CusACK — Temperature — Phosphorescent Substances. 537 



at 300° C. the spar did not phosphoresce as brilliantly as at normal, 

 and that when heated to about 500° C. it did not phosphoresce at all. 



Fluorspar was abandoned at this point as it was found difficult to 

 deal with, inasmuch as when powdered it would not phosphoresce as 

 well as when in pieces, and the pieces were difficult to handle. Instead 

 I tried, along with other salts, a calcium sulphide which phosphoresces 

 violet, and with which it was much easier to work. 



Before leaving the subject of the fluorspar, I may say that several 

 specimens of artificial fluorspar which I succeeded in making in the 

 dark did not phosphoresce when the temperature was raised. A 

 gelatinous precipitate of fluorspar was obtained in the dark. This 

 precipitate was then taken and boiled in water for many hours, until 

 it had assumed a partially crystalline form; it was then carefully 

 dried, and, when dry, heated to about 300° C. No phosphorescence 

 was observed. The above operations were all carried on in the dark, 

 but even when carried on, as they subsequently were in full daylight, 

 no glow was observed when the calcium fluoride was heated. 



To see whether the artificial calcium fluoride would phosphoresce 

 at all, I exposed some of it to the Leyden jar spark, and found that 

 it phosphoresced with considerable brilliancy, though not nearly so 

 brilliantly as the natural fluoride. The operation of precipitating the 

 fluoride was then carried on in the light of the Leyden jar sparks, 

 and the digesting of the gelatinous fluoride thus obtained was carried 

 on in the dark. The calcium fluoride obtained thus did not glow 

 when the temperature was raised. It must be remembered, however, 

 that the gelatinous compound might not retain light when raised to the 

 boiling point of water ; certainly the gelatinous fluoride phosphoresces 

 when exposed to the sparks at ordinary temperature, but the glow 

 dies more rapidly than in the case of the crystalline, neither is it 

 brilliant. 



The whole operation for obtaining the semi-crystalline calcium 

 fluoride was now carried out under the Leyden jar sparks, or rather 

 I should say as much as possible of it, for the induction coil could 

 not conveniently be kept working all the time the gelatinous precipa- 

 tate was being digested, and was only worked at intervals. The 

 fluoride thus obtained phosphoresced as would be expected when the 

 temperature was raised. Now although these experiments tend to 

 show that fluorspar has to be exposed to light before it phosphoresces, 

 it would be very surprising to find that a piece of spar taken from the 

 bottom of a mine where it could never have been exposed to light 

 failed to phosphoresce when heated. This is an experiment I hope 



