JRadio-activity of Uranmm, 



411 



Not one of these minerals showed the slightest action on the sensi- 

 tive plate. 



4. Having obtained negative results with barium compounds, I 

 went through every mineral in my cabinet — a somewhat extensive 

 collection, numbering many fine specimens. Large photographic 

 plates were covered with black paper, and the minerals were laid on 

 them as close as they could conveniently be placed, accurate note of 

 their names and positions being recorded. They were exposed in 

 total darkness for forty-eight hours. By this means a list of radio- 

 active minerals was ultimately obtained. They were then tested for 

 order of intensity of action. The following is a list of active minerals 

 arranged in order, the most active heading the list : — 



It will be observed that these minerals all contain either uranium or 

 thorium. 



5. Pitchblende was the most radio-active mineral, but it varied 

 much in different parts. A slice was cut from a piece of pitchblende 

 from Cornwall and the surface was polished. A sensitive photo- 

 graphic plate was pressed against it, and after twenty-four hours the 

 plate was developed. The impression showed the structure of the 

 mineral in a remarkable manner, every little piece of pitchblende 

 showing black, those portions in which the radio-active substance was 

 not so operative showing in half tint, while the felspar, quartz, pyrites, 

 &c., having no radio-activity, left the plate transparent (see Plate 5). 



Pitchblende from different localities differed greatly in action. 



6. A large crystal of orangite from Arendal was ground flat and 

 polished at one end and side, and a piece of sensitive celluloid film, 

 cut half through so as to allow it to bend sharply, was put on the 

 polished surfaces, one half pressing against the end and the other 

 half against the side. The exposure was continued for seventy-two 

 hours. On developing, no difference could be seen in the intensity of 

 the impression, whether made by the end or the side of the crysta,l. 

 The impression also was uniform over the surface, the cracks in the 

 surfaces not having impressed themselves. 



These experiments were repeated with the interposition of a thin 

 sheet of celluloid between the mineral and the sensitive plate. The 

 results were practically the same as before. 



1. Pitchblende. 



9. Broggerite. 



10. Monazite. 



11. Xenotime. 



12. Arrhenite. 



13. Sipilite. 



2. Uranite. 



3. Autunite. 



4. Orangite. 



5. Thorite. 



6. Euxenite. 



7. Samarskite. 



8. Alvite. 



14. Fergusonite, 



15. Chalcolite. 



16. Hielmite. 



