602 PROCEEDINGS OP THE AMERICAN ACADEMY. 



influence in the emission of light by the substance, 22 and especial weight 

 has often been laid on the analogy with the necessary presence of small 

 quantities of impurities (usually copper, bismuth, or manganese, and a 

 salt of sodium or potassium) in order that phosphorescence shall be 

 exhibited by the sulphides of calcium, barium, and strontium. Since 

 most colored minerals lose their color, their power of fluorescing and 

 phosphorescing, and their content of included gases at about the same 

 temperature and after about the same duration of heating, a direct 

 connection between these three properties has often been suggested, 

 and a relation of this kind is usually accepted as a probability by 

 every one who studies the phenomena in question. 



Spectroscopic investigations on the fluorescence and thermo-luminescence 

 of the mineral fluorspar have yielded many facts of interest, and several 

 new and wholly unexpected discoveries. 23 It seems necessary to ex- 

 amine the included gases and liquids and the other impurities in the 

 naturally occurring mineral as minutely as possible, with especial ref- 

 erence to some chemical factor which might exert an influence on the 

 luminescence phenomena. 



Tlie Color of Minerals. 



The mineral fluorite is remarkable for the variety of colors which it 

 may exhibit. It occurs in fact in nearly every color of the entire spec- 

 trum, with the possible exception of bright red, and not only do fluorites 

 from different localities show this difference in color, but even the most 

 homogeneous and perfect crystal often shows a series of layers of differ- 

 ent colors sharply separated from each other, and usually lying in planes 

 parallel to the natural faces of the crystal. The same crystal often 

 shows as many as four or five sharp differences in color, and the colored 

 strata often repeat in sets in which the colors take different sequences 

 until the number of visible separate layers reaches twenty or more. 

 Sir David Brewster 24 was the first to call attention to this structure 

 of fluorite, and even at that early date he suspected a relation between 

 these strata and the remarkable phenomena of fluorescence which he 

 was investigating. 



The general question of the cause of color in minerals has been made 

 the subject of a great deal of investigation, without any satisfactory or 



22 See Becquerel, C. r., 112, 657 (1891). 



23 See the two preceding divisions of this paper. 



2 * Edinb. Phil. Trans., 16, [2], 111 (1812) ; Edinb. Jour. Sci., 6, 115 (1821). 



