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SCIENCE 



[N. S. Vol. XLV. No. 1162 



served to confirm the century-old evidence 

 of the spectroscope as to the fact of the 

 complexity of the atom, and to educate the 

 public into a readiness to accept it, \vith- 

 out at first adding much information as to 

 its nature. These studies did reveal, how- 

 ever, two types of bodies, the alpha and beta 

 particles, as atomic constituents, though 

 they said nothing at first as to their num- 

 ber, their arrangement, or their condition 

 within the atom. 



It was the study by Barkla of a radia- 

 tion problem, namely the problem of the 

 secondary X-radiations scattered by atoms, 

 which furnished the first important evi- 

 dence as to the number of electronic consti- 

 tuents within an atom. He found that the 

 number of electrons which can act as scat- 

 tering centers for X-rays is about half the 

 atomic weight.^ This conclusion was bril- 

 liantly confirmed by the simultaneous study 

 in the Manchester laboratory of the scat- 

 tering of the alpha rays in passing throiigh 

 matter,^ and out of the converging evidence 

 of these two types of research there emerged 

 with considerable definiteness the Ruther- 

 ford nucleus atom, consisting of a central, 

 positively charged body of extraordinarily 

 minute dimensions, its diameter being not 

 over a ten thousandth of the diameter of 

 the atom, surrounded in the outer regions 

 of the latter by a number of negative elec- 

 trons equal to about half the atomic weight. 

 In this statement the "diameter of the 

 nucleus" means the diameter of that por- 

 tion of the atom which is found by experi- 

 ment to be impenetrable to the alpha rays, 

 while the diameter of the atom means the 

 average distance of approach of the centers 

 of two atoms in thermal encounters. 



But it was again the study of a radia- 

 tion problem which had to be called upon 

 to furnish unquestionable information as 



2 Barkla, Phil. Mag., 21, 648, May, 1911. 



3 Eutherford, Phil. Mag., 21, 669, May, 1911. 



to the exact value of this number, and at 

 the same time to provide the most convinc- 

 ing evidence that we have of the general 

 correctness of the conception of the nucleus 

 atom. 



In a research* which is destined to rank 

 as one of the dozen most brilliant in con- 

 ception, skilful in execution, and illumi- 

 nating in results in the history of science, 

 a young man but twenty-six years old threw 

 open the windows through which we can 

 now glimpse the subatomic world with a 

 definiteness and certainty never even 

 dreamed of before. Had the European war 

 had no other result than the snuffing out of 

 this young life, that alone would make it 

 one of the most hideous and most irrepara- 

 ble crimes in history. 



For the proof that there exist but 92 

 elements, from the lightest known one, 

 hydrogen to the heaviest known one, ura- 

 nium, and that these are built up one from 

 the other by the successive addition of one 

 and the same electrical element to the 

 nucleus, this proof comes alone from 

 Moseley's discovery (checked and extended 

 as it has been by de Broglie clear up to ura- 

 nium) that the square roots of the char- 

 acteristic X-ray frequencies of the elements 

 progress by almost exactly equal steps from 

 the lightest observable one to the heaviest. 

 Moseley proved this in a general way for 

 both the alpha and the beta emission lines 

 of the hardest characteristic X-ray of the 

 elements, the so-called K rays, and also for 

 the alpha and beta lines of the next softest 

 series, the L series. De Broglie^ and Hull^ 

 have recently shown that Moseley's law 

 holds for the frequencies above which the 

 different elements absorb the general radia- 

 tion of tungsten. This critical frequency 



i Moseley, Phil. Mag., 26, 1024, Dee., 1913, and 

 27, 703, 1914. 



5 de Broglie, Compte Mendu, 165, 87, 352, 1917. 



6 Hull, Proc. Nat. Acad, of Sciences, 2, 265, 1916. 



