596 PROCEEDINGS OF THE AMERICAN ACADEMY. 



brilliantly fluorescent or phosphorescent under the conditions to which they 

 have been subjected, but both are very strongly thermoluminescent, the 

 chlorophane emitting a very strong bright green light, easily visible even 

 in a well-lighted room, and the other fluorite emitting a lavender or pur- 

 ple light, not very different to the eye from the light emitted by the same 

 substance in fluorescence and phosphorescence. 



Spectroscopic examination of the light from the chlorophane shows 

 what appears to be a continuous spectrum with a maximum in the green- 

 yellow. Carefully rested eyes enable some other sharper maxima to be 

 seen, but little detail can be made out. 



The light from the other fluorite shows several broad bands of a rather 

 diffuse nature, and superimposed over these some sharper lines can be 

 made out. The broad bands of the two spectra are the source of most 

 of the emitted light, and the sharp lines are much obscured by their 

 presence. 



The spectrum of fluorites much like the New Hampshire variety has 

 been described by a few observers. The oldest reference found was in a 

 brief note in Poggendorfs Annalen, "from a letter from Heir Kindt." *7 

 In this note the broad bands of the thermo-luminesccnce spectrum are 

 mentioned, and the remark is made that the bands are as strong as the 

 absorption bands in the spectrum of the didymium salts. Later references 

 to the same spectrum are to be found in the researches of Hagenbach 18 

 and Liveing, 19 both of whom observed the broad bands and give their 

 approximate wave-lengths. No reference to photography or more minute 

 examination of the question has been found. 



There are at least three distinct stages in the thermo-luminescence of 

 the chlorophane. At a low temperature (50° to 100°) the light is bright 

 green and strong. After the light has begun to grow dim and the tem- 

 perature is increased and maintained for some time, a yellowish color of 

 less intensity and short duration takes its place, to be in turn replaced by 

 a weak lavender luminescence at a still higher temperature. This final 

 color is very much like that of the fluorescence under excitation by light. 



In the case of the other specimen chosen for examination (the colorless 

 crystal from New Hampshire), no such marked differences in the color 

 emitted at different stages of heating were found, but it is evident that 

 the light produced at higher temperatures, toward the end of the lumi- 

 nescence, is of a deeper purple color than that emitted at lower temper- 



« Pogg. Ann., 131, 160 (1867). " Archiv. de Geneve [2], 60, 297 (1877). 



19 Proe. Camb. Soc, 3, 96 (1877). 



