RADIOACTIVITY AND MATTER. 271 



we nowhere meet such an exclusiveness as here would be brought to 

 view. An exception might possibly be made as to the thorium, the 

 radioactivity of which, according to G. F. Barker, is independent of its 

 co-occurrence with uranium. However K. A. Hofmann and F. Zerban 

 contradict this opinion, and F. Zerban believes that he has detected 

 a small amount of uranium in the monazite-sand supposed entirely 

 free from it. The method he used is not quite unobjectionable ; it was 

 originally devised (by Laube) for the working of uranium residues and 

 not for analytical purposes. 



The material thus far used for obtaining radium and other radio- 

 active substances has mainly been the residue left by the K. K. 

 Austrian Uran works after extracting uranium from pitchblende. This 

 residue remains after pitchblende has been roasted, ignited with soda 

 and niter, and extracted with water and dilute sulphuric acid. It 

 consists mainly of gangue, silica, ferric oxide, basic iron sulphate and 

 lead sulphate, but contains also some bismuth and silver. It amounts 

 to 40 per cent, of the uranium ore worked, and we may estimate that 

 in the 50 years of their existence the uranium works have dumped 150 

 to 200 tons of this residue. It is most difficult to estimate the total 

 amount of radioactive substances in this dump, but the amount of 

 radium will count only by grams, that of radiotellurium only by frac- 

 tions of a gram. 



The residue referred to possesses 4.5-fold the activity of metallic 

 uranium. According to S. Curie, 1,000 kilos of the residue yielded 

 10 to 20 kilos crude sulfates of 30 to 60 activity, and these again 8 

 kilos of barium chloride containing radium also of 60 activity. This 

 seems to prove that the radioactivity does not keep pace with the con- 

 centration of the barium compounds, but lags far behind. A reliable 

 measure of the radium-amount is not furnished. Even a preparation 

 of 3,500 activity must have consisted mainly of barium chloride, since 

 it yielded 140 for the atomic weight which is only slightly in excess 

 of that of barium. As to the activity of the purest radium chloride, 

 we find no statements, but it is said that the best radium preparations 

 have an activity of 50,000 to 100,000 times that of uranium. 



Now arises the question: in what form of combination is radium 

 contained in pitchblende, and in the final solution of the residue? 

 In the latter it is certainly as sulphate, probably also in the former, 

 since heavy spar is most commonly associated with pitchblende. Since 

 now the solubility of the sulphates of the metals of the alkaline earths di- 

 minishes with the increase of the atomic weight, radium sulphate ought 

 to be the most insoluble of all in this group. From this and from the 

 isomorphism of barium and radium compounds, we must conclude that 

 the heavy spar associated with pitchblende really must carry the 

 radium of the Joachimsthal ores. But William Crookes has been un- 



