Velocities of a Particles from Radioactive Substances. 553 



It was concluded that the a particles from different radio- 

 active products were identical in mass but differed in their 

 initial velocity of projection. No change in the mass of the 

 a. particle was observed after its passage through absorbing- 

 screens. The value o£ E/wi obtained for the a. particle 

 suggested that it must be a projected helium atom carrying 

 two unit positive charges. This was definitely established by 

 the experiments of Rutherford and Koyds *, who found that 

 the a. particles fired through a thin glass tube consisted of 

 helium atoms, and by Rutherford and Geiger f by actual 

 measurement of the charge carried by each a particle. 

 The agreement between the calculated and experimental 

 values of the rate of production of helium by radium, as 

 shown by the experiments of Sir James Dewar % and 

 Boltwood and Rutherford §, still further confirmed these 

 conclusions. 



The object of the present experiments was to make an 

 accurate determination of the velocity of expulsion and the 

 ratio Fi/m of the « particle from radioactive substances.. 

 Although, as we have seen, the identity of the a particle 

 with a helium atom deprived of two electrons bas been 

 established by several independent methods of experiment, 

 the two quantities mentioned above are important and funda- 

 mental radioactive constants which require to be known with 

 the greatest possible precision. Moreover, a considerable 

 discrepancy exists between the value E/m for the a particle 

 deduced from radioactive measurements and the theoretical 

 value for the helium atom, deduced from the atomic and 

 electrochemical data. The most recent determinations of the 

 atomic weight of helium have been made by Watson ||, who 

 found a value 3"994 (0 = 16). and by Heuse ^[, who obtained 

 a value 4*002. Taking the atomic weight as the mean of 

 these two values, viz. 3'998 and the value of e\m for the 

 hydrogen atom as 9570, it follows that, for the helium atom 

 carrying two unit charges, the value of E/m should be 4826, 

 while, as we have seen, the experimental value for the 

 « particle is 5070. 



We are not aware of any precise experiments to settle 

 whether the value of e/m for a charged atom moving; with 



* Rutherford and Royds, Phil. Mag-, xvii. p. 281 H909). 

 f Rutherford and Geiger. Proc.Rov. Soc. A. Ixxxi. pp. 141, 162 (1908). 

 J Dewar, Proc. Roy. Soc. A. Ixxxi. p. 280 (1908) ; Ixxxiii. p. 404 (1910). 

 § Boltwood and Rutherford, Wien. Ber. cxx. p. 313 (1911). Phil. Rfag. 

 xx'ii. p. 586 (1911). 



|| Watson, Journ. Cheni. Soc. xcvii. p. 810 (1910). 



If Heuse, Ber. Deutsch. Phi/sikal. Gex. xv. p. 518 (1913). 



