and the Theory of Atomic Structure. 1101 



On the other hand, we may see from a comparison of the 

 diagrams I. and II. and the tables of: Part II. that the lines, 

 which according to the diagrams should fall out between 

 niton and xenon (i. e. in the L-series j3 s and y 6 ) were actually 

 not found for the elements in the neighbourhood of xenon. 

 In the same way we may see from the diagrams II. and III. 

 compared with the tables of Part II. that the lines Ly 4 , L/3 2 

 and Ly 1? which according to the diagrams should fall out 

 between xenon and krypton, were not observed for Rb and Sr. 

 . § 3. We will now proceed to a closer comparison of Bohr's 

 theory of the development of groups of electrons in the atom 

 with the experimental results as regards the first appearance 

 of the lines in the X-ray spectra. 



Starting from niton the lines which first disappear are 

 /3 5 andy 6 . For the elements in the neighbourhood of niton 

 both the lines /3 5 and <y 6 are fairly strong ; in the neigh- 

 bourhood of the Pi metals their intensity decreases very 

 appreciably ; for W, and especially for Ta, which elements 

 both gave very good plates, they belong to the faintest known 

 lines in the X-ray spectra. In the rare-earth metals and for 

 Ba and Os not the slightest truce of these lines has as vet 

 been found. On several plates, however, I found white lines 

 on a dark background in the place where we should expect to 

 find /3 5 or y 6 . As mentioned in II. § 9, these white lines are 

 the characteristic absorption lines * Lj and L 2 due to the 

 absorption of the Cu radiation in the salts which were used 

 upon the anticathode. Though even this is not an absolute 

 proof t that the lines /3 5 and y 6 do not exist, it seems on the 

 whole to be very probable that these lines disappear for 

 the rare-earth metals. This is in agreement with the theory. 

 According to this 5 3 orbits probably appear for the first time 

 for L« (57) +, but in the rare earths the electrons moving in 

 these orbits are very loosely bound in the form of valency 

 electrons, which are hardly present in the salts used in the 

 experiments. It is only in Ta (73) and the following ele- 

 ments, that a 5 3 electron can first be expected to be present 

 under the conditions of the experiments and to correspond to 

 a binding sufficiently strong for the 5 3 level to be detected. 



* On carefully exposed photographs of an absorption edge usually a 

 white line may be seen. This white line implies that the electron 

 " prefers r ' the absorption of a frequency which is just able to bring it 

 outside the atom. 



t In studying the L x and L 2 absorption edges of tungsten, Duaue and 

 Patterson have also made use of the absorption in the tungsten of the 

 anticathode. (See Proc. Nat. Ac. Sci. Washington, Sept. 1920.) 



+ Compare N. Bohr, Zeitschr.f. Physik, ix. p. 1 (1922). 



