and Phosphorescence. 119 



the large crystal, which had been exposed to light all day long. 

 The luminosity was brilliant, and lasted about two hours. The 

 small piece, however, did not become phosphorescent any more, 

 even when I raised the temperature a good deal. The large piece, 

 after ceasing spontaneously to shine four, and even five times, 

 easily became brightly luminous again, and, indeed, the phospho- 

 rescence lasted longer each time; the temperature, however, 

 required to be raised a little. The sixth phosphorescence did not 

 come to an end, although I waited for it more than three 

 hours. I now exposed the small piece to the light of a stearine 

 candle, and then put it upon the stove-plate, but still it did not 

 shine. I next exposed it for half an hour while cold to the light 

 of a petroleum lamp, which, as is well known, is not very poor 

 in the most refrangible rays ; but even this treatment did not 

 confer upon it the capability of again becoming phosphorescent. 

 Exposure to the light of burning magnesium, which is very rich 

 in the most refrangible rays, produced no greater effect. Lastly, 

 I exposed the small piece for a whole day to diffused sunlight, 

 and to a small extent to the direct rays of the sun, and tried 

 again the following night to cause it to phosphoresce. Still it 

 did not do so; whereas the large piece, which had not been ex- 

 posed to light, easily became luminous. After the small piece 

 had lain for two days more exposed to light, I examined it again. 

 Placed on a plate, the temperature of which I gradually raised 

 very considerably, it at last became luminous, but the light was 

 very weak and soon died away. The power of becoming phos- 

 phorescent had accordingly been restored, though only to a small 

 extent ; but this was certainly due, not to the insolation, but to 

 the long rest. The larger piece, which had not been exposed to 

 light, shone much more strongly upon a plate that was not so 

 much heated, and continued shining as long as I had patience to 

 watch it. The piece that I had made phosphorescent forty times 

 during the first night, and which was always removed from the 

 hot plate immediately it had become luminous and carefully pro- 

 tected from light, I caused to become luminous by heat some 

 dozens of times more during the second night, as well as several 

 times during the third night. I could not perceive that its readi- 

 ness to become luminous was in the least degree diminished. The 

 small piece had evidently at one time been too hot, and its capacity 

 of becoming phosphorescent had been thereby impaired. In other 

 respects it remained externally unchanged, and, in particular, it 

 still retained its yellow colour. From these observations I may 

 therefore conclude that yellow fluor-spar can become phospho- 

 rescent by the action of heat alone, without any necessity for its 

 previous exposure to light. Observations made by Von Grotthuss 

 on a reddish-violet fluor-spar from Nertschink (pyrosmaragdite or 



