Apkil 6, 1917] 



SCIENCE 



323 



coincides for each element, nearly, if not 

 exactly, with the highest emission frequency 

 characteristic of that element. De Broglie 

 has measured accurately these critical ab- 

 sorption frequencies for all the heavy ele- 

 ments clear up to thorium, thus extending 

 the K series from N = 60, where he found 

 it, to N^90 — a notable advance. It is to 

 be noted, however, that in going up from 

 bromine, atomic number 35, to uranium, 

 atomic number 92, the length of the step 

 does change by a few per cent. 



Now it is these radiating and absorbing 

 properties of atoms and these alone which 

 justify a series of atomic numbers differ- 

 ing from and more fundamental than the 

 series of atomic weights. Our present 

 series of atomic numbers is simply this 

 Moseley series of steps based on square 

 root frequencies. It is true that a series of 

 atomic numbers coinciding with the series 

 of atomic weights was suggested earlier, 

 indeed 100 years earlier, by Prout, and by 

 many others since then, and it is true, too, 

 that changes in the chemical properties of 

 radioactive substances accompanying the 

 loss of alpha and beta particles led van den 

 Broek,^ just before Moseley 's work ap- 

 peared, to suggest that position in the pe- 

 riodic table might be a more fundamental 

 property than atomic weight, but since this 

 position is in some instances uncertain, and 

 since the number of elements was wholly 

 unknown, no definite numbers were or could 

 be assigned to all the elements until Mose- 

 ley 's discovery was made, and the only evi- 

 dence which we now have as to just how 

 many elements there are between hydrogen 

 and uranium, and as to just where each 

 one belongs, is the evidence of the X-ray 

 spectra. It is true that between helium, 

 atomic number 2 and sodium, atomic num- 

 ber 11, we have no evidence other than the 

 order of atomic weights, the progression of 



7 Van den Broek, PJiys. Zeit., XIV., 32, 1913. 



chemical properties and the number of 

 known elements in this region to guide us 

 in completing the table, but since in the re- 

 gion of low atomic weights the progression 

 in the Moseley table is always in agreement 

 with the progression in the periodic table 

 there can be little doubt about the correct 

 number of each element even in this region 

 which is as yet inaccessible to X-ray meas- 

 urements. Moseley 's name must then be 

 set over against one of the most epoch ma- 

 king of the world's great discoveries. And 

 I wish to call attention to some important 

 conclusions as to atomic structure which 

 are rendered extremely probable by it. 



The first is this : If we may assume that 

 the ordinary law of inverse squares holds 

 for the forces exerted by the atomic nucleus 

 on negative electrons near it — and this 

 time-honored law, so amply verified in ce- 

 lestial regions, has been fully verified for 

 subatomic regions as well by the work done 

 at the University of Manchester on the 

 scattering of alpha rays — then the Moseley 

 law that the square roots of the highest 

 frequencies obtainable from different atoms 

 are proportional to the nuclear charge' 

 means, without any quantum theory, that 

 the distances from the nucleus of each type 

 of atom to the orbit of the inmost electron 

 is inversely proportional to the charge on 

 the nucleus, i. e., to the atomic number. To 

 see this it is only necessary to apply the 

 Newtonian law connecting central force eE, 

 orbital frequency n and radius a, namely, 



—- = {27my-ma, 



£2 0^3 



(1) 



and then to set as the statement of Mose- 

 ley 's experiment 



8 This is the proper statement of the Moseley 

 law, as he himself interpreted his experimental re- 

 sults. He knew and was careful to state, that there 

 is not an exact linear relation between the atomic 

 numbers and the square roots of the frequencies, 

 but the lack of exactness of equation (2) both as 



