and Phosphorescence. 113 



was rather warm) phosphorescence began, 



In the uncovered crystal after ... 70 seconds. 

 In the crystal in the salt cell after . .170 „ 

 In the crystal in the glass cell after . 330 „ 



I did not find that the times required by the several crystals 

 in order to become phosphorescent were constant in different 

 experiments, nor even that these times were in a constant ratio. 

 But the uncovered crystal always became luminous first ; then, 

 after from two to three times as long an interval, the crystal in 

 the salt cell began to shine, and that in the glass cell invariably 

 last of all. Both cells always became very hot, so that in taking 

 them quickly out of the heated space, in spite of a thick leathern 

 glove, I generally burnt my fingers, and several times there was 

 a perceptible smell of singed leather. 



5. Very nearly an equal number of equally intense rays of 

 heat are received in equal times by the uncovered piece of fluor- 

 spar and by the piece in the salt cell. It is true that a small 

 loss takes place owing to reflexion at the surfaces of the rock- 

 salt plates and to absorption (8 per cent, according to the usual 

 estimate founded on Melloni's experiments) ; but, on the other 

 hand, the rock-salt itself becomes heated and radiates from a 

 shorter distance upon the crystal, so as, no doubt, to make up 

 for the loss. Seeing that rock-salt has very little, or almost no 

 power of absorbing radiant heat, it gets heated chiefly by con- 

 tact with the hot air. In consequence of this heating, one 

 might be induced to believe that the crystal in the salt cell actu- 

 ally received more heat than the uncovered crystal. We must 

 remember, however, that the radiating-power of rock-salt is ex- 

 cessively small, its absorbing-power being so very slight. Hence, 

 if this phosphorescence was really an instance of negative fluo- 

 rescence, the fluor-spar in the salt cell and that in the open air 

 ought to begin to be luminous almost simultaneously. But all 

 my observations contradict this in the most indubitable way. 

 The crystal in the glass cell receives scarcely any rays of heat 

 direct from the walls of the stove, since glass absorbs rays of the 

 kind here concerned almost completely. The glass, however, 

 becomes heated, partly by absorption and partly by contact with 

 the hot air, and the glass plates themselves then radiate upon the 

 crystal. The intensity of the radiation from the glass can never 

 become equal to that from the sides of the stove, since the emis- 

 sive power of glass is much less than that of graphite, with which 

 the sides of the stove are covered; and, moreover, the tempera- 

 ture of the glass always remains very considerably lower than 

 that of the stove itself. The maximum temperature of the glass, 

 and consequently the maximum of its radiation, is certainly not 



Phil Mag. S. 4. Vol. 34. No. 228. Aug. 1867. I 



