Manchester MeDwirs, Vol. Ihi. (1908), No. \. 



I. The Nature of the n Particle. 



By Professor E. RUTHERFORD, F.R.S., 



AND 



T. ROYDS, M.Sc. 



Received and read N'ove?nber jrd, igo8. 



The nature of the o particle from radioactive 

 substances has, for several years, been one of the most 

 important questions in Radioactivity. The evidence as a 

 whole indicates that the a particle is an atom of helium 

 carrying a positive charge. Recent experiments of 

 Rutherford and Geiger (Proc. Roy. Soc, 1908) have sub- 

 stantiated this conclusion. An additional proof of the 

 correctness of this point of view is afforded by the good 

 agreement between the rate of production of helium 

 calculated by Rutherford and Geiger, and the rate of 

 production recently measured by Sir James Dewar {Proc. 

 Roy. Soc, 1908). 



This evidence is, however, of too indirect a character 

 to prove decisively that the a particle is an atom of 

 helium. It might be possible, for example, that the ex- 

 pulsion of an a particle led to the liberation of helium 

 from the active matter, but that the a particle itself was 

 not an atom of helium. In order to give a definite proof 

 of the identity of the a particle with a helium atom, it is 

 necessary to show that helium can be obtained from 

 accumulated a particles, quite independentl}' of the active 

 matter from which they are expelled. This has been 

 done in the following way : — 



Purified emanation, corresponding to the equilibrium 



November ipiA, igo8. 



