240 PHYSICAL SCIENCE 



found to be hurled aside as by a mighty force. 

 Rutherford examined this scattering of the a 

 particles, and found that it would be accurately 

 explained if we suppose that the atoms of the gas 

 through which they passed were formed of elec- 

 trons revolving about a central, very minute, but 

 relatively massive nucleus with a positive charge, 

 which repelled the flying positively electrified 

 a particles as they passed, in the orthodox 

 manner according to the inverse square of the 

 distance, and thus swung them aside in hyperbolic 

 orbits. 



Since the electrons are of negligible mass com- 

 pared with the nucleus, the mass of the nucleus is 

 very nearly the atomic weight of the atom. Thus 

 the mass of the uranium nucleus is about 238 times 

 that of the nucleus of hydrogen. The size may 

 be estimated by measuring the amount of scatter- 

 ing of a particles by different atoms at large angles. 

 A heavy atom seems to have a nucleus with a 

 radius not greater than 6 x lO""^^ centimetres and 

 those of light atoms would be yet smaller. 



The electric charge on the nucleus may also be 

 estimated by a study of the same scattering effect. 

 It seems to increase with the place of an atom in 

 the periodic table. But this fundamental result was 

 first discovered, and has been most satisfactorily 

 demonstrated by another line of research. 



As we saw on page 1 39, the X-rays discovered 

 by Rontgen have been proved to consist of waves 

 similar in kind to those of light, but of very much 

 shorter wave-length and greater frequency of 

 vibration. Their wave-lengths may be measured 

 by analysing them by a crystal, the layers of mole- 

 cules in which act towards X-rays as the parallel 



