604 Dr. H. Geiger and Mr. E. Marsden on the Laws of 



But whatever value is attached to the numerical evidence by 

 those competent to judge, the broad conclusion from the 

 direction of the changes is unaffected. 



I should like also to reply to one criticism which has been 

 made. It is not necessary to the hypothesis that the atomic 

 free frequency should be absolutely invariable throughout 

 all chemical changes. If it were the case that a slight 

 change (say 1 per cent.) in the atomic free frequency would 

 account for all the observed changes of refractivity and dis- 

 persion, the criticism would have force. But any one who- 

 examines the figures in the table for (e. g.) hydrogen, nitrogen, 

 and ammonia, will see that no slight change in a frequency 

 can possibly account for the observed changes. For hydrogen 

 n 2 x 10~ 27 = 12409, for nitrogen 17095. For ammonia the 

 number which expresses its average value is 8135, an 

 enormous drop. At the same time the refractivity has gone 

 up only 3J- per cent. But if the number of vibrators had 

 remained constant while the average free frequency decreased, 

 the increase of the refractivity must have been much greater 

 than this. Hence, to account for the observed changes, one 

 must also assume that the number of vibrators (? electrons) 

 has fallen off in about the same proportion as the frequency. 



These two hypotheses seem much more improbable than 

 that which is here put forward. 



x — 



LXI. The Laivs of Deflexion of a Particles through Large 

 Angles*. By Dr. H. Geiger and E. Marsden f. 



IN a former paper J one of us has shown that in the 

 passage of a particles through matter the deflexions are, 

 on the average, small and of the order of a few degrees 

 only. In the experiments a narrow pencil of a particles fell 

 on a zinc-sulphide screen in vacuum, and the distribution 

 of the scintillations on the screen w r as observed when different 

 metal foils were placed in the path of the a particles. From 

 the distribution obtained, the most probable angle of scat- 

 tering could be deduced, and it was shown that the results 

 could be explained on the assumption that the deflexion of 

 a single a particle is the resultant of a large number of very 

 small deflexions caused by the passage of the a particle 

 through the successive individual atoms of the scattering 

 substance. 



* Communicated to 7c. d.-Jc. Akad. d. Wiss. Wien. 

 t Communicated by Prof. E. Rutherford, F.R.S. 

 % II. Geiger, Roy. Soc. Proc. vol. lxxxiii. p. 492 (1910) ; vol. lxxxvi. 

 p. 235 (1912). 



