120 Prof. C. Bohn on Negative Fluorescence 



chlorophane) lead to the same conclusion. He found that this 

 mineral could be rendered luminous by a very gentle heat after 

 having been kept in the dark for months together. He kept 

 one piece, that had been previously insolated, in the dark, and 

 examined it from time to time by night. On the thirty-sixth 

 night after the exposure to light, a faint luminosity (Lichtschim- 

 mer) was excited by a temperature of 40° R. On previous nights, 

 even so high a temperature as this was not needed ; the heat of 

 the hand was sufficient. Von Grotthuss surprisingly concludes 

 "that chlorophane, as well as other phosphorescent minerals, 

 never become luminous except as a consequence of previous ex- 

 posure to light, or, in other words, that during phosphorescence 

 only that light is developed from them which they had absorbed 

 during previous exposure to light at some time or other, and 

 which had been incorporated with their ponderable matter"*. 

 I should have thought that, as the chlorophane received no new 

 supply by further exposure, the store of " absorbed light" 

 must have been exhausted by the thirty-sixth night, though this 

 cannot have been the case, since the phosphorescence still oc- 

 curred. In my experiments the larger piece was luminous 

 altogether for nearly ten hours, for the most part shining very 

 brightly without any fresh exposure to light. This long-con- 

 tinued and abundant emission of light would have exhausted a 

 very considerable store, but yet I could not detect any diminu- 

 tion of capacity ; at most a somewhat higher temperature was 

 required in order to evoke the luminosity when only a short in- 

 terval was allowed to elapse between two experiments ; but even 

 this difference disappeared if the mineral was left longer in re- 

 pose : it then became luminous at the same time as a piece 

 freshly broken off which had been exposed to light only a short 

 time before. 



I never heated the pieces of fluor-spar suspended in the cells 

 so strongly as to cause an alteration of their properties, nor 

 did I ever allow them to remain luminous long enough for their 

 luminosity to die out of itself while the external conditions re- 

 mained the same. 



15. The stove made use of in this experiment has two ovens, 

 placed one above the other like two stories. The bottom plate 

 of the upper one is 31 centims. above that of the lower one, and 

 the top plate of the whole stove is at the same distance above the 

 bottom plate of the upper oven. This latter was used as the 

 heated space in the experiments described in §§ 2, 4, 7> 9, and 

 11; and in general the fire was so regulated that the bottom 

 plate of the lower, unused oven was slightly red-hot. With the 



* Gehler's Physikalisches Wbrterbuch, vol. vi. p. 252, quoted from 

 Schweigger's Journal, vol. xiv.p. 154, and vol. vi. p. 172. 



