the Cause and Nature of Radioactivity. 57$ 



careful determination, using 20 grams of finely-powdered 

 thorium nitrate, this worked out to be only 1*8 per cent, of 

 the emanating power of thoria. Dissolved in water, however, 

 and tested for emanation bv bubbling a current of air through 

 the solution, it gives about three times as much emanation as 

 thorium oxide. That is, solution in water increases the 

 emanating power of thorium nitrate nearly 200 times. The 

 emanating power, as in the case of solids, is proportional to 

 the weight of substance present, and within the limits tried 

 is not much affected by dilution, for a solution of 10 grams 

 made up to 25 c.c. in volume possessed a similar value when 

 diluted four times. 



V. Tlie Cause of the Emanating Power of Thorium. 



The separation from thorium of ThX, detailed in the first 

 part of this communication, showed that not only the radio- 

 activity but also the emanating power of thorium is connected 

 with the presence of a non-thorium type of matter, ThX. 

 The solutions from which thorium hydroxide had been pre- 

 cipitated by ammonia possessed, when concentrated, about as 

 much emanating power as the solutions from which they 

 were prepared, while the precipitated hydroxide was more or 

 less completely de-emanated. On allowing these preparations 

 to stand, the emanating power of the filtrates gradually dis- 

 appeared, while that of the hydroxide in most cases rose 

 steadilv with time, till at the end of a fortnight thev had 

 attained a maximum between three and four times that of 

 ordinary thoria. This recovery of the emanating power in 

 the case of the hydroxide was noticed long before the similar 

 change of its radioactivity was observed, but the two 

 phenomena admit of a similar explanation. If, in the 

 precipitation by ammonia, care is taken to remove the ThX 

 completely, the thorium hydroxide is at first almost devoid of 

 emanating power. The small faction that remains— only a 

 few per cent, of the maximum — can be accounted for by the 

 reproduction of ThX during the time taken to dry the 

 precipitate. 



The Rate of Recovery and Decay of Emanating Power. — 

 The rate of decay of the emanating power of ThX, and the 

 recovery of this property by the thorium from which it had 

 been separated, were then investigated in parallel with the 

 similar experiments on radioactivity already described. One 

 quarter of the concentrated filtrate used for the latter purpose 

 was taken, and the decrease of its emanating power with 

 time measured. The increase of emanating power of the 

 thorium hydroxide from which ic had been prepared was also 



