118 Prof. C. Bohn on Negative Fluorescence 



The last became luminous in 100 seconds; the smaller of the two 

 which had previously spontaneously ceased shining became lumi- 

 nous in 160 seconds (more or less, for I was only counting by the 

 beats of my pulse) ; but the larger piece did not begin to shine even 

 after 25 minutes. By this time the smaller piece, after shining 

 tolerably brightly, had again ceased of its own accord to be 

 luminous. I took it out and let it cool. In the meantime the 

 fire was increased, so that a cotton thread which was put into 

 the heated space wound upon a knitting-needle was blackened, 

 but still retained its continuity. The larger piece then at once 

 began to shine, and remained splendidly luminous for more than 

 an hour. It had not been at all exposed to light any more than 

 the small piece ; for when I removed this I found it by feeling 

 for it with the tongs, guided by the faint gleam of a cigar. More- 

 over the small piece, which by this time was cold again, when 

 put back into the heated space, soon became luminous again — 

 that is, for the third time, after having twice ceased shining of 

 its own accord, and without having been exposed to light in the 

 interval. As I was trying to take out the larger piece with the 

 tongs after its luminosity had died out, I let it slip, and imme- 

 diately I remarked three small points which shone very brightly 

 for a long time. They were small fragments separated by the 

 slight fall of the piece which escaped from the tongs. I was able 

 to move these luminous points about with the tongs, and one of 

 them remained hanging on to them. This accidental observation 

 consequently proved that, although the luminosity of a whole 

 piece may have died out, its particles may still be capable of 

 shining. But whether a renewal of the surface or a diminution 

 of the mass is a necessary condition of this continued luminosity, 

 I cannot say. After an interval of a quarter of an hour, during 

 which the pieces had cooled down, they were replaced upon the 

 stove-plate, which by this time was not quite so hot. The large 

 piece soon became luminous, but the small one did not get lumi- 

 nous again in this way. My supply of fuel had come to an end, 

 and I could not readily get any more; otherwise I should have been 

 glad to have raised the temperature again a little. Just as day 

 began to dawn in the east, the phosphorescence of the large piece 

 had so far subsided that, although my eyes had become very 

 sensitive, there remained only a barely perceptible glimmer ; the 

 small piece was not luminous; the piece mentioned in § 13 

 began in one minute to shine for the fortieth time. I now shut up 

 all the pieces in the dark until the following night. The large 

 piece, whose luminosity had already died out of itself three times, 

 and which had not been again exposed to light, became luminous 

 simultaneously with the piece which had previously been phos- 

 phorescent forty times, and with a perfectly fresh piece broken off 



