594 PROCEEDINGS OP THE AMERICAN ACADEMY. 



3. The time during which the thermo-luminescence persists appears 

 to be a quite definite function of the temperature of heating. Some 

 phosphorescent sulphides may be exhausted of thermo-luminescence at a 

 high temperature in a few seconds, while the luminescence of a crystal 

 of fluorspar may be either exhausted in a short time (a minute or two) 

 at a red heat or may persist for hours or days with a nearly constant 

 intensity if the temperature be kept at 100°. Measurements of the rela- 

 tion between temperature, time of heating, and intensity of the emitted 

 light are now being made. 



4. The temperature at which light begins to be emitted by a sub- 

 stance may be a very low one. Some specimens of chlorophane and 

 certain diamonds (especially Brazilian ones) are so sensitive that they 

 show visible light plainly when warmed by the hand, and many other 

 substances show the effect plainly at temperatures below 200°. 



5. In many cases the store of light contained in the substance appears 

 not to oecome exhausted by heating to a temperature below a certain 

 definite maximum, no matter how long the heating is continued. For 

 after all emission has ceased at the lower temperature, more may be 

 produced by raising the temperature to a higher point, and when the 

 light supply has been exhausted at this higher temperature more light is 

 emitted when the temperature is raised still higher. There appears, 

 however, to be a certain maximum temperature at which all of the 

 store of light in the body is exhausted, and further increase in tem- 

 perature beyond that point does not result in the production of further 

 luminescence. 15 



6. On standing in the dark, the power to emit light on being heated is 

 not regenerated. This is clearly shown by heating a crystal of fluorite 

 to a definite temperature below the maximum until no further lumines- 

 cence is visible. If the crystal is now cooled and allowed to remain in 

 the dark, and then brought back to the same temperature as before, no 

 light is emitted. If it is heated to a higher temperature, the lumines- 

 cence begins again. 



7. With respect to the regeneration of the thermo-luminescence by 

 light, various substances show differences so great as to make it certain 

 that there is more than one kind of thermo-luminescence. The phospho- 

 rescent sulphides are easily regenerated by a short exposure to light, and 

 this effect would naturally be expected from the method of their prepara- 



15 Le Bon, Revue Scientif., 14, 289 and 327 (1900) ; Becquerel, C. r., 112, 557 



(1891). 



