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



and the size or density of these particles will determine the particular 

 colours of light absorbed and transmitted. 



Disturbance of the molecular structure of the crystal by heat, 

 daylight, etc., enables the dissociated particles to approach each 

 other and recombine, with consequent loss of colour. 



The quantity of impurity may be exceedingly small, since the 

 dissociated body appears to act as a powerful dye, and in many 

 cases it may be impossible to determine its chemical nature largely 

 owing to the difficulty of obtaining a perfectly pure sample of the 

 original substance. 



The question as to whether a perfectly pure substance is capable 

 of showing these colours is still open to doubt, although Goldstein's 

 work seems to point to the conclusion that such is possible. With 

 potassium chloride Goldstein obtained a deep violet coloration in 

 cathode rays, and the authors of this work had no difficulty in repeat- 

 ing the experiment. With potassium bromide, Goldstein obtained a 

 deep blue coloration, while the authors obtained a green colour ; also 

 this sample required twenty minutes' treatment before any appreci- 

 able colour was obtained, while Goldstein's colour was obtained in a 

 few seconds. It seems still possible therefore that the colours may 

 be due to traces of impurity, which are always present in the purest 

 obtainable samples. Goldstein estimates that certain impurities, 

 amounting to not more than one part in a million, may produce quite 

 perceptible colour effects under the influence of cathode rays. He 

 has also shown that the same impurity may give rise to different 

 colours when present in different solids, a fact which well illustrates 

 the danger of attempting to utilise the colour as a guide to the 

 nature of the impurity in minerals. 



At first sight, the production of the Giesel salts by heating 

 pure salts in sodium or potassium vapour, having the same colour 

 as those produced by cathode rays, would seem to be indisputable 

 evidence that the colour is due to metallic potassium or sodium. 

 When the intensity of the colour is considered, however, the evidence 

 appears much, less satisfactory. If a trace of metallic potassium, 

 so minute that its presence is beyond detection by chemical means, 

 is sufficient to colour a large mass of potassium chloride dark violet, 

 then the quantity present in Giesel salts should render them so 

 black that the colour is indistinguishable. This is not the case. 

 It is quite possible therefore that metallic potassium, introduced 

 either from outside or from within by the action of cathode rays, 

 is only a reagent which causes the dissociation of the colouring 

 material whatever that may be, and is not really the colouring agent 

 itself. This theory would explain why long-continued action of the 

 cathode rays never carries the depth of colour beyond a certain 

 limit, although the quantity of free metal is steadily increasing all 

 the time. It also explains why those colours which Goldstein terms 

 " after colours of the first class " should be identical in appearance 

 with '' after colours of the second class." 



The emission of light on heating the radiated crystals is 

 probably due to intense vibrations set up by the dissociated atoms 



