184 PHYSICS OF THE ELECTRON 



appears. The considerations advanced to account for radium apply 

 equally well to actinium, which in all probability, when isolated, will 

 prove to be an element of the same order of activity as radium. The 

 most important problem at present in the study of radioactive min- 

 erals is not the attempt to discover and isolate new radioactive sub- 

 stances, but to correlate those already discovered. Some progress 

 has already been made in reducing the number of different radio- 

 active substances and in indicating the origin of some of them. For 

 example, there is no doubt that the " emanating substance " of Giesel 

 contains the same radioactive substance as the actinium of Debierne. 

 In a similar way, there is very strong evidence that the active con- 

 stituent in the polonium of Mme. Curie is identical with that in the 

 radio-tellurium of Marckwald. The writer has recently shown that 

 the active constituent in radio-tellurium or polonium is, in all prob- 

 ability, a disintegration product of radium (radium E). The same 

 considerations apply to the radio-lead of Hofmann, which is probably 

 identical with the product radium D. It still remains to be shown 

 whether or no there is any direct family connection between the 

 radioactive substances uranium, thorium, radium, and actinium. 

 It seems probable that some at least of these substances will prove to 

 be lineal descendants of a single parent element, in the same way 

 that the radium products are lineal descendants of radium. The 

 subject is capable of direct attack by a combination of physical and 

 chemical methods, and there is every probability that a fairly definite 

 answer will soon be forthcoming. 



X. Radioactivity of the Earth and Atmosphere 



It is now well established, notably by the work of Elster and Geitel, 

 that radioactive matter is widely distributed both in the earth's 

 crust and atmosphere. There is undoubted evidence of the presence 

 of the radium emanation in the atmosphere, in spring water, and in 

 air sucked up through the soil. It still remains to be settled whether 

 the observed radioactivity of the earth's crust is due entirely to 

 slight traces of the known radioactive elements or to new kinds of 

 radioactive matter. It is not improbable that a close examination 

 of the radioactivity of the different soils may lead to the discovery of 

 radioactive substances which are not found in pitchblende or other 

 radioactive minerals. The extraordinary delicacy of the electro- 

 scopic test of radioactivity renders it possible not only to detect the 

 presence in inactive matter of extremely minute traces of a radio- 

 active substance, but also in many cases to settle quickly whether 

 the radioactivity is due to one of the known radio-elements. 



The observations of Elster and Geitel render it probable that the 

 radioactivity observed in the atmosphere is due to the presence of 



