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The fubftance I am fpeaking of is ijland cryjlal , 
(which is well known for its fingular property of a 
double refraction) on a piece of which, I have made 
the following obfervations. ift, After this piece of 
cryftal has been rubbed, when the heat of the air is 
moderate, it (hews figns of eledtricity, though not 
very ftrong ones : 2d obf. If the heat is increaled, fo 
as to be a little greater than that ol the hand, it de- 
ftroys its eledtric power entirely : tgd obf. By cooling 
the ftone again, the eledtric power ,s reftored. 
I immerfed this piece of cryftal into a veffel filled 
with quicklilver, and furrounded by ice, where it 
remained near two hours, when the weather was 
very cold : upon taking it out with a pair of tongs, 
(that it might not be altered by the heat of my hands) 
and rubbing it again, it was more ftrongly eledtric than 
I had at any other time experienced ; but, on placing 
it for a few minutes on the hearth, at fome diftance 
from the fire, its eledtric property was again deftroyed, 
for rubbing would not occasion any figns thereof. 
Thus we fee two different kinds of fixed bodies, 
the one of which acquires an eledtric property, with 
the fame heat, with which another lofes it ; while a 
third fet of fubftances, as glafs, &c. retain their eledt- 
ricity, through both the degrees of heat, neceffary to 
the other two. 
Some pieces of ifland cryftal, which I have pro- 
cured from different places, have not the property 
of lofng their eledincity by a moderate heat. I have, 
in particular, a piece of that cryftal, one part whereof, 
when gently heated, becomes non-eledtric, while 
the other part with the fame heat (or even with a 
much greater one) remains perfedtly eledtric. 
There 
