Structure of the Atom. 489> 



in which practically all the mass of the atom was concentrated. 

 The nucleus was supposed to be surrounded by a distribution 

 of electrons to make the atom electrically neutral, and 

 extending to distances from the nucleus comparable with 

 the ordinary accepted radius of the atom. Some of the 

 swift a particles passed through the atoms in their path and 

 entered the intense electric field in the neighbourhood of the 

 nucleus and were deflected from their rectilinear path. In 

 order to suffer a deflexion of more than a few degrees, the 

 a particle has to pass very close to the nucleus, and it 

 was assumed that the field of force in this region was not 

 appreciably affected by the external electronic distribution. 

 Supposing that the forces between the nucleus and the 

 a particle are repulsive and follow the law of inverse squares, 

 the a particle describes a hyperbolic orbit round the nucleus- 

 and its deflexion can be simply calculated. 



it was deduced from this theory that the number of 

 a. particles falling normally on unit area of a surface and 

 making an angle <j> with the direction of the incident rays 

 is proportional to 



(1) cosec 4 <f>/2 or l/<£ 4 if <£ be small ; 



(2) the number of atoms per unit volume of the scattering 



material ; 



(3) thickness of scattering material t provided this is 



small; 



(4) square of the nucleus charge N<? ; 



(5) and is inversely proportional to (mu 2 ) 2 , where m is 



the mass of the a particle and u its velocity. 



From the data of scattering on a particles previously 

 given by G-eiger *, it was deduced that the value of the 

 nucleus charge was equal to about half the atomic weight 

 multiplied by the electronic charge. Experiments were 

 begun by Geiger and Marsden f to test whether the laws of 

 single scattering of a particles were in agreement with the 

 theory. The general experimental method employed by 

 them consisted in allowing a narrow pencil of a particles to 

 fall normally on a thin film of matter, and observing by the 

 scintillation method the number scattered through different 

 angles. This was a very difficult and laborious piece of 

 work involving the counting of many thousands of particles. 

 They found that their results were in very close accord with 

 the theory. When the thickness of the scattering film was 

 very small, the amount of scattering was directly proportional 



* Proc. Roy. Soc. A. lxxxiii. p. 492 (1910). 



t Geiger and Marsden, Phil. Mag. xxv. p. 604 (1918). 



