Badio'-activity of Uranium, 



413 



A sensitive film is laid face upwards at the bottom of the wooden 

 tray ; on this are put the lead screens, and then the experimental cells 

 of radio-active bodies in order, careful note being taken of their rela- 

 tive positions. 



9. Wishing to prepare compounds of radium and polonium, the 

 very curious bodies discovered by M. and Mdme. Curie, I arranged 

 with my friend Mr. Tyrer, Stirling Chemical Works, for the systematic 

 working up of half a ton of pitchblende. It was necessary to examine 

 every precipitate and filtrate in each stage of the operation, and for 

 convenience of registration they had to be compared with a standard 

 cell filled with a substance unvarying in action. After many trials, I 

 selected crystallised uranium nitrate as being strongly radio-active and 

 easily prepared pure. This led me to the observation which forms the 

 subject of the present paper. 



10. The following compounds of uranium were tested simultaneously, 

 being put into glass cells and arranged on a lead screen with seven 

 holes ; — 



1. Metallic uranium, from M. Moissan. 



2. Uranium nitrate, U02(N03)2.6H20. 



3. Uranium acetate. . . 



4. Uranium persulphate. 



5. Uranium protosulphate. 



6. Uranium oxide (green), UO22UO3. 



7. Uranium oxide (black), UO2UO3. 



, For twenty-four hours the sensitive plate was exposed to the 

 influence of these bodies. With the exception of metallic uranium, 

 which showed least action, there was not much difference between the 

 effect produced by any of the others. 



11. In order to prepare uranium nitrate of great purity for a 

 standard, I took some pounds of the commercial salt and purified it, 

 first by solution in ether (13), and then by repeated crystallisation. 

 After many operations a cell was filled with the crystals, and it was 

 used as a standard. To my surprise, after having acted on a sensitive 

 plate for twenty-four hours, on development not a trace of image 

 could be seen. 



12. Thereupon I tried the following experiments to ascertain if the 

 great radio-activity of some uranium compounds and the absence of it 

 in others might be caused by some variation of physical, crystalline, 

 or chemical condition. 



Commercial uranium nitrate was taken, and — 



(1) A portion was heated with excess of nitric acid to dryness on 

 the water-bath. It was powdered and put in a cell. 



(2) A similar portion of the salt was dissolved in alcohol, and the 

 solution evaporated to dryness over the water-bath. The resulting 

 orange-coloured pasty material was ground and put in a cell. i ; 



