Stipplement to '' Nature,'' July 7, 1923 



35 



times that of the hydrogen nucleus can therefore be 

 represented by the formula : 



\n ^ n • 



If in this formula we put N = 2, we get a spectrum 

 which contains a set of lines in the visible region 

 which was observed many years ago in the spectrum 

 of certain stars. Rydberg assigned these lines to 

 hydrogen because of the close analogy with the series 

 of lines represented by the Balmer formula. It was 

 never possible to produce these lines in pure hydrogen, 

 but just before the theory for the hydrogen spectrum 

 was put forward, Fowler succeeded in observing the 

 series in question by sending a strong discharge 

 through a mixture of hydrogen and helium. This 

 investigator also assumed that the 

 lines were hydrogen lines, because 

 there existed no experimental 

 evidence from which it might be 

 inferred that two different sub- 

 stances could show properties 

 resembling each other so much as 

 the spectrum in question and that 

 of hydrogen. After the theory 

 was put forward, it became clear, 

 however, that the observed lines 

 must belong to a spectrum of 

 helium, but that they were not 

 like the ordinary helium spectrum 

 emitted from the neutral atom. 

 They came from an ionised helium 

 atom which consists of a single 

 electron moving about a nucleus 

 with double charge. In this way 

 there was brought to light a new 

 feature of the relationship between 

 the elements, which corresponds 

 exactly with our present ideas of 

 atomic structure, according to which the physical and 

 chemical properties of an element depend in the first 

 instance only on the electric charge of the atomic 

 nucleus. 



Soon after this question was settled the existence 

 of a similar general relationship between the properties 

 of the elements was brought to light by Moseley's 

 well-known investigations on the characteristic X-ray 

 spectra of the elements, which was made possible 

 by Laue's discovery of the interference of X-rays in 

 crystals and the investigations of W. H. and W. L. 

 Bragg on this subject. It appeared, in fact, that 

 the X-ray spectra of the different elements possessed 

 a much simpler structure and a much greater mutual 

 resemblance than their optical spectra. In particular^ 

 it appeared that the spectra changed from element 



to element in a manner that corresponded closely to 

 the formula given above for the spectrum emitted 

 during the binding of an electron to a nucleus, pro- 

 vided N was put equal to the atomic number of the 

 element concerned. This formula was even capable 

 of expressing, with an approximation that could not 

 be without significance, the frequencies of the strongest 

 X-ray lines, if small whole numbers were substituted 

 for n' and n". 



This discovery was of great importance in several 

 respects. In the first place, the relationship between the 

 X-ray spectra of different elements proved so simple that 

 it became possible to fix without ambiguity the atomic 

 number for all known substances, and in this way to 

 predict with certainty the atomic number of all such 



70 12 /4 16 18 20 Z2. 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 



Afe/hl^M5,P5a^rl(CaScnMrMeMiCuMa£ekSeBrMMZrmo-RuMAj(kMnSbTelXCsMai:eP^ 



Fig. 3. 



hitherto unknown elements for which there is a place 

 in the natural system. Fig. 3 shows how the square 

 root of the frequency for two characteristic X-ray 

 lines depends on the atomic number. These lines 

 belong to the group of so-called K-lines, which are 

 the most penetrating of the characteristic rays. With 

 very close approximation the points lie on straight 

 lines, and the fact that they do so is conditioned 

 not only by our taking account of known elements, but 

 also by our leaving an open place between molybdenum 

 (42) and ruthenium (44), just as in Mendeleeff's 

 original scheme of the natural system of the elements. 

 Further, the laws of X-ray spectra provide a con- 

 firmation of the general theoretical conceptions, both 

 with regard to the constitution of the atom and the 

 ideas that have served as a basis for the interpretation 



