and Phosphorescence. 115 



retardation of the phosphorescence of the crystal in the glass 

 cell might be anticipated, on my view of the phenom non, as a 

 necessary consequence. 



7. In the experiments recorded in § 4, the fluor-spar and the 

 cells were cold when they were put into the heated space ; but I 

 now arranged the experiments so that they were both at a high 

 temperature before they were introduced. When one of the 

 fragments had become luminous, I placed it with its holder upon 

 the stove, where it was still kept warm, but where its luminosity 

 soon ceased. In these new experiments the crystals began to 

 shine much more quickly than when they were put in cold. 

 But a more important result is that the pieces in the glass cell 

 and in the salt cell became luminous almost simultaneously, 

 often exactly at the same instant; and, indeed, on one occasion 

 the piece in the glass cell began to glow before that in the salt 

 cell. The previous heating thus did away with the retardation 

 before observed. 



8. When the crystals and the sides of the cells are already 

 hot — already nearly as hot as they can become after equilibrium 

 of temperature has been established between them and the oven — 

 the further elevation of temperature is due much less to absorp- 

 tion of radiant heat than to contact with the heated air. The 

 conditions of further heating are therefore equally favourable for 

 the crystal in the salt cell and for that in the glass cell. Accord- 

 ingly the observed fact that the luminosity of both crystals 

 begins at about the same time agrees with my explanation of the 

 phenomenon. If, in accordance with the opposite view, a direct 

 transformation of rays takes place, the fluor-spar in the salt cell 

 must always be ahead of the piece in the glass cell in beginning 

 to glow, since it receives the radiation from the sides of the 

 stove, whereas this radiation is cut off from the other piece by 

 the glass. 



9. A piece of easily phosphorescing fluor-spar was put cold 

 into the cold salt cell and then transferred to the heated space. 

 In two minutes the mineral had become luminous. The cold 

 glass cell was now put into the oven, and the glowing fluor- 

 spar was taken out of the salt cell and put into the glass cell. 

 The luminosity ceased almost immediately, and did not begin 

 again until after ten minutes. I then replaced the still shining 

 crystal in the salt cell (which had cooled down in the meantime); 

 here its luminosity ceased at first, but appeared again after 2| 

 minutes. 



10. If the luminosity were due directly to the rays received by 

 the crystal, one cannot see why it should cease when the glow- 

 ing crystal was transferred from the glass cell to the salt cell, 

 since the rays of heat must have had freer access to it in the 



12 



