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LXIV. The Cause and Nature of Radioactivity. — Part II. 

 • By E. Rutherford, M.A., B.Sc, Macdonald Professor of 



Physics, and F. SoDDY, B. A. (Oxon.), Demonstrator in 



Chemistry, Me Gill University, Montreal*. 



Contents. 

 I. Introduction. 



II. Method of Measuring Emanating Power. 

 III. The De-emanation of Thoria and the Regeneration of the 



Emanating Power. 

 IV. The Effect of Conditions upon Emanating Power. 

 V. The Cause of the Emanating Power of Thorium. 

 VI. The Chemical Nature of the Emanation. 

 VII. The Nature of Emanating Power. 

 VIII. The Excited Radioactivity from Thorium. 

 IX. Eurther Theoretical Considerations. 



I. Introduction. 



THE investigation of the radioactivity of thorium, detailed 

 in the first part of this commtmicationf , arose out of an 

 examination of the power possessed by thorium compounds 

 of giving out a radioactive emanation. The nature of this 

 property and its relation to the radioactivity of thorium 

 remain to be considered. 



A -hort resume of what was known at the commencement 

 of the work may be of interest. Thorium radioactivity was 

 discovered by Schmidt and Curie independently in 1898, and 

 Owens in the following year investigated its nature in detail 

 (Phil. Mag. 1899, p. 360). He observed the inconstancy of 

 the radiation and the effect of air currents in reducing its 

 value. The discovery of the thorium emanation which ex- 

 plained these results, and its power of exciting activity on 

 surrounding matter, followed shortly after (Rutherford, Phil. 

 Mag. 1900, pp. 1 & 161). 



It was shown that the radiation from the emanation decays 

 rapidly, but at a perfectly defined rate, falling to about one- 

 half the original value at the end of one minute. The ema- 

 nation passes unchanged through cotton -wool, weak and 

 strong snlphnric acid, and aluminium and other metals in the 

 form of foil, but not through' an extremely thin sheet of 

 mica. The emanating power of thoria is independent of the 

 surrounding atmosphere, but is destroyed to a large extent 

 by intense ignition, and does not return when the substance 

 is kept. 



There is a very close connexion between the excited radio- 

 activity produced by thorium compounds and the emanation. 



* Communicated bv the Authors. 



t For Part I. see Phil. Maor. Sept. 1902. 



Phil. Mag. S. 6. Vol. 1. No, 23. Sov. 1902. 2 P 



