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XXXVIII. The Diffusion of Actinium and Thorium Ema- 

 nations. By Sidney Russ, B.Sc, Demonstrator in Physics, 

 Manchester University *. 



Introduction. 



"IVfO direct determinations of the molecular weights o£ the 

 jLi radioactive emanations have yet been made. Owing 

 to the very small quantities of these substances available, 

 experimental work on this subject has been almost com- 

 pletely restricted to a study of the way in which the emana- 

 tions diffuse, and by a comparison of the diffusion coefficients 

 so obtained with those of gases of known molecular 

 weights, values of the molecular weights for the emanations 

 themselves may be inferred by means of Graham's law. 



The work of Curie and Dannef, Rutherford J, Rutherford 

 and Miss Brooks §, and Makower || on these lines has indi- 

 cated, for the emanations of radium and thorium, a molecular 

 weight of the order one hundred. Bumstead and Wheeler If, 

 however, give a rather higher value in the case of radium, 

 while for the emanation of actinium, Debierne** obtained a 

 molecular weight of seventy. 



On the disintegration theory radium and thorium emana- 

 tions should have very approximately the sama molecular 

 weight, namely, 222. There is thus a very large discre- 

 pancy between the theoretical and experimental numbers. 



In order to account for this it has been suggested that the 

 ordinary laws of diffusion do not perhaps hold when very 

 minute quantities of the substances in question are being- 

 dealt with. There being no obvious remedy in this direction, 

 it was thought that by a study of the diffusion of the emana- 

 tions in gases differing considerably in molecular weight, 

 any existing discrepancies would be brought to light. With 

 this object in view, the diffusion coefficients of actinium 

 emanation in air, hydrogen, carbon dioxide, and sulphur 

 dioxide were determined. While the work was in progress 

 there appeared in the American Journal of Science, June 

 1908, a paper by P. B. Perkins, in which the diffusion 

 of radium emanation was compared directly with that of 

 mercury vapour, the experiments giving a molecular weight 



* Communicated by the Physical Society : read November 27, 1908. 



t Comptes Rendus, 1903, p. 114. 



X i Radioactivity,' p. 276. 



$ Trans. Roy. Soc. of Canada, 1901-2. 



|| Phil. Mag. 1905, p. 56^ 



^1 Le Radium, June 1907. 



** American Journal of Science, Feb. 1901. 



