Radioactivity of Radium and Thorium. 449 



in all cases, as we have already shown for the thorium 

 emanation. M. Curie, however, apparently overlooked the 

 initial period of increase, and states that the rate of decay is 

 uninfluenced by an alteration of the time of " activization " 

 (or exposure to the original radium) from 15 minutes to one 

 month. As we have seen, for any period under 5 or 6 hours 

 the excited activity will increase at first instead of decaying, 

 and for a period as short as 15 minutes this effect would be 

 very marked. This increase of activity, due to the steady 

 production of excited activity when the emanation is trans- 

 posed from one vessel to another, is of course only one example 

 of many similar ones that have now been accumulated. But 

 it would be difficult to get in any single experiment a better 

 illustration of the real nature of the phenomena occurring in 

 radioactivity. A quantity of the emanation is removed from 

 the radium that produced it, and, mixed with air, is stored 

 like an ordinary gas over mercury in a gas-holder. Several 

 weeks after, it may be, a portion is removed to a new vessel, 

 when its activity is found to rise steadily to double its original 

 value in the course of a few hours, showing that all the time 

 it has been manufacturing out of itself the fresh active matter 

 which causes the excited activity. When the maximum is 

 reached, it is not because the process of manufacture has 

 stopped, but because an equilibrium has been reached between 

 the rate of supply of new active matter and the rate of decay 

 of that already deposited. 



§ 3. The Occlusion of the Emanations. 



In the solid state radium compounds give out so small an 

 amount of emanation that special methods must be employed 

 to detect it. As in the case of thorium compounds, heat or 

 moisture, but especially solution in water, increases the 

 emanating-power of radium ; but these effects are far more 

 marked in the latter case. The same considerations apply 

 equally to the power of radium to excite or induce activity on 

 bodies in its neighbourhood ; for the activity excited under 

 any conditions is proportional only to the amount of emanation 

 present. These variations in the power of exciting activity 

 in radium compounds were observed by Dorn and by M. and 

 Mine. Curie. 



Radium compounds can be de-emanated by ignition, and 

 the de-emanated compounds recover their power as soon as 

 they are brought into solution, exactly in the same manner 

 as we have shown for thorium compounds (M. and Mine. 

 Curie, Comp, Rend, cxxxiv. p. 85, 1902). It may be stared 



