Electricity upon Minerals. 79 



and similarly exposed to ordinary light, did not phosphoresce 

 when heated, until it had been electrized, when it was faintly 

 luminous, with a deep-purple light. 



Apatite was then experimented with, and likewise deprived 

 of its phosphorescent property by calcination ; but, upon 

 electrifying it, and applying heat, it was found to have re- 

 sumed the power, and evolved a lemon-coloured light, which 

 rendered the figure of the fragment distinctly visible. 



With apatite, as well as with chlorophane, the light repro- 

 duced was in proportion to the discharges made. A fragment 

 of apatite answers better than the mineral powder. 



These experiments proved, that the phosphorescent pro- 

 perty, when destroyed by heat, can be restored by electricity 

 to minerals which had thus been deprived of it. 



I was therefore led to investigate how far other mineral 

 substances which phosphoresced by heat could have this pro- 

 perty increased and restored to them ; and also whether some 

 substances, which did not possess this property naturally, 

 could have it imparted to them by electric action. The 

 following experiments were accordingly made: 



A colourless variety of fluor spar was tried, which gave not 

 the least indication of light when heated ; but after six dis- 

 charges had been made from the Leyden jar, it was capable of 

 evolving a beautiful flame-coloured or orange light. In this 

 case, the property was conferred upon a substance which pro- 

 bably never possessed it previously. 



The experimental results obtained with other specimens are 

 given in the form of a table, under the respective heads. 



