308 Collision of a particles with Hydrogen Atoms. 



such close collisions when the nuclei must approach within 

 3 x 10~ l3 cm. of each other. This problem is being attacked 

 in the Cavendish Laboratory by two methods. The electric 

 balance method, as devised by Mr. McAulay (v. Phil. Mag. 

 Dec. 1919), has the great advantage that it does not involve 

 the eyestrain of counting, and, if the electrical effect is 

 sufficiently large, is practically independent of the probability 

 variations which make it necessary to count so many 

 particles in the scintillation method. This electrical method 

 should yield approximate quantitative data and has certain 

 advantages over the scintillation method for examining the 

 effect of K atoms near the end of their range where their 

 ionization per unit path is increased due to their reduction 

 of velocity. The relation between the number of H atoms 

 shot out at different angles with the primary beam of « rays 

 for different velocities of the particle is under direct exam- 

 ination by Mr. Chadwick and Mr. Bieler using the scintil- 

 lation method. The general results so far obtained by these 

 new methods indicate that the number of H atoms liberated 

 is much larger than the theoretical number to be expected 

 from point nuclei and possibly even greater than my original 

 estimate. They also confirm the observation that the H 

 atoms tend, for the high velocities of the a particle, to be 

 shot forward in the direction of the a particle but not to the 

 same extent as the preliminary experiments showed. The 

 direct method used by Chadwick and Bieler indicates that 

 even for swift a particles there is present a certain proportion 

 of lower velocity H atoms. The determination of the 

 relation between the number of H atoms and angle of ejection 

 should afford valuable data for calculation in order to throw 

 light on the structure of the nuclei and forces between them. 

 Unfortunately such counting experiments involve a large 

 amount of time, so that there will be some delay before the 

 necessary data are available for adequate comparison of 

 theory with experiment. 



I would like to take this opportunity of drawing attention 

 to a slip in my original paper (loc. cit. p. 546) which 

 fortunately is of little importance to the main argument. 

 In calculating the emergent range for H atoms scattered at 

 an angle 9 with the direction of the a particle, the correcting 

 factor for absorption under the experimental conditions was 

 taken as cos 9. As Mr. 0. Darwin pointed out to me some 

 time ago, this is only a rough approximation and the average 

 value for the correcting factor is a complex function of 9 

 which can readily be calculated. 



E. Rutherford. 



