6S6 Prof. E. Rutherford on the 



the small fraction of the incident {3 particles scattered through 

 a large angle. In this way, possible errors due to small 

 scattering will be avoided. 



The scattering data for the /3 rays, as well as for the 

 u rays, indicate that the central charge in an atom is 

 approximately proportional to its atomic weight. This falls 

 in with the experimental deductions of Schmidt *. In his 

 theory of absorption of /3 rays, he supposed that in traversing 

 a thin sheet of matter, a small fraction a of the particles are 

 stopped, and a small fraction /3 are reflected or scattered 

 back in the direction of incidence. From comparison of the 

 absorption curves of different elements, he deduced that 

 the value of the constant /3 for different elements is propor- 

 tional to ?iA 2 where n is the number of atoms per unit volume 

 and A the atomic weight of the element. This is exactly the 

 relation to be expected on the theory of single scattering if 

 the central charge on an atom is proportional to its atomic 

 weight. 



§ 7. General Considerations. 



In comparing the theory outlined in this paper with the 

 experimental results, it has been supposed that the atom 

 consists of a central charge supposed concentrated at a point, 

 and that the large single deflexions of the « and (3 particles 

 are mainly due to their passage through the strong central 

 field. The effect of the equal and opposite compensating 

 charge supposed distributed uniformly throughout a sphere 

 has been neglected. Some of the evidence in support of 

 these assumptions will now be briefly considered. For con- 

 creteness, consider the passage of a high speed a particle 

 through an atom having a positive central charge Ntf, and 

 surrounded by a compensating charge of N electrons. 

 Remembering that the mass, momentum, and kinetic energy 

 of the a, particle are very large compared with the corre- 

 sponding values for an electron in rapid motion, it does not 

 seem possible from dynamic considerations that an a particle 

 can be deflected through a large angle by a close approach 

 to an electron, even if the latter be in rapid motion and 

 constrained by strong electrical forces. It seems reasonable 

 to suppose that the chance of single deflexions through a 

 large angle due to this cause, if not zero, must be exceedingly 

 small compared with that due to the central charge. 



It is of interest to examine how far the experimental 

 evidence throws light on the question of the extent of the 



* Annal. d. Fhys. iv. 23. p. 671 (1907). 



