386 PROCEEDINGS OP THE AMERICAN ACADEMY. 



pure ammonic su]phhydrate added in excess. The color of the result- 

 ing precipitate of uranium sulphide varies greatly with the temperature, 

 lu warm solution it was at first reddish brown, while that precii)itated in 

 the cold varied from bright red to brownish yellow. On washing, all 

 turn black, the sulphide being decomposed into uranous oxide and sul- 

 phur. After thorough washing the resulting mixture of oxide and 

 sulphur was ignited in a porcelain dish, the green urano-uranic oxide 

 being the product. 



The oxide was then dissolved in a platinum dish in redistilled nitric 

 acid, evaporated, and recrystallized from nitric acid solution. Uranyl 

 nitrate does not crystallize well from aqueous solution, but it was found 

 that if a little nitric acid is added, it crystallizes readily in fairly large 

 monoclinic prisms. This recrystallization was repeated ten times from 

 acid solution, and finally twice from aqueous solution. Finally the pure 

 nitrate was converted to the oxide by ignition in platinum. A second 

 sample, used in the preliminary series, was prepared by repeated 

 fractionation of the mother liquors of the first sample. 



Since this work was carried out. Sir William Crookes * has published 

 the account of several methods by which he was able to prepare specimens 

 of uranyl nitrate which were not radio-active. The radio-activity of 

 uranium has hitherto been supposed to be characteristic of this element. 

 Crookes has shown, however, that this is not the case, but that the 

 active element can be separated by treatment with ether, by fractional 

 crystallization, or by treatment with excess of ammonium carbonate. 

 Unfortunately none of the pure oxide prepared for this investigation 

 remained, hence it is impossible to test directly its radio-activity. Since 

 two of Crookes's methods were used in purifying our material, viz. the 

 ammonium carbonate treatment and fractional crystallization, it is highly 

 improbable that our oxide was radio-active. In repeating Crookes's 

 work with nitrate made from some of the same material used in pre- 

 paring our best nitrate, it was found that a sample of the ^/i/i!A crystalliza- 

 tion gave no trace of action on twenty-four hours exposure to a quick 

 photographic plate. The material used in this experiment had not been 

 submitted to the ammonium carbonate treatment. "When it is con- 

 sidered that the material used for our atomic weight determinations was 

 first put through the carbonate process, — in itself sufficient to remove the 

 radio-active element, — and then was recrystallized twelve times as 

 nitrate, it would seem that our pure oxide must have been free from 

 all radio-active material. 



* Proceed. Lond. Royal Soc, 66, 409 (1900). 



