612 PROCEEDINGS OP THE AMERICAN ACADEMY. 



intensity of the color, for some of the whitest and clearest crystals have 

 the largest number of cavities, aud some deeply colored green and purple 

 ones are almost wholly free from them. The change in appearance of 

 some of these deeper colored crystals, on being heated, is a very marked 

 one : the colored strata become milky and opaque, and lose their color 

 completely at a temperature not far above 300°. Those parts of the 

 crystal which were previously uncolored remain perfectly clear and 

 show not even a trace of milkiuess. Other crystals, especially those o 

 yellow or green coloring, are comparatively without stratification, and so 

 far as the experiments have shown, these are the ones which yield but 

 little gas on being heated. 



Conclusion. 



Although many facts brought out during the study of the fluorescence 

 and thermo-luminesceuce of fluorite appear to point toward impurities 

 which might determine the luminescence, the present examination of the 

 gaseous products obtained by heating these same fluorites in vacuo reveals 

 nothing out of the ordinary among the constituents of the mineral. The 

 analyses have only confirmed what was already known ; that is, that 

 practically all fluorites contain organic materials. The old ^question as 

 to the cause of the color is of course not definitely settled by these 

 experiments, which show only that color, gas content, and fluorescence 

 disappear at the same temperature, and that the gases produced while 

 the color is disappearing are such as would be obtained from an organic 

 distillation of a destructive sort. Taking into account the accurate work 

 of other observers, the necessary conclusion seems to be that the hydrogen, 

 carbon dioxide, and carbon monoxide which are produced when fluorspar 

 is heated are the result of the decomposition of some organic coloring 

 substance which is present in the mineral. Oxygen and nitrogen are in 

 no case present in these analyses in the proportions in which they exist 

 in air, and this suggests that the greater part of the oxygen has been 

 used up to form carbon dioxide and water, whether during the progress 

 of the analyses or not cannot be directly decided. 



No proof whatever that these organic substances have anything to 

 do with the fluorescence and thermo-luminescence of these crystals has 

 been found. In fact, the regeneration of a part of the thermo-lumines- 

 cence power of chlorophane, described in the preceding section of this 

 paper, seems to indicate an inorganic source for at least a part of the 

 emitted li<jht. 



