492 Sir E. Rutherford on the 



falling into the nucleus". This result is of great interest, for 

 it may offer an explanation of the disappearance of swift 

 /3 particles in their passage through matter. In addition, it 

 must be borne in mind that the swiftest (3 particle expelled 

 from radium C possesses only about one-third of the energy 

 of the corresponding a. particle, while the average energy of 

 the /5 particle is less than one-sixth of that of the a particle. 

 It is thus to be anticipated that the large angle scattering of 

 a /3 particle by the nucleus will take place in regions where the 

 ol particle will only suffer a small deflexion — regions for which 

 the application of the simple theory may not have been accu- 

 rately tested. For these reasons, it is of great importance to 

 determine the laws of large angle scattering of j3 particles of 

 different speeds in passing through matter, as it should throw 

 light on a number of important points connected with atomic 

 structure. Experiments are at present in progress in the- 

 laboratory to examine the scattering of such swift /3 particles 

 in detail. 



It is obvious that a /3 particle in passing close to an 

 electron will occasionally suffer a large deflexion. The 

 problem is mathematically similar to that for a close encounter 

 of an « particle with a helium atom of the same mass, which 

 is discussed by Mr. Darwin in the accompanying paper. 

 Such large deflexions due to electronic encounter, however* 

 should be relatively small in number compared with those 

 due to the nucleus of a heavy atom. 



Scattering in Hydrogen. 

 Special interest attaches to the effects to be expected when 

 a particles pass through light gases like hydrogen and helium. 

 In a previous paper by Mr. Nuttall and the author *, it has 

 been shown that the scattering of a particles in hydrogen and 

 helium is in good agreement with the view that the hydrogen 

 nucleus has one positive charge, while the a particle, or 

 helium, has two. Mr. Darwin has worked out in detail the 

 simple scattering to be anticipated when a particles pass- 

 through hydrogen and helium. It is only necessary here to 

 refer to the fact that on the nucleus theory a small number 

 of hydrogen atoms should acquire, as the result of close 

 encounters with a particles, velocities about 1*6 times that of 

 the velocity of the a particle itself. On account of the fact 

 that the hydrogen atom carries one positive charge while the 

 a particle carries two, it can be calculated that some of the 

 hydrogen atoms should have a range in hydrogen of nearly 

 four times that of the a particle which sets them in motion. 

 * Butlierford and Nuttall, Phil. Mag. yxvi. p. 702 (1913). 



