490 Sir E. Rutherford on the 



to the thickness and varied inversely as the fourth power of 

 the velocity of the incident a particles. A special study was 

 made of the number of a particles scattered through angles 

 varying between 5° and 150°. Although over this range the 

 number decreased in the ratio 200,000 to 1, the relation 

 between number and angle agreed with the theory within 

 the limit of experimental error. They found that the scat- 

 tering of different atoms of matter was approximately 

 proportional to the square of the atomic weight, showing 

 that the charge on the nucleus was nearly proportional to 

 the atomic weight. By determining the number of a. par- 

 ticles scattered from thin films of gold, they concluded that 

 the nucleus charge was equal to about half the atomic weight 

 multiplied by the electronic charge. On account of the 

 difficulties of this experiment, the actual number could not 

 be considered correct within more than 20 per cent. 



The experimental results of Geiger and Marsden were 

 thus in complete accord with the predictions of the theory, 

 and indicated the essential correctness of this hypothesis of 

 the structure of the atom. 



In determining the magnitude of single scattering, I 

 assumed in my previous paper, for simplicity of calculation, 

 that the atom was at rest during an encounter with an 

 a. particle. In an accompanying paper, Mr. 0. Darwin has 

 worked out the relations to be expected when account is 

 taken of the motion of the recoiling atom. He has shown 

 that no sensible error has been introduced in this way even 

 for atoms of such low atomic weight as carbon. Mr. Darwin 

 has also worked out the scattering to be expected if the law 

 of force is not that of the inverse square, and has shown that 

 it is not in accord with experiment either with regard to the 

 variation of scattering with angle or with the variation of 

 scattering with velocity. The general evidence certainly 

 indicates that the law of force between the a particle and the 

 nucleus is that of the inverse square. 



It is of interest to note that 0. T. R. Wilson *, by photo- 

 graphing the trails of the a particle, later showed that the 

 a particle occasionally suffers a sudden deflexion through a 

 large angle. This affords convincing evidence of the 

 correctness of the view r that large deflexions do occasionally 

 occur as a result of an encounter with a single atom. 



On the theory outlined, the large deflexions of the « particle 



are supposed to be due to its passage close to the nucleus 



where the field is very intense and to be not appreciably 



■affected by its passage through the external distribution of 



* C. T. E. Wilson, Proc. Roy. Soc. A. lxxxvii. p. 277 (1912). 



