492 BELL SYSTEM TECHNICAL JOURNAL 



secuti\e elements had to be reversed, and at several points it was 

 necessary to leave vacant spaces between apparently-adjacent 

 elements, imagining undiscovered ones to separate tlicm. Thus it 

 became clear that the true arrangement of the elements was con- 

 trolled by something deeper and more fundamental than the com- 

 bining weights; yet there was no adequate reason for preferring one 

 of the measurable physical or chemical properties above all the others 

 as the fundamental one. Moseley then discovered that the square 

 root of the most conspicuous X-ray frequency increased at a steady 

 and even pace from one element to the next, throughout almost the 

 entire list of elements. \\'here the order of combining weights dis- 

 agreed with the order of physical and chemical properties, the order 

 of X-ray frequencies agreed with the latter and not with the former; 

 where the succession of chemical and physical properties suggested 

 that an element was missing from the list, the exces.sive leap of the 

 root of the X-ray frequency in passing from the element below to the 

 element above the suspected gap ga\'e a striking confirmation. This 

 important quality of the elements, advancing by equal steps from 

 one to the next, testified far more impressively than the periodic 

 variations of the various chemical and physical qualities to the close 

 affiliation among them. 



Measurements of the deflections of aliilia-pariicles by atoms had 

 shown that the atom has a massive nucleus bearing a positive charge; 

 as there are also electrons surrounding the nucleus, and as no one 

 has proved the existence of negati\e electricity otherwise than in 

 electrons, it was inevitable to believe that the positive nuclear charge 

 is balanced by and balances the charges of the surrounding electrons, 

 and so is an integer multiple of the electron-charge e. Moseley 's 

 law could be interpreted to mean that the nuclear charge increases by 

 e in passing from one element to the next. Fajans and Soddy had 

 already found that wlien one of the radioacti\'e elements is trans- 

 formed into another, the transformation is always such that an 

 increase of e in nuclear charge goes with an advance of one step along 

 the series of elements. Therefore, it would be possible to assign 

 the nuclear charges of all tiie elements if the nuclear charge of one, 

 or preferably of several, could be absolutely determined. The experi- 

 nuiHs upon scattered alpha-particles did show for several elements 

 that (lie nuclear charge of the n\\\ element is at least as close to ne 

 as to any other integer multiple of e\ direct measurement of the nuclear 

 charge of the second element showed that it is quite accurately 2e; 

 and Bohr's theory, of which the interpretation of Moseley 's law was 

 an offshoot, derived its own successes partly from the essential as- 



