.370 Prof. E. Rutherford and Mr. F. Soddy on 



be a mere point ; the region of misfit (to borrow an expressive 

 term from Prof. Osborne Reynolds) might, if necessary 3 have 

 definite extension and structure. Hypotheses of this type 

 are most naturally (indeed, tfs it seems to me, unavoidably) 

 expressed in terms of an aether which is only locally disturbed 

 by each moving ion ; so that a congeries of connected atoms 

 like the Earth does not push it along bodily and establish any 

 finite flow. But there may be philosophers who prefer not to 

 employ the term aether at all, who are satisfied with a colourless 

 phenomenology, and who manage to escape the consideration 

 of the possibility of an aether whose parts maintain their 

 positions notwithstanding the motion of matter through it, by 

 saying merely that if a certain scheme of formal relations 

 between variables which are symbols of things unknown is 

 altered in a certain formal way, probably originally suggested 

 by the use of dynamical analogies such as have been referred 

 to, the scheme will continue to group the facts under the wider 

 conditions, and they would thus feel freed from any necessity 

 -of considering images or models, probably imperfect, of things 

 which being outside ourselves we cannot intrinsically know. 

 Cambridge, August 7,' 1902. 



XLI. The Cause and Nature of Radioactivity. — Part I. By 

 E. Rutherford, M.A., D.Sc, Macdonald Professor o 

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

 Chemistry, McGill University, Montreal*. 

 Contents. 

 I. Introduction. 

 II. Experimental Methods of investigating Radioactivity. 



III. Separation of a Radioactive Constituent (ThX) from Thorium 



Compounds. 



IV. The Rates of Recovery and Decav of Thorium Radioactivity. 

 V. The Chemical Properties of ThX. 



VI. The Continuous .Production of ThX. 



VII. The Influence of Conditions on the Changes occurring in 

 Thorium. 

 VIII. The Cause and Nature of Radioactivity. 

 IX. The Initial Portions of the Curves of Decay and Recovery. 

 X. The Non-separable Radioactivity of Thorium. 

 XI. The Nature of the Radiations from Thorium and ThX. 

 XII. Summary of Results. 

 XIII. General Theoretical Considerations. 



I. Introduction. 



THE following papers give the results of a detailed 

 investigation of the radioactivity of thorium com- 

 pounds which has thrown light on the questions connected 



* Communicated "by the Authors. Accounts of these researches, 

 during the progress of the investigation, have already been given to the 

 London Chemical Societv. 



