6 Newbery — Lupton, Radio-activity and Coloration of Minerals. 



A purple crystal also failed at first to regain its original colour 

 under the action of radium but acquired the same peculiar blue, 

 which again changed to purple in a similar manner. 



The colourless Alston Moor specimen showed traces of a lighter 

 blue colour after twenty days' treatment with 25 mg. of radium, 

 but immediately under the radium tube the crystal was broken up 

 and a yellow stain produced. This yellow colour slowly penetrated 

 the whole crystal on further treatment. 



The colourless Pyrenean specimen was uncoloured by the 

 radium treatment given to it, but it rapidly acquired a deep purple 

 colour with a brilliant bronze surface tarnish under the 'action of 

 the cathode rays. 



The most striking colour effect in this work was obtained with 

 the colourless Matlock fluor. After only twenty-four hours under 

 50 mg. of radium, the whole crystal attained a wonderful deep 

 blue colour, resembling, but deeper than, that of a copper sulphate 

 crystal. This particular type of colour is very rare, if it occurs at 

 all, in any natural fluor. It is quite permanent if the crystal is kept 

 in the dark, but is rapidly destroyed (in about three hours) in direct 

 sunlight, more slowly by diffused daylight. It is also destroyed by 

 gentle heating, but may be restored as often as desired by renewed 

 radium treatment. Strange to say, this species of fluor is difficult 

 to colour by cathode rays, which only produce a faint purple on the 

 surface. It appears therefore that the purple colour of fluor-spar 

 may be produced by /3 radiation, while this blue colour, which is 

 similar to though stronger than that produced in the decolourised 

 yellow and purple crystals, is due to the action of y rays. The 

 green, yellow and blue colours were produced right through the 

 crystals, while the purple was never more than 1 mm. deep. No 

 colour other than purple was given to any of these fluors by cathode 

 rays. 



The thermo-luminescent effect after treatment with iadium 

 was very striking. 



The coloured varieties, on strongly heating, completely lost 

 their power of emitting light. After treatment with radium this 

 power was restored, but in addition a new capacity for emitting a 

 bright green light at a comparatively low 1 emperature was imparted 

 to all the specimens whether previously heated or not. The quantity 

 of light emitted varied with the time of exposure to radium and 

 also in different specimens. The Matlock fluor, after colouring 

 blue with radium, gave a comparatively feeble green light on heating 

 and this light soon faded without changing colour. The Pyrenean 

 specimen, which was quite uncoloured by long exposure to radium, 

 gave a wonderfully brilliant and lasting green light. Some pieces 

 left on the table 6 in. away from the radium were nearly as brilliant 

 as the fully treated specimens, although none showed more than an 

 extremely faint glow before thus treating. This shows conclusively 

 that the y rays are responsible for generating this green thermo- 

 luminescence. 



All the other specimens after treatment with radium gave at 



