542 Prof. J. W. Nicholson on the High-frequency 



supposed* that his formula for spectra, applied to the steady 

 states of an inner ring of four electrons rotating round a 

 nucleus whose charge corresponds to the "atomic number" 

 of the element, will account for the principal line found by 

 Moseley in the X-ray spectra of the elements from calcium 

 to zinc in the Periodic Table. The present paper is in no 

 way intended as a criticism of Bohr's theory of the origin 

 of spectra in general, and of the hydrogen and helium spectra 

 in particular, but it puts forward the view that this particular 

 application of the theory cannot be made in the manner 

 contemplated, without a serious conflict with the recent 

 results obtained by Rutherford, Barkla, and others as to the 

 number of electrons in atoms, which seem to indicate that 

 the number is about half the atomic weight. 



Recently there has been a striking change of opinion as 

 to the inner significance of the Periodic arrangement of the 

 elements. It is now largely believed that the position of an 

 element is not determined by its atomic weight so much as 

 by its " atomic number " — defined as the electron-equivalent 

 of the positive nucleus of the atom. This view appears to 

 be due originally to van den Broekt, and will be referred to 

 as van den Brook's hypothesis. It has been developed by 

 Soddy and others, and appears to throw a light upon the 

 true positions of the elements of the rare-earth group and 

 the radioactive products. For, according to this hypothesis, 

 several elements may occupy the same position in the Periodic 

 Table, provided that they possess nuclei with equal charges, 

 but different arrangements of electrons circulating round 

 the nuclei in possible steady motions, and slightly different 

 atomic weights, so that the number which defines the atomic 

 weight as found in the laboratory is actually the mean value 

 of a set of numbers each corresponding to a more or less 

 stable variety of the element. There is some evidence for 

 the truth of this view, for it seems that the spectra of two 

 elements different in their degree of activity may be iden- 

 tical, as the investigation of Rossi and Russell J on the 

 spectrum of ionium tends to show. 



But to admit that there is truth, in a general way, in 

 van den Broek's hypothesis, does not of necessity contain 

 the further admission that the atomic number of an element 

 corresponds exactly to its position in the Table as we now 

 know it. It is still possible, for example, to take the view 

 that other elements may exist between hydrogen and lithium, 

 in addition to helium; and if an element like potassium is 



* Moseley, Phil. Mag. Dec. 1913. 



t Phys. Zeit. xiv. p. 32 (1913). 



X Roy. Soc. Proc. A. lxxxvii. p. 478 (1912). 



