220 



SCIENCE 



[Vol. LV, No. 1418 



scattering element and e is the fundamental 

 unit of electric charge. Elaborate experiments 

 by Geiger and Marsden^ on the scattering of 

 a rays confirmed this view. The validity of 

 the theory was also established in a convincing 

 manner by C. G. Darwin* who made a thorough 

 mathematical investigation of the deflexions 

 which could ensue from an intimate encounter 

 between an alpha particle in motion and a 

 nucleus. In this investigation he showed that 

 the I'esults of the scattering experiments of 

 Geiger and Marsden could not be reconciled 

 with any law of central force except that of 

 the inverse square. 



In another entirely difderent field of investi- 

 gation, namely, that of the scattering of X rays 

 by light elements, Barkla^ had shown in 1911 

 that the number of electrons in an atom which 

 took part in the scattering of the X rays was 

 equal to about one half of the atomic weight 

 of the element. 



Both lines of investigation therefore led to 

 the view that the charge on the nucleus of an 

 atom was given by -i^ A e and that the number 

 of electrons in an atom surrounding the nucleus 

 was % A. It was the experiments on the scat- 

 tering of a rays, however, which led to the 

 view that the positively charged portions of 

 atoms were nuclear in character with dimen- 

 sions small compared with those of the atoms 

 themselves, and that by far the greater part of 

 the mass of the atoms was concentrated in the 

 nucleus. 



(6) Nuclear charge and atomic number. 



In 1913 Van den Broeek^ put forward the 

 suggestion that the scattering of ct particles 

 was not inconsistent with the view that the 

 charge on the nucleus of an atom was equal to 

 Ne where N is the atomic number of the atom 

 of the element concerned, i. e., the number of 

 the element when the elements are arranged in 

 order of increasing atomic weight. A refer- 

 ence to a table of atomic weights will show 

 that N is approximately equal to % A. The 



3 Geiger and Marsden, Eoy. Soe. Proe. A., Vol. 

 82, p. 495, 1909. 



4 Darwin, P7ii7. Mag., Vol. 27, p. 499, 1914. 

 sBarkla, Phil. Mag., Vol. 21, p. 648, 1911. 

 oVan den Biocek, Fliys. Zcit., Vol. 14, p. 703, 



1914. 



importance of this suggestion was soon made 

 evident by the remarkable work of Moseley^ 

 on X ray spectra which followed in 1913 and 

 in 1914. In this work Moseley showed that 

 the frequencies of the vibrations of correspond- 

 ing lines in the X ray spectra of the elements 

 depended on the squares of numbers which 

 varied by unity with the successive elements. 



This relation, it was seen, could be readily 

 explained by assuming that the nuclear charge 

 of an atom varied bj' unity in passing from an 

 atom of one element to that of another, and by 

 assuming that the nuclear charge was given 

 numerically by N, the atomic number. 



The importance of Moseley's work was 

 enhanced when it ivas seen that it gave us a 

 new method of regarding the periodic classi- 

 fication of the elements based on the assump- 

 tion that the atomic number or its equivalent, 

 the nuclear charge, was of more fundamental 

 importance than the atomic weight. As a 

 result of Moseley's work it became possible 

 not only to fix definitely the number of pos- 

 sible elements and the position of undeter- 

 mined elements, but also to show that the 

 properties of an atom were defined by a num- 

 ber which varied by unitj' in successive 

 elements. 



In Moseley's work the frequency of vibration 

 of corresponding lines in the X ray spectra of 

 the elements was not found to be exactly pro- 

 portional to N^ where N is the atomic number 

 but to (N — a)- where a was a constant which 

 had different values depending on whether the 

 K or L series of characteristic rays was 

 measured. 



The investigations of Bohr' on the origin of 

 radiations emitted by atoms are entirely in 

 keeping with the assumptions that the nuclear 

 charge is given by Ne, for he has shown that 

 the frequency formula for X ray spectral lines 

 must include a term (N — a)- with "a" having 

 values approximately equal to those found by 

 Moseley. In Bohr's investigation he showed 

 that X rays originated in disturbances given 

 to certain classes of extra nuclear electrons 

 and that the quantity "a" represented a modifi- 



" Moseley, Phil. Mag., Vol. 26, p. 1024, 1913 ; 

 Phil. Mag., Vol. 27, p. 703, 1914. 



8 Bohr, Phil. Mag., Vol. 26, p. 476, Sept., 1913. 



