﻿572 Spectra of X-rays and Theory of Atomic Structure. 



spectrum of the rare-earth metals (compare Part IV. § 11 

 and Part V. § 5). If these lines may be explained in the 

 above way, we must expect that they also appear as absorp- 

 tion-lines. In this case, however, we should have the 

 remarkable fact that the difference between these absorption- 

 lines and the corresponding normal absorption-edges should 

 be quite considerable (i. e., in the case of L : for Tb about 

 40 X.U. corresponding to 250 volts). I intend to start an 

 experimental investigation about this matter. 



Summary. 



This paper has been divided into 5 Parts : Parts I., II., 

 and III. have already been published in Phil. Mag. xliii. 

 p. 1070 (1922). 



In Part I. the general laws governing the emission of the 

 characteristic X-ray spectrum have been dealt with and 

 the relation between these laws and Bohr's theory of atomic 

 structure has been discussed. 



Parts II. and IV. contain the new experimental results, 

 Part II. gives the new measurements in the L-series of the 

 elements Hb-Ba, Part IV. those of the elements La-Lu. 

 These measurements comprise : 1°, a great number of lines 

 which may be arranged in a simple diagram, these are 

 denoted as diagram lines ; 2°, some non-diagram lines ; 

 3°, some absorption discontinuities. 



Parts III. and V. give a theoretical discussion of the new 

 measurements. From this discussion it appears that the 

 new results as regards the diagram lines are in beautiful 

 agreement with Bohr's theory as regards the successive 

 development of the shells of electrons in the atom (Part III. 



§§ 1 - 7 )-. 



According to Bohr, at different stages of the periodic table 

 we meet with atoms for which an inner shell of electrons is 

 completed. Thus the M-shell is completed in the neigh- 

 bourhood of the iron group, the inner N-shell is partly 

 completed in the region of the Pd group and again definitely 

 for the rare-earth metals. The appearance of the Pt metals 

 is connected with the partial completion of the inner 

 O-shell. This conception is found to be in agreement with 

 the experimental results. Especially for the completion of 

 the N-shell in the region of the rare-earth metals several 

 experimental proofs have been given (Part III. § 8). 



Again, according to Bohr, we must expect that, where an 

 inner shell is being completed, the most loosely bound elec- 

 trons of this shell are bound not more firmly than the 



