124 Journal of the Mitchell Society [Jan. 



parable with the diameter of the atom, does not seem capable, 

 without modification, of producing the large deviations of an a 

 particle which are observed. 



" The single large scattering of a particles, however, could be 

 explained by supposing that the atom consisted of a concentrated 

 positive charge at its centre and surrounded by a distribution 

 of electrons to render it electrically neutral. It was necessary to 

 suppose that the greater part of the mass of the atom was asso- 

 ciated with the positive charge which for distances up to 

 3 X 10~^^ cm. behaved as if it were concentrated at a point. 



" The large angle scattering of the a particles is then almost 

 entirely due to the passage of the a particle through the intense 

 electric field surrounding the central charge. Supposing the 

 electrical force to vary as the inverse square of the distance from 

 the central charge, the positively charged a particle, in passing 

 close to the centre of the atom describes a hyperbolic orbit, the 

 angle of deflection being greater the nearer the a particle passes 

 the centre of the atom." 



The laws of scattering to be expected on this hypothesis have 

 been worked out and verified experimentally. The angle of 

 deflection has been found to depend upon the atomic weight and 

 from examination of the metals from gold to aluminum the 

 positive charge of the nucleus was deduced as I/2 ^^^ 



If the a particles are allowed to bombard the light atoms of 

 hydrogen the smaller nucleus of the hydrogen atom will be 

 projected and this has been proved by experiment. The hydro- 

 gen nuclei produce scintillations at a range four times as 

 great as that of the a particles. As calculated by G. C. Dar- 

 win, the a particle or helium nucleus must have approached 

 the hydrogen nucleus so that their centres were not more than 

 1.7 X 10~^^ cm. distant from one another. The hydrogen 

 atom is taken as composed of one unit charge of negative, 

 electrons and one unit charge of positive electrons. 



The Rutherford atom then is made up of a nuclear mass 

 associated with positive electrons (1 unit charge to 2 units mass 

 form a central nucleus which is .0001 of the diameter of the 

 atom). A number of negative electrons in value equal to the 



