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this jlone. The Tourmalin I now fpeak of, is plain on 
one fide, and concave on the other j with a fmall flat 
border round it. It was fquare, but one of the cor- 
ners had been broken off. When I examined it, the 
broken part was plus , and the corner oppofite to the 
broken part minus : fo that here alfo the eledlric cur- 
rent ran through the hone in a diagonal line. 
Exp. f i . Every one of thefe Tourmalins , except 
that in the Britijh Mufeum , I greafed all over, and, 
w’hilft they were warm enough to preferve the greafe 
liquid, I tried each Tounnalin feparately, but found, 
no alteration in the virtue of the ftone, except weak- 
ening it a little ; though it is well known, that moift- 
ure, of any fort, readily conducts the eledtric fluid ; 
and therefore, if the Tourmalin had not a fixed kind 
of eledtricity, the plus and minus obferveable on 
the two fides of the flone, mufl, by this treatment, 
have united, and deftroyed each other : the plus fide 
parting with as much of the fluid, as the minus , on 
the other fide, wanted, to reftore the equlibrium. 
Upon the whole, all thefe experiments do moft 
clearly prove, that the Tourmalin Juff'ers the electrical 
fluid to paj's through it only in one direction, and fo 
far it bears fome analogy to the loadjlone. And as the 
had/lone lofes its virtue by being made red-hot, I was 
defirous to fee what would be the event in the Tour- 
malin under the fame treatment. 
Exp. 52. I therefore put one of the flat Tourma- 
lins into a ftrong fire, for half an hour ; but could not , 
afterwards, perceive the leaf alteration. I made the 
fame experiment upon another Tourmalin , with the 
fame fuccefs. 
Vol. LI. 
Xx 
