290 Prof. R. R. Salmi on the 



The method employed by Mr. Biedermann for the gene- 

 ralization of his theory to allow for propagation effects 

 is very ingenious, but hardly susceptible of strict mathe- 

 matical specification. The assumption of a finite velocity of 

 propagation involves the further assumption of a reacting 

 agent to effect the propagation ; and no theory of the 

 interaction of radiating particles can be valid if it does not 

 take account of the action of this agent and its associated 



I am, 



Yours very truly, 



G. H. Livens. 



XXVIII. The Scattering of a Particles by Gases. By 

 R. R. Sahni, M. A., Professor, Government College, Lahore* '. 



[Plate VI.] 



AS mentioned in a previous paper f , the writer was 

 engaged for the best part of a year at the Physical 

 Laboratories of the University of Manchester, on the deter- 

 mination of the scattering of a particles in their passage 

 through gases. The method adopted consisted in allowing 

 a particles from the tip of a very fine needle activated with 

 radium to pass through a small hole (about 70 /ul in diam.) 

 made in a platinum diaphragm, and then to strike a photo- 

 graphic plate placed in a suitable chamber at a known 

 distance. The chamber contained the gas with which expe- 

 riments were to be conducted at a known pressure. The 

 photographic plate was exposed to the particles for a period 

 varying from two to three hours and then developed. The 

 plate was then examined under the microscope and the distri- 

 bution of the silver grains in equal concentric areas from the 

 centre of the image outwards was determined and recorded. 



Some forty plates were prepared in this manner, with the 

 apparatus finally adopted. The detailed examination of the 

 plates was reserved till after the writer's return to India 

 towards the close of last year. Unfortunately many of the 

 plates were found spoiled, and the remaining ones, while 

 giving promising results, are not considered to afford 

 sufficient experimental data from which the angle of scat- 

 tering could be calculated with any approach to certainty. 



As it may be some time before the experiments can be 

 repeated with radium 0, it is considered advisable to publish 

 this preliminary paper on the work already done. Meanwhile,, 



* Communicated bv Sir E. Rutherford, F.R.S. 

 t Sahni, Phil. Mag! Jane 1915. 



