the Cause and Nature of Radioactivity. 387 



■emanating power are here absent. The separation of UrX 

 and the recovery of the activity of the uranium with time 

 appear, however, analogous to these processes in thorium, 

 and the rate of recovery and decay of uranium activity are 

 at present under investigation. It i* proposed to test the 

 influence of conditions on the rate of change more thoroughly 

 in the case of uranium,, as here secondary changes do not 

 interfere. 



VIII. The Cause and Xature of Radioactivity. 



The foreooino- conclusions enable a great generalization to 

 he made in the subject of radioactivity. Energy considera- 

 tions require that the intensity of radiation from any source 

 should die down with time unless there is a constant supply 

 of energy to replace that dissipated. This has been found to 

 hold true in the case of all known types of radioactivity with 

 the exception of the " naturally " radioactive elements — to 

 take the best established cases, thorium, uranium, and radium. 

 It will be shown later that the radioactivity of the emanation 

 produced by thorium compounds decays geometrically with 

 the time under all conditions, and is not affected by the most 

 drastic chemical and physical treatment. The same has been 

 shown by one of us (Phil. Mag. 1900, p. 161) to hold for the 

 excited radioactivity produced by the thorium emanation. 

 This decays at the same rate whether on the wire on which 

 it is originally deposited, or in solution of hydrochloric or 

 nitric acid. The excited radioactivity produced by the 

 radium emanation appears analogous. All these examples 

 satisfy energy considerations. In the case of the three 

 naturally occurring radioactive elements, however, it is 

 obvious that there must be a continuous replacement of the 

 dissipated energy, and no satisfactory explanation has yet 

 been put forward. 



The nature of the process becomes clear in the light of the 

 foregoing results. The material constituent responsible for 

 the radioactivity, when it is separated from the thorium which 

 produces it, then behaves in the same way as the other types 

 of radioactivity cited. Its activity decays geometrically with 

 the time, and the rate of decay is independent of the mole- 

 cular conditions. The normal radioactivity is. however, 

 maintained at a constant value by a chemical change which 

 produces fresh radioactive material at a rate also independent 

 of the conditions. The energy required to maintain the 

 radiations will be accounted for if we suppose that the energy 

 of the system after the change has occurred is less than it 

 was before. 



