and Phosphorescence. 121 



intention of bringing the top plate of the stove to about the 

 same temperature as the bottom plate of the space into which 

 the pieces of fluor-spar with their cells were introduced had pre- 

 viously possessed, I now increased the fire so much that the bot- 

 tom plate of the upper oven became slightly red-hot. I then 

 placed the crystals with their cells and the same tripods upon the 

 stove. Here they did not become luminous, not even when I 

 made the fire still stronger. Only a single mineral, a white 

 fluor-spar with brownish-black specks, and with more of a fatty 

 than the true vitreous lustre, which phosphoresced more easily 

 than any of the other specimens that I examined — more easily 

 even than a chlorophane belonging to the mineralogical collection 

 of the University of Giessen, with which I had previously expe- 

 rimented — only this one piece became luminous when placed 

 upon the stove, hanging freely in the air (not in a cell) ; and 

 even it required to be brought within a quarter of an inch of 

 the hot stove-plate. If the crystals had been so long inside the 

 oven, where they received the radiation from five surfaces, as to 

 have become luminous and were then placed upon the stove, they 

 continued luminous for some time — as much as 2J minutes.' 



16. In these experiments the under surface of the crystal was 

 exposed to just the same radiation as when it was inside the 

 oven ; but the other surfaces of the fluor-spar were there also ex- 

 posed to strong radiation. If the phosphorescence consisted in 

 a direct transformation of rays, and if it be further assumed that 

 the radiation must attain a certain degree of intensity in order 

 that the phenomenon may be perceptible, the crystals ought in 

 the last arrangement to become luminous, at least, at their under 

 surfaces. But as this was not the case, this phenomenon is evi- 

 dence against the analogy of phosphorescence and fluorescence. 

 On the other hand, if elevation of temperature is the only ex- 

 citing cause of luminosity, this result cannot be expected to occur 

 when the crystals are placed upon the stove; for in this case they 

 are far from being as hot as when they are inside the oven. 

 For, when placed upon the stove, the crystals radiate in all direc- 

 tions, except straight downwards, against relatively cold surfaces, 

 and thus lose in these directions more heat than they receive. 

 Moreover they are exposed to a current of air which ascends 

 without any obstacle, and does not acquire an equally high tem- 

 perature with the more confined air in the oven. The observa- 

 tion that pieces which are already hot enough to be luminous 

 continue to shine for a short time when placed upon the stove 

 where they can only cool slowly, is in perfect accordance with 

 my view of the nature of this phosphorescence. 



17. A thin rusty iron dish was placed upon a tripod in the 

 oven, and a polished flat-iron upon another. When these had 



