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LXXV . Excited Radioactivity and Ionization of the Atmosphere . 

 By E. Rutherfokd, M.A., D.Sc, Macdonald Professor of 

 Physics, and S. J. Allen, M.Sc, Demonstrator in Physics, 

 McGill University, Montreal*. 



THE experiments of Elster and Geitelf and C. T. R. 

 Wilson % have shown that a well insulated charged 

 conductor placed inside a closed vessel gradually loses its 

 charge, and that this loss of charge is due to a small spon- 

 taneous ionization of the volume of air inside the closed 

 vessel. Wilson calculated from his results that about 19 ions 

 per c.c. are produced in the air per sec. In a later paper § 

 Wilson has shown that the ionization in different gases varies 

 approximately as the density and pressure of the gas. These 

 results point to the possibility that the ionization observed in 

 gases may be due, in part at least, to the emission of an 

 ionizing radiation from the walls of the containing vessel. 



Recently Elster and Geitel || made the interesting discovery 

 that a negatively charged conductor, placed in the open air, 

 becomes temporarily radioactive. This radioactivity decays 

 in the course of a few hours. The phenomena appear to be 

 closely analogous to the " excited " radioactivity produced 

 by the radioactive emanations of thorium and radium. The 

 excited activity from the air can be concentrated on the 

 negative electrode in exactly the same way as one of the 

 authors 1[ has shown for thorium-excited activity. 



In addition Elster and Geitel have shown that the substance 

 responsible for the radioactivity can be removed by solution 

 in acid. On evaporating the solution to dryness an active 

 residue, which deca} T s with time, is left behind in the vessel. 

 This also is in striking agreement with what one of us {he. cit.) 

 had previously shown for thorium-excited activity. 



In the experiments of Elster and Geitel, and Wilson, the 

 amount of ionization of air has been determined by observing 

 the rate at which the leaves of a charged electroscope of 

 special construction fall together. This method, while very 

 simple and adAantageous for some experiments, is, in general, 

 slow, and in many cases does not allow of sufficient variation 

 of experimental conditions. 



In the present investigation the authors have utilized a 



* Communicated by the Authors. [Communicated to the American 

 Phvsical Societv, Dec. 27, 1901 ; Abstract published iu the Phys. Zvit. 

 No! 11, 1902. 



t Phys. Zeit. Nov. 24, 1900. 



X Proc. Rov. Soc. March 1901. § Ibid. Dec. 1901. 



|| Phys. Zeit. iii. p. 76 (1901) : xl. p. 590 (1901). 



% Phil. Mag. Feb. 1900. 



