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CHAPTER III. 



PHOSPHOEESCENCE BY CLEAVAGKE, FEICTION, PEE- 

 CUSSION, CEYSTALLIZATION, AND MOLECULAE 

 OE CHEMICAL CHANGE. 



IT has been observed that numerous minerals and 

 chemical products emit light when they are split, 

 i. e. during the process of cleavage ; others, by 

 friction ; others again, by percussion or whilst 

 they crystallize, etc. 



When a lamina of mica, for instance, is divided 

 by cleavage, and the operation proceeds in a dark 

 room, a feeble emission of light is perceived at the 

 moment the separation of the two plates occurs. 

 Each of the two plates thus separated is found 

 afterwards to be electric : the one shows positive 

 electricity, the other negative electricity. Some- 

 thing similar is observed in the cleavage of feld- 

 spar, which, according to Landrin and some other 

 authors, emits a feeble light in this circumstance, 

 the phosphorescence lasting only a few moments. 



Another instance is afforded by boracic acid. 

 When boracic acid is melted in a crucible and then 



