BY HEAT. 23 



pipe. As soon as copper is thoroughly melted (at 

 the inner flame), it glows with a greenish-yellow 

 light, similar to that of the glow-worm. On cool- 

 ing a little, it rapidly loses this property, and at 

 the same time a molecular change is observed on 

 the surface of the metal. I have also found that 

 the mineral lepidolite is as brilliantly phospho- 

 rescent by heat as fluor-spar. But to observe this 

 phenomenon properly, it should be viewed through 

 a piece of glass coloured blue by oxide of cobalt. 

 In these circumstances, the phosphoric light of 

 lepidolite before the blowpipe is very fine. The 

 blue glass extinguishes the orange-red light of 

 the heated charcoal. 



Among organic salts it has been observed that 

 sulphate of quinine and sulphate of chinconine be- 

 come phosphorescent under the influence of heat. 

 M. Bottger has remarked that these salts do not 

 shine in the dark as long as their temperature con- 

 tinues to rise; they become phosphorescent only 

 when, after being heated, the temperature begins 

 to decrease and they remain in a luminous state 

 for some minutes whilst cooling. Pure quinine 

 and sulphate of quinine are very phosphorescent 

 in this manner : the phosphoric light given out by 

 sulphate of quinine whilst cooling is sufficiently 

 strong to enable one to read by it. Pure dncho- 

 nine does not appear to be phosphorescent by heat, 

 but sulphate of cinchonine is so, though to a less 

 degree than sulphate of quinine. 



