378 SCIENCE PROGRESS 



attainment of knowledge concerning the structure of the atom. 

 Several papers of importance have recently appeared in the 

 Physical Review, bearing on these subjects. In the August 

 number is printed Prof. R. A. Millikan's address to the American 

 Physical Society, delivered last December. It is the facts of 

 radiation, he remarks, which have provided reliable informa- 

 tion about the inner structure of the atom. Even in the days 

 when one relied entirely on the spectroscopy of luminous and 

 ultra-violet radiation, it was the complexity of the spectra 

 even of simple gases which prevented the physicist from assent- 

 ing to the dogma of the indivisible atom and caused him 

 to insist that the atom must have an intricate structure, as 

 intricate, in Rowland's phrase, "as a grand piano." When 

 the facts concerning X-rays and 7-rays gradually became 

 known, and especially the recent work on X-ray spectroscopy, 

 a sort of rough outline began to show itself. The work of 

 Barkla on secondary X-radiations and of Rutherford on the 

 scattering of alpha rays in passing through matter gradually 

 focussed scientific opinion on the nuclear atom, consisting of 

 a central, positively charged body of excessively minute dimen- 

 sions surrounded in the outer regions of the atom by a number 

 of negative electrons equal to about half the atomic weight ; 

 the " diameter of the nucleus," i.e. the diameter of that portion 

 of the atom which is found by experiment to be impenetrable 

 to alpha rays, is not over a ten-thousandth of the " diameter of 

 the atom," i.e. the average distance of approach of the centres 

 of two atoms in a thermal encounter. It was, however, the 

 now famous researches of the young British physicist, Moseley 

 (whose death in Dardanelles campaign has ended what promised 

 to be a brilliant career), which revealed some facts about the 

 subatomic world with a definiteness and certainty never attained 

 before {Phil. Mag. December 191 3, January 1914). Moseley 's 

 work, checked and extended by De Broglie (Comptes Rendus, 

 *65, 87, 352 (191 7)), proves that there exist but ninety-two 

 elements from the lightest, hydrogen, to the heaviest, uranium, 

 and that these are built up one from the other by the succes- 

 sive addition of one and the same electrical element to the 

 nucleus. This conclusion rests on Moseley's discovery that 

 the square roots of the characteristic X-ray frequencies of the 

 elements progress by almost equal steps from lightest to heaviest ; 

 this law he proved in a general way for the a and @ lines of 



