168 G. F. Barker — Radioactivity of Thorhtvi Minerals. 



thorium, of a gas or vapor itself temporarily radioactive and 

 capable of emitting rays, has thrown mncli light upon this 

 so-called induced radioactivity. This " emanation," as Ruth- 

 erford calls it, diffuses without change through gases and 

 liquids, and even porous substances, but not through glass or 

 mica. It is chemically inert, and may be passed through 

 ignited platinum black and lead chromate without change. It 

 appears to be a new member of the argon family, with an 

 atomic mass between forty and one hundred. The emanations 

 from thorium and radium differ somewhat in pro^Derties, that 

 from the former liquefying at — 120° and that from radium at 

 — 150°. Moreover, the radioactivity of the thorium emanation 

 rapidly decays, falling to half its value in one minute ; while 

 that of the radium emanation retains its active properties for 

 several weeks. On the other hand, the " excited " radioactiv- 

 ity, as Rutherford prefers to call it, produced by the former 

 emanation, is much more permanent than that produced by the 

 latter. Since excited radioactivity can be produced on bodies 

 if the emanation be present, even in the absence of a radioac- 

 tive substance, and since the amount of effect is directly pro- 

 portional to the amount of the emanation, it follows, first, that 

 the production of excited radioactivity is a property of the 

 emanation and therefore is always produced on bodies when 

 the radioactive emanations from thorium and radium are pres- 

 ent ; and second, that uranium and polonium wdiich do not 

 give oif any emanation, do not possess the power of exciting 

 radioactivity. In the present view of science, therefore, it 

 would not seem probable that the radioactivity of thorium is 

 a secondary or excited radioactivity due to the uranium asso- 

 ciated with it in the minerals above mentioned. 



