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



It was shown that the amount of the former produced under 

 various conditions was proportional to the amount of the 

 latter, and if the emanating power of thoria be destroyed by 

 ignition, its power to excite radioactivity correspondingly 

 disappears. Simultaneously with the appearance of the 

 papers referred to, Curie showed that radium also possessed 

 the power of exciting activity on surrounding objects. Later, 

 Dorn (Abk. der Naturforsch. Ges. fur Ilalle-a.-S., 1900) re- 

 peated the work quoted for thoria, and extended it to include 

 two preparations of radioactive barium compounds (radium) 

 prepared by P. de Haen, and a preparation of radioactive 

 bismuth (polonium). He found that radium, but not polo- 

 nium, gave an emanation, especially on warming, and this 

 possessed the power of exciting activity on surrounding 

 objects. Radium and thorium are in this respect completely 

 analogous and different from other radioactive substances, 

 but the phenomena in the two cases are quite different. The 

 emanation from radium retains its activity for many weeks, 

 while the excited radioactivity it produces, on the other hand, 

 decays much more rapidly than that from thorium. 



One of the most interesting advances in this connexion 

 was made during the progress of the work by Elster and 

 Geitel (Phys. Zeit. 1901, ii. p. 590), who found that it is 

 possible to produce excited radioactivity from the atmosphere, 

 without further agency, by simply exposing a wire highly 

 charged to a negative potential in the air for many hours, 

 and that this also possesses the property of being dissolved 

 off by acids, and of being left behind unchanged on the 

 evaporation of the latter. But here again the rate of decay 

 is different from that of the excited radioactivity produced 

 by thorium. 



At the commencement of the work the presumption seemed 

 to be in favour of considering emanating power as a separate 

 phenomenon not directly connected with the ordinary radio- 

 activity of thorium. The former could be destroyed in 

 thorium oxide by ignition without reducing the latter. Later 

 many external conditions were found to affect the value of 

 emanating power without influencing the radioactivity. The 

 nature of the phenomenon had been fully examined from this 

 point of view with very puzzling results, but the conclusion 

 was arrived at that emanating power is probably the mani- 

 festation of a change of the nature of a chemical reaction. 



The discovery of ThX and its continuous production, how- 

 ever, revealed the true interpretation of the results, and 

 enables a fairly complete explanation of the phenomenon to 

 be oiven. 



