586 Profs. B. B. Boltwood and E. Rutherford on 



vapour -of ice, but -jL is larger and L^ smaller in the ease of 



ice than in the case of water. The equations cannot there- 

 fore agree with the facts in each case, which indicates that 

 the matter is not evenly distributed in space in one or both 

 cases. 



Cambridge, March 10th, 1911. 



LV. Production of Helium by Radium. By Prof. B. B. 

 Boltwood and Prof. E. Rutherford, F.R.S* 



Introduction. 



THE connexion of helium with the transformation of 

 radioactive material has been a problem of great 

 interest and importance for the past eight years, and has 

 been the subject of a number of investigations. 



In 1908, Rutherford and Socldy f advanced the theory of 

 the disintegration of radioactive matter and suggested that 

 the helium, which occurs in such relatively large quantities 

 in radioactive minerals, was probably one of the products 

 of transformation of the radio-elements contained in these 

 minerals. In 1903, Ramsay and Soddy t showed experi- 

 mentally that helium could be obtained from a salt of radium 

 and that helium was produced from the emanation of radium. 

 In these early experiments, the helium was present in minute 

 quantity and was detected by spectroscopic tests. These 

 observations were shortly afterwards confirmed by a number 

 of independent experimenters. Somewhat laier Debierne 

 found that helium was produced also from preparations of 

 actinium. 



At the time of the discovery of the production of helium 

 by radium, it was of fundamental importance to fix the 

 position of helium in the general scheme of radioactive 

 transformations. In 1903, Rutherford § showed that the 

 a, rays emitted by radium consisted of positively charged 

 particles moving with a high velocity. The determination 

 of e/m — the ratio of the charge to the mass of the a particle — 

 indicated that the a particle was of atomic dimensions and 

 of apparent mass about twice that of the hydrogen atom. 



* A preliminary note on the subject was communicated by the 

 Authors to the Manch. Lit. and Phil. Soc. No. 6, 1909. The complete 

 paper was published in the Wiener Berichte y cxx. II. a, March 1911. 



t Phil. Mag. [6] vol. iv. p. 582 (1902). 



% Proc. Roy. Soc. vol. lxxii. p. 204 (1903). 



§ Phil. Mag. [6 J vol. v. p. 177 (1903). 



