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



In light of these results, and the view that has already 

 been put forward of the nature of radioactivity, the speculation 

 naturally arises whether the presence of helium in minerals 

 and its invariable association with uranium and thorium may 

 not be connected with their radioactivity. 



VII. The Nature of Emanating Power. 



The foregoing results therefore find their simplest expression 

 on the view that, just as a chemical change is proceeding in 

 thorium whereby a non-thorium material is produced, so the 

 latter undergoes a further reaction, giving rise to a gaseous 

 product which in the radioactive state constitutes the 

 emanation. 



It will be seen at once that this secondary change is of a 

 different kind from the primary, for it is affected apparently 

 by the conditions in a very marked manner. It was shown 

 that moisture, the state of aggregation, and temperature in- 

 fluenced the value of the emanating power. From — 80° to a 

 red heat the latter regularly increases in the ratio of 1 : 40 

 in the case of thorium oxide, while the ratios between the 

 values for thorium nitrate in the solid state and in solution is 

 as 1 : 200. The secondary reaction appears therefore at first 

 sight much more nearly allied to ordinary chemical reaction 

 than the primary. It must not be forgotten, however, that 

 the laws controlling the manifestation of the two phenomena — 

 radioactivity and emanating power — are of necessity very 

 different. In the former we deal with the intensity of 

 radiations emitted by a solid, in the latter with the rate of 

 escape of a gas into the surrounding air from either a solid or 

 a liquid. Since this gas is detected by its radioactivity, and 

 this decays extremely rapidly with time, a very slight delay 

 in the rate of its escape will enormously affect the experi- 

 mental value obtained for emanating power. 



On the other hand, it is now well established by experiment 

 that sometimes thorium compounds de-emanated chemically 

 by removal of ThX do not recover their normal emanating 

 power with time, but remain constant at a lower value. On 

 one occasion a carbonate was prepared which possessed hardly 

 any emanating power until it was again dissolved and preci- 

 pitated. In another experiment two samples of hydroxide 

 prepared from different nitrates were tested together for rise 

 of emanating power. That of the one rose normally to its 

 maximum (as in fig. 2), which was twenty times the minimum. 

 The other started from the same minimum, but rose to a 

 maximum only one-fourth as great. Wken the experiment 



