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XV. 



THE ErPECT OF CHANGE IX TE:MPERATrRE OX PHOS- 

 PHOEESCEXT SUBSTAXCES. Br E. S. CTSACK. 



(Plaie TIL) 



[cOMiTCSTCATED BY GEO. F. FII2 GEEALD, D.^c] 

 [Read April 26, 1897.] 



Chemical compounds that possess the property of storing up light- 

 energy when exposed to certain ethereal vibrations are generally 

 known as phosphorescent substances. Such phosphorescence must not 

 be confused with phosphorescence in the animal kingdom, which is due 

 to combustion. 



Of the many minerals that possess the property of, as it were, 

 bottling up light, one of the most remarkable is fluorspar ; it was 

 while making experiments on crystals of fluorspar with the object of 

 expelling the light from the crystal without cracking the latter that 

 the quantity of light-energy which it was possible for a phospho- 

 rescent substance to store up seemed to me to depend on the tempe- 

 rature at which the substance was exposed to the source of light. 

 And also that the amount of light that it was possible for a body to 

 hold was dependent on the temperature. 



Before entering into these latter experiments I have to give an 

 account of the experiments which led to them, and which were made 

 with the object of ascertaining if possible whether fluorspar or any 

 other phosphorescent substance increased in volume when "charged" 

 with light-energy. It was at the suggestion of Dr. Joly that I first 

 started the investigation, as he thought it possible that a substance, 

 when it had stored up light-energy, would increase in volume. The 

 difficulties to be overcome were numerous, but especially that arising 

 from change in temperatrure, and as was afterwards found the heat of the 

 finger at a distance of nearly ten centimetres produced considerable 

 effect on the apparatus. The apparatus first devised was a flat glass 

 bottle with a capillary tube ground into the stopper. Into this bottle 

 the broken up fragments of a fluorspar crystal were put, and the 



