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PAR is known from Talc by its Want of Elaticity;

from Selenite by its Want of Flexibility;

from Crytal by its Dullnes, and by fermenting with Acids.

It is heavier than any of the three other pellucid Foils; and is known from all Bodies in the World (when pure enough to be een through) by its doubling Lines laid under; and viewed through it.

This lat Property has been uppofed peculiar to that Species of Spar called Iland Crytal: And the greatet Writers, Linnæus, Wallerius, Crontedt, and the long et cætera, have eparated that