Scattering of a and ft Particles by Matter. 687 



distribution of the central charge. Suppose, for example, 

 the central charge to hi composed of N unit charges dis- 

 tributed over such a volume that the large single deflexions 

 are mainly due to the constituent charges and not to the 

 external field produced by the distribution. It has been 

 shown (§ 3) that the fraction of the a particles scattered 

 through a large angle is proportional to (N^E) 2 , where N<? is 

 the central charge concentrated at a point and E the charge 

 on the deflected particle. If, however, this charge is dis- 

 tributed in single units, the fraction of the a particles 

 scattered through a given angle is proportional to Ne 2 instead 

 of N'V. In this calculation, the influence of mass of the 

 constituent particle has been neglected, and account has only 

 been taken of its electric field. Since it has been shown that 

 the value of the central point charge for gold must be about 

 100, the value of the distributed charge required to produce 

 the same proportion of single deflexions through a large 

 angle should be at least 10,000. Under these conditions the 

 mass of the constituent particle would be small compared 

 with that of the a. particle, and the difficulty arises of the 

 production of large single deflexions at all. In addition, 

 with such a large distributed charge, the effect of compound 

 scattering is relatively more important than that of single 

 scattering. For example, the probable small angle of de- 

 flexion of a pencil of a particles passing through a thin gold 

 foil would be much greater than that experimentally observed 

 by Greiger (§ b—c). The large and small angle scattering 

 could not then be explained by the assumption of a central 

 charge of the same value. Considering the evidence as a 

 whole, it seems simplest to suppose that the atom contains 

 a central charge distributed through a very small volume, 

 and that the large single deflexions are due to the central 

 charge as a whole, and not to its constituents. At the same 

 time, the experimental evidence is not precise enough to 

 negative the possibility that a small fraction of the positive 

 charge may be carried by satellites extending some distance 

 from the centre. Evidence on this point could be obtained 

 by examining whether the same central charge is required 

 to explain the large single deflexions of a and ft particles ; 

 for the ex. particle must approach much closer to the centre 

 of the atom than the ft particle of average speed to suffer 

 the same large deflexion. 



The general data available indicate that the value of this 

 central charge for different atoms is approximately propor- 

 tional to their atomic weights, at any rate for atoms heavier 

 than aluminium. It will be of great interest to examine 



