1915^ Radio-Activity and Periodic System 33 



importance to these numbers, but the efforts had little to 

 commend them. Lately it has been suggested by van den 

 Broek that this is a fundamental and important number. 

 Beginning with 1 for H, the numbers would be 2 for He, 

 3 for Li, 4 for Be, etc. The question then naturally arises, 

 can these numbers be reliably determined without reference 

 to the atomic weights and correcting the manifest mistakes 

 made in following the simple order of these weights ? 



One method for doing so, though with limitations, lies in 

 the measuring of the scattering of the « particles when passing- 

 through different kinds of matter. Geiger found that the 

 angle of the scattering seemed to depend very largely upon 

 the atomic weight of the scattering metal. A very small 

 fraction are scattered through such a large angle that they 

 return on the side of incidence. This deflection is, of course, 

 both a volume and surface effect. For equal thickness of 

 screen calculations based on Rutherford's conception of the 

 atom and his belief that this large angle scattering is due to 

 the near approach of the positively charged a particle to the 

 positive nucleus of the atom of the screen would make the 

 scattering vary as the product of the density by the atomic 

 weight. Thus Rutherford calculated that the scattering by 

 gold should be about fifty times that by aluminium. This 

 has been confirmed by the experiments of Geiger and Mars- 

 den, and the relative scattering has been determined for a 

 large number of elements. The phenomenon is manifestly 

 one determined by the electrical content of the atom. 



The nuclear charge of the Rutherford atom can be calcu- 

 lated from the a particle scattering at various angles. This 

 charge is found to be one-half the atomic weight multiplied 

 by the charge of an electron. The same value was reached 

 by Barkla by observations on X-rays. Soddy concludes that 

 it is the nuclear charge rather than the atomic mass which 

 fixes the position of the element, basing his conclusion largely 

 upon the work of Barkla, Sadler and Moseley, which will be 

 briefly outlined further on. This in reality agrees with the 

 hypothesis of van den Broek that the number of electrons in 



