MORSE. STUDIES ON FLUOR1TE. 595 



tion. The thermo-luminescence of substances of this class depends on 

 something which is not destroyed by the action of heat to such an extent 

 or in such a way that it cannot be reformed under the exciting influence 

 of light. But the thermo-luminescence of fluorite is of a wholly different 

 kind. When the store of light has once been exhausted, it may be in a 

 slight degree recovered under the influence of light, but the light pro- 

 duced by a subsequent heating is not the same as the original thermo- 

 luminescence light. This point will be brought out more clearly in the 

 description of my own experiments. 



8. With respect to the regeneration of thermo-lumiuescent power under 

 the influence of the electric spark, these same differences in the two classes 

 of substances are evident. The alkaline earth sulphides are easily re- 

 generated, probably by the light of the spark rather than by any special 

 electric action. Some fluorites, especially those which are usually called 

 chlorophanes, which give a strong green thermo-luminescent light, showing 

 a spectrum which is described in a later part of this paper, cannot be 

 so regenerated that the original light is emitted. The power of emittiug 

 light is indeed restored under the excitation of the electric spark, but 

 the light emitted is never the same as that produced during the first 

 heating. 16 



9. Many substances change during heating, so that the light, while in 

 some cases remaining apparently nearly constant in intensity, changes in 

 color during the experiment. Others show a great change in intensity, 

 together with changes in the color of the emitted light. It will be seen 

 later that the spectrum of the light from certain fluorites consists of two 

 distinct parts, one of which persists after the other has practically dis- 

 appeared ; and this fact goes one step toward an explanation of the 

 changes in color during heating. 



Experimental Data. 



The present paper contains data on the spectra of thermo-luminescence 

 of two distinct varieties of fluorite, — one a "chlorophane " from Amelia 

 Court-House, Va., and the other a common, clear, colorless (or very slightly 

 greenish) specimen from Westmoreland, N. EL Both are slightly fluo- 

 rescent under excitation by ultraviolet light, the fluorescence being of the 

 usual lavender color common apparently to all fluorites under these con- 

 ditions, and both are phosphorescent, showing this lavender-colored light 

 for some time after removal from the exciting source. Neither is at all 



16 Pearsall, Journ. of Royal Institution, 1, 77 and 209 (1830). 



