MORSE. — STUDIES ON FLUORITE. 593 



rated from the spark lines which have crept in. The bands in this fluores- 

 cence spectrum have every appearance of being made up of unresolved 

 fine lines just as the ordinary bands in metallic and gaseous spectra are. 



These lines of fluorescence have, in fact, all the characteristics of 

 sharp spectral lines, comparable with those of metals. 



This result is so important and so wholly at variance with the existing 

 ideas about fluorescence that the whole matter deserves minute investi- 

 gation, and further study is already under way with the assistance of a 

 grant from the Rumford Fund of the American Academy. 



II. The Thermo-luminescence of Fluorite. 



The more obvious facts about the thermodurainescence of substances 

 have been known for a very long time. That many substances emit 

 light on being heated to a temperature far below that of incandescence 

 has been known since the time of Boyle, and some crystals having this 

 property were sent to the French Academy of Sciences in 1724. These 

 were evidently fluorspar crystals, and this substance forms the basis of 

 most of the older experiments on the subject, although long lists of thermo- 

 luminescent substances are to be found in the researches of Wedgwood, 9 

 Brewster, 10 and Pearsall. 11 Later researches of importance have been 

 carried out by Becquerel, 12 Le Bon, 13 and Wiedemann and Schmidt. 14 



General Facts. 



The general facts concerning the production of a thermoduminescence 

 may be briefly summarized as follows : — 



1. Certain minerals (fluorite, diamond, leucophane, apatite, scheelite, 

 etc.) and many artificially prepared sulphides of the alkaline earth metals, 

 as well as many salts, emit light when heated to a temperature below 

 that of incandescence. 



2. These substances appear without exception to exhibit fluorescence 

 or phosphorescence, or both, under excitation by light, but many of the 

 most brilliantly thermo-luminescent substances are not at all brilliant in 

 fluorescence or phosphorescence, and many brilliantly fluorescent or phos- 

 phorescent substances exhibit no measurable thermo-luminescence. 



9 Phil. Trans., 82, 28 (1792). Eefers to still older literature. 



10 Edinb. Phil. Journal, 1, 383 (1819). 



11 Journ. of Royal Institution, 1, 77 and 267 (1830). 

 " C. r., 112, 557 (1891). 



« Revue Scientif., 14, 289 and 327 (1900). 

 « Wied. Ann., 56, 201 (1895). 



VOL. XLI. 38 



