and Phosphorescence. 117 



a certain limit, the mineral becomes luminous again under re- 

 newed heating, though perhaps not quite so easily as before. 

 Now it has been asserted by some physicists that phosphores- 

 cence can be excited by heat only after previous exposure to 

 light; others, on the contrary, regard any such preliminary 

 insolation as unnecessary. M. Fiebig has briefly collected to- 

 gether these contradictory views*, and has arrived from experi- 

 ments of his owu, made with a piece of green fluor-spar, at the 

 conclusion that phosphorescence cannot be excited by heat with- 

 out the previous action of rays of light. But as the same piece 

 of fluor-spar became repeatedly luminous, in my experiments 

 described in §§ 9 & 11, without having been exposed to light 

 in the intervals, I was induced to make a few special experi- 

 ments in order to acquire further information as to the question 

 of the possibility of phosphorescence by heat without previous 

 exposure to light. 



13. During a dark night I caused one and the same fragment 

 of fluor-spar to phosphoresce forty times in succession by laying 

 it on the moderately hot plate of a stove ; and the phosphorescence 

 was as bright and as easily produced the fortieth time as the 

 first. No exposure to light took place at all : I sat the whole 

 night in the dark. Every time, as soon as the luminosity had 

 become quite distinct, I removed the piece of spar from the 

 heated stove-plate and allowed it to cool down to the tempera- 

 ture of my hand on a small metal dish before making a new ex- 

 periment. It continued luminous in the cold metal dish for 

 nearly a minute on an average, and it appeared to me sometimes 

 as though it shone, when first put into the metal dish, more 

 brightly than it had been doing previously. 



14. Two other fragments, one of them of about 1^ cub. centim. 

 and another smaller, were broken off the same crystal from which 

 the piece just mentioned was taken. These likewise shone well, 

 but I left them so long on the stove-plate that their luminosity 

 ceased of itself. This took place in from forty to forty-five minutes. 

 The stove had remained all the time at nearly the same tempe- 

 rature ; and when the luminosity of these pieces came to an end, 

 the piece above-mentioned began to glow after lying on the stove 

 for one minute. I now allowed the pieces to cool. The stove 

 had previously been gently heated ; a fine cotton thread which 

 was tied round the smaller piece was found, when examined the 

 next day, unbrowned and in other respects uninjured. I now 

 moderated the heat of the stove still more, and put in again the 

 two pieces of fluor-spar, together with the one mentioned in § 13. 



vol. vi. p. 246; Becquerel, Trait 6 d' Electricite, vol. i. p. 421, according 

 to Pearscal. 

 * Pogg. Ann. vol. cxiv. p. 292. 



