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 radioactive 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 fractions 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 concentration 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 diminishes 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-
