explained on Kossel's view. The simple 

 reverse of the process corresponding to the emission of, for 

 instance, K a would necessitate the direct transfer of an elec- 

 tron from ring 1 to ring 2, but this will obviously not be 

 possible unless at the beginning of the process there was a 

 vacant place in the latter ring. For the excitation of any 

 line in the K radiation, it is therefore necessary that the 

 electron should be completely removed from the atom. 

 Another consequence of Kossel's view is that it should be 

 impossible to obtain the K series of an element without the 

 simultaneous emission of the L series. This seems to be in. 



* Maimer, Phil. Mag. xxviii. p. 787 (1914). 



t See Kossel, loc. cd. p. 960. 



% Bragg, Phil. Mag. xxix. p. 407 (1915). 



