PHYSICS 533 



" Similarly, the period-luminosity law of Cepheid variation 

 would be meaningless ; Kapteyn's researches on the structure 

 of the local cluster would need new interpretation, because his 

 luminosity laws could be applied locally but not generally ; 

 and a very serious loss to astronomy would be that of the 

 generality of spectroscopic methods of determining star 

 distances, for it would mean that identical spectral characteris- 

 tics indicate stars differing in brightness by loo to i, depending 

 only upon whether the star is in the solar neighbourhood or 

 in a distant cluster." 



PHYSICS. By James Rice, M.A„ University, Liverpool. 



In a former number of Science Progress (54, Oct. 191 9) an 

 account was given in this section of experiments which had been 

 carried out by Prof. Rutherford and his co-workers which 

 gave considerable reason for the view that the atom of the 

 element nitrogen could be disintegrated by bombardment 

 with the very swift a-particles emitted by radium C. This work 

 has been continued at the Cavendish Laboratory during the two 

 years intervening, and further interesting and extremely impor- 

 tant information is to hand showing that certain other atoms 

 can be similarly disintegrated, and providing us with some data 

 bearing on hypotheses which we may frame concerning the 

 structure of an atomic nucleus. 



Thus in No. 240 of the Phil. Mag. (Dec. 1920) Mr. Chad- 

 wick, at the suggestion of Prof. Rutherford, has performed 

 a series of experiments to test more closely than had been pos- 

 sible the well-known hypothesis of Van der Broek that the 

 charge on the nucleus of an atom bears to the electron charge a 

 ratio equal to the atomic number of the element, i.e. the number 

 of the element when all are arranged in order of increasing atomic 

 weight. It is now some years since Barkla first showed by 

 experiments on the scattering of X-rays by the light elements 

 that the number of electrons in a light atom agreed approxi- 

 mately with half the atomic weight. Further evidence in 

 support of this was forthcoming when Geiger and Marsden 

 published their paper [Phil. Mag., 25, p. 604 (191 3)] on the 

 scattering of a-particles by matter, and showed that if one 

 adopted the nuclear structure theory (which had just been 

 put forward by Rutherford), the nuclear charge was approxi- 

 mately ^ Ae, where A is the atomic weight and e the electronic 

 (positive) charge. The order of approximation was about 20 

 per cent. It was, however, the work of Moseley on X-ray 

 spectra [Phil. Mag., 26, p. 1024 (191 3), and 27, p. 703 (1914)] 

 which clearly indicated that the important number for an 

 atom was not so much its atomic weight as the number indicating 

 its place in a periodic classification. He showed experimentally 



35 



