« Pat tides with Light Atoms, 545 



of the nature and cause of this luminosity will be postponed 

 till a later paper. In a similar way, mica was found to cause 

 a, good deal of luminosity, apparently due to gamma rays. In 

 addition, as is to be expected, mica gives rise to numerous 

 H atoms and swift oxygen atoms. For these reasons, mica 

 is unsuitable for an absorbing screen for a particles in this 

 type of experiment. 



§ 7. Theory of Collision of a. particles with light atoms. 



It will be seen later that the number of H atoms and their 

 distribution with velocity differ markedly from the results 

 to be expected theoretically. It i^ consequently desirable to 

 ■consider first with some detail the results to be anticipated 

 on simple theoretical grounds, before discussing the experi- 

 mental results. 



The effect of collision of swift a pai tides with light atoms 

 has been worked out by C. Darwin *. 



a particle : M mass, E charge, v initial velocity, <£ angle 

 of scattering from original direction. 



Light atom : m mass, e charge, u velocity after impact, 

 6 angle of deflexion from original direction 

 of a. particle. 



From considerations of simple impact, it follows that 



M 

 11 = 2v mT^ • cos > C 1 ) 



M + 



m 



, m sin 26 



tan ^ = M-mco S 2g & 



If there is no loss of energy in the impact we should 

 consequently expect u = %vco$0 for the hydrogen atom, 

 *juite independently of any assumption as to the nature and 

 magnitude of the forces involved in the impact. In order, 

 however, to calculate the number of H atoms scattered within 

 a given angle 6, it is necessary to make special assumptions 

 as to the magnitude and direction of the forces. Assuming 

 that the forces arise from the charges carried by the atomic 

 nuclei which are to be regarded as points, and that the forces 

 vary as the inverse square, Darwin has shown that 



P = flt2Lll0, (3) 



where p is the perpendicular distance from the atom 

 on the initial direction of motion of the a. particle and 



/* = 



_E*/ 



v* \m 



■ii M/ 



* C. Darwin, Phil. Mag. (be. cit.). 



