﻿720 Dr. H. Stanley Allen on an 



adding together the appropriate multiples of the atomic 

 susceptibilities and a constant term depending on the 

 structure of the molecule. Further, he has shown that the 

 elements chlorine, bromine, iodine, and fluorine (and some 

 others) contain a common aliquot part in the specific 

 susceptibility. Certain compounds of the halogens show 

 less susceptibility than would be expected from the additive 

 law. This diminution can be expressed in terms of the same 

 aliquot part, whose value is 0*2468 x 10~ 7 . If we identify 

 this quantity with the effect of a single electron in the ring, 

 we are led to the conclusion that the radius of the ring is 

 about the same for the elements in question, and that its 

 value is of the order of magnitude to be expected. 



The difficulties connected with the explanation of para- 

 magnetic properties are, of course, left untouched by these 

 suggestions. No one has as yet explained how the orbits 

 become tilted when under the influence of an external 

 magnetic field. We may note that a similar difficulty is 

 found in connexion with Kitz's theory of the Zeeman effect, 

 Avhich is attributed to a precessional motion of the elementary 

 magnet, no explanation being forthcoming of the way in 

 which the precessional motion is set up. 



The Quantum llieory of Spectral Series. 



The success of Bohr's theory in explaining the ordinary 

 Balmer's series in the spectrum of hydrogen, and especially 

 in obtaining close agreement between the observed and the 

 calculated values of Rydberg's constant, raises a strong 

 presumption in its favour. The essential feature of the 

 theory is the emission of exactly one quantum of energy as 

 monochromatic radiation in the passage between one steady 

 state of motion and another. This leads directly to an 

 expression for the frequency, v, as the difference between 

 two " sequences,'"' the form of the expression being 



When, however, an attempt is made to apply the theory to 

 the spectral series of elements other than hydrogen, serious 

 difficulties are encountered. These have been discussed by 

 Nicholson * with special reference to the spectra of helium 

 and of lithium. In the first place, it is necessary to suppose 

 that every electron concerned in the emission of radiation 

 emits one quantum, instead of supposing that one quantum 



* Phil. Mag. vol. xxvii. pp. 541- 5C4, vol. xxviii. pp. £0-103 (1914). 



