Manchester Memoirs, Vol. Ixii. (1918), No. 1C. 9 



faces, and by reason of these properties (probably electrical) were 

 able to attract certain impurities which had no effect upon the 

 colour of the crystal before the radium treatment was applied. 

 As the crystal grew, therefore, the path of growth of these faces was 

 traced out by the presence of the impurities, and the impurities 

 were then made visible by the action of the 7 rays from the radium. 



Anhydrite, Celestine and Barytes. — These anhydrous sulphates 

 of the alkaline earths all occur in the state of clear glassy crystals, 

 which are sometimes colourless and sometimes blue. Celestine in 

 fact owes its name to the sky-blue colour of many specimens. 



Since this blue colour can usually be discharged by heat, it 

 was expected that colourless specimens would, in some cases at least, 

 be coloured blue by radium. This expectation was fulfilled with 

 all three minerals, but to different degrees. Anhydrite in clear 

 crystals is rare, and the only specimen the authors were able to get 

 required six days under the action of 30 mg. of radium before any 

 decided blue colour was obtained. The crystal emitted a green 

 glow during exposure to the radium, and was slightly phosphorescent 

 afterwards when heated gently. 



Celestine treated in the same way was coloured pale blue more 

 readily than the anhydrite, but further treatment did not increase 

 the colour appreciably. 



One specimen of barytes showed no colour after similar treat- 

 ment, but a large clear crystal from Cumberland was coloured a 

 fine deep indigo blue after nine days under the action of 25 mg. 

 of radium. The blue colour was fairly uniform throughout the 

 whole crystal and it is therefore probably due to the action of the 

 y rays alone. 



This conclusion is- rendered more probable by the fact that a 

 similar crystal of barytes was unchanged under the action of cathode 

 rays. The fact that certain specimens of barytes and celestine are 

 uncoloured, or hard to colour, by radium indicates that the blue 

 colour is due to changes in some impurity and not to - dissociated 

 particles of the pure substance. 



Anglesite was chosen for experiment as being the only available 

 colourless crystalline anhydrous sulphate which could be compared 

 with those of the alkaline earths. 



The action of radium was very slight. After twelve days under 

 30 mg. of radium a barely visible blue colour was produced. 



The action of cathode rays on this mineral is peculiar. When 

 the tube is exhausted until cathode rays just begin to appear the 

 crystal emits a bright blue glow, while a fluor-spar crystal by the 

 side of it is hardly affected. As exhaustion proceeds and the rays 

 become harder, the fluor rapidly increases in brightness, but the 

 anglesite glow diminishes until it is barely visible under very hard 

 rays. No other crystal used in this work showed this peculiarity. 



Apatite and Phosphorite. — These two minerals, though having 

 identical chemical composition, show considerable differences under 

 the action of radium. 



Several specimens were used, including brown and green crystals 



