116 Prof. C. Bohn on Negative Fluorescence 



latter than in the former. On the other hand, if elevation of 

 temperature is the cause of the luminosity, the observed result 

 of changing the place of the crystal is just what might have 

 been expected. The heated mineral is cooled by radiation to- 

 wards the cold sides of the little cell, whether they consist of 

 glass or of rock-salt ; and its temperature does not rise again 

 sufficiently for the luminosity to begin again until the sides of 

 the cell have become hot. The fact of the phosphorescence of the 

 crystal in the glass cell having been interrupted for ten minutes 

 proves how slowly the inner surfaces of the glass plates acquire 

 the high temperature. Even although the temperature of the 

 stove may have been higher in the experiments described in § 4 

 than during those described in § 9, the admissibility of the as- 

 sertion made in § 5, to the effect that the glass plates had not 

 attained their maximum temperature, and consequently their 

 greatest emissive power for heat, after the lapse of 5| minutes, is 

 established. 



11. I transferred an already luminous piece of fluor-spar from 

 one cell to another that was previously heated and standing in 

 the flue of the stove. Generally no interruption of the phos- 

 phorescence occurred; a few times only there was a diminution 

 of brilliancy. It made no difference in the result whether the 

 crystal was changed from the glass cell to the salt cell, or vice 

 versa. This observation is also very plainly in favour of the 

 assumption that the luminosity is an affair of temperature, and 

 that no direct transformation of rays takes place ; for if it were 

 otherwise, the phosphorescence must have been always more in- 

 tense in the salt cell than in the other, in consequence of the 

 ease with which the rays of heat could penetrate to the inside ; 

 while in the glass cell it must always have diminished, since the 

 rays from the blackened sides of the stove could not pass through 

 it, and the radiation from the sides of the cell itself is not, as 

 we have already pointed out, a full substitute. 



12. If a piece of fluor-spar is made to shine in the dark by 

 heating it, either by laying it upon a hot conducting plate, or 

 by exposing it while hanging in the badly conducting heated air 

 to radiant heat, the brightness of the emitted light goes on for 

 some time increasing ; it then diminishes, until at last the faintest 

 gleam disappears, although external circumstances have remained 

 entirely unaltered. If the mineral has been very much heated, 

 its power of becoming phosphorescent is thereby destroyed ; re- 

 newed heating does not make it luminous any more. Never- 

 theless it is stated that the phosphorescent quality may be re- 

 stored by moistening with water acidulated with hydrofluoric 

 acid, or by the electric spark*. If the heating has not exceeded 



* According to Placidus Heinrich, Gehler's Physikalisches Worterbuch, 



