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XX. The Passage of a. Particles through Hydrogen. — II.. 

 By Prof. E. Marsden, D.Sc, and W. C. Lantsberry, 

 B.Sc* 



IN a previous paper f by one of us, evidence was given 

 that in the passage of a particles through hydrogen,, 

 certain intimate collisions take place with the nuclei of the 

 hydrogen atoms. These encounters result in the nuclei 

 attaining velocities greater than those of the a particles 

 impinging on them, giving them a correspondingly greater 

 penetrating power or range. The maximum value of this 

 range was found to be about 100 cm. in hydrogen at ordinary 

 temperature and pressure. The absorption by various metals 

 was also studied, and relations were found to hold for the 

 hydrogen particles similar to Bragg' s laws of absorption for 

 ol particles. 



As Rutherford % has pointed out, these encounters between; 

 a. particles and hydrogen nuclei are very important from the 

 point of view of ihe estimation of their dimensions. If the 

 laws of their relative directions after impact follow simple- 

 theory, as given by Darwin §, it can be predicted that the 

 radii of the nucleus of the hydrogen atom and of the a. par- 

 ticle are certainly less than 10 ~ 13 cm., i. e. less than the 

 radius of an electron. This follows from a calculation of 

 the distance they are apart at the moment of nearest approach 

 in a collision. 



All the experiments described in the previous paper show ed 

 a good qualitative agreement with Darwin's calculations.. 

 It seemed desirable, however, to make quantitative measure- 

 ments of the number and distribution of the H particles 

 when a parallel beam of a particles of definite velocitv and 

 intensity falls on a known thickness of hydrogen gas/ The 

 present experiments were undertaken with this aim in view,, 

 but unfortunately we have not been able to complete them. 

 However, certain points of interest have arisen, and it seems 

 advisable to publish them. 



The formula derived by Darwin for the number v and. 

 distribution 6 of the H particles is 



1 e 2 E 2 

 i/ = QNfo. ^ ^p- . 25 sec. 3 <9, 



where E and M are the charge and mass of the a particle,, 



* Communicated by the Authors. 

 t Phil. Mag. xxvii. p. 824 (1914). 

 % Phil. Mag. xxvii. p. 488 (1914). 

 § Phil. Mag. xxvii. p. 499 (1914). 



