Radioactivity of Radium and Thorium. 453 



In the case, therefore, of both thorium and radium the 

 manufacture o£ emanation takes place at the same rate in 

 non-emanating as in highly emanating compounds. 



The effect of heating solid non-emanating radium com- 

 pounds is precisely analogous to the effect of dissolving 

 them. It has long been known that the emanating-power 

 of solid radium preparations is increased to the order of a 

 hundred thousand times by heat. As in the case of solution, 

 the occluded emanation is liberated ; and when this has passed 

 off the effect again falls to a value approximating the true 

 emanating-power, i. e. the amount of emanation produced 

 per second. 



A compound like thorium oxide, possessing in the solid 

 state one-third to one-fourth of the emanating-power of the 

 amount of thorium in solution, has its emanating-power 

 increased 3 or 4 times at a dull red heat. A compound like 

 the hydroxide or carbonate, on the other hand, which pos- 

 sesses as much emanating-power in the solid state as when 

 dissolved, does not suffer much increase of emanating-power 

 with rise of temperature. 



The changes of emanating-power that are produced in 

 thorium and radium compounds by ignition, moisture, solu- 

 tion, &c, are therefore to be ascribed solely to changes in 

 the rate of escape of the gaseous emanation into the sur- 

 rounding medium from the substance producing it. This 

 result is of great importance in the general theory of radio- 

 activity, for it brings into conformity what might otherwise 

 have been regarded as an exception to the view that the 

 processes which maintain radioactivity lie outside of the 

 sphere of known molecular forces. 



Attention may here be drawn to the fact that the general 

 phenomenon of occlusion of a gas by a solid is not connected 

 at all with the radioactive properties of the matter in question, 

 although in the present instances radioactivity has furnished 

 a convenient means of accurately studying the problem. The 

 helium is given off from the mineral f ergusonite, for example^ 

 in part when it is heated, and completely by dissolving the 

 mineral. 



It is therefore to be expected that if any of the unknown 

 ultimate products of the changes of a radioactive element are 

 gaseous, they would be found occluded, possibly in consi- 

 derable quantities, in the natural minerals containing that 

 element. This lends support to the suggestion already put 

 forward (Phil. Mag. 1902, iv. p. 582) that possibly helium is 

 an ultimate product of the disintegration of one of the radio- 

 active elements, since it is only found in radioactive minerals. 



Phil. Mag. S. 6. Vol. 5. No. 2S. April 1903. 2 H 



