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XXI. The Nature of the a Particle from Radioactive Sub- 

 stances. By Professor E. Rutherford, F.R.S., and 

 T. Royds, M.Sc, 1851 Exhibition Science Scholar*. 



TflHE experimental evidence collected during the last 

 JL few years has strongly supported the view that the 

 a. particle is a charged helium atom, but it has been found 

 exceedingly difficult to give a decisive proof of the relation. 

 In recent papers, Rutherford and Greiger f have supplied still 

 further evidence of the correctness of this point of view, 

 The number of a. particles from one gram of radium have 

 been counted, and the charge carried by each determined. 

 The values of several radioactive quantities, calculated on the 

 assumption that the a. particle is a helium atom carrying two 

 unit charges, have been shown to be in good agreement with 

 the experimental numbers. In particular, the good agree- 

 ment between the calculated rate of production of helium by 

 radium and the rate experimentally determined by Sir James 

 DewarJ, is strong evidence in favour of the identity of the 

 a. particle with the helium atom. 



The methods of attack on this problem have been largely 

 indirect, involving considerations of the charge carried by 

 the helium atom and the value of e/m of the a particle. 

 The proof of the identity of the u particle with the helium 

 atom is incomplete until it can be shown that the a. particles, 

 accumulated quite independently of the matter from which 

 they are expelled, consist of helium. For example, it might be 

 argued that the appearance of helium in the radium emana- 

 tion was a result of the expulsion of the « particle, in the 

 same way that the appearance of radium A is a consequence 

 of the expulsion of an « particle from the emanation. If 

 one atom of helium appeared for each a. particle expelled, 

 calculation and experiment might still agree, and yet the 

 u particle itself might be an atom of hydrogen or of some 

 other substance. 



We have recently made experiments to test whether helium 

 appears in a vessel into which the a particles have been fired, 

 the active matter itself being enclosed in a vessel sufficiently 

 thin to allow the a. particles to escape, but impervious to the 

 passage of helium or other radioactive products. 



* Communicated bv the Authors. 



t Proc. Roy. Soc. A. lxxxi. pp. 141-173 (1903). 



\ Proc. Ttoy. Soc. A. lxxxi. p. 280 (1908). 



