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CHAPTER III. 
PHOSPHORESCENCE BY CLEAVAGE, FRICTION, PER- 
CUSSION, CRYSTALLIZATION, AND MOLECULAR 
OR CHEMICAL CHANGE. 
Ir has been observed that numerous minerals and 
chemical products emit light when they are split, 
1.e. durmg 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. 
Hach 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 ight 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 
