£ 444 ] 
of them are fomething like the ruby, others are more 
pale, and there is one inclining to the orange colour. 
In point of hardnefs and luftre, they are nearly the 
fame with the topaz. ? 
Six of thefe gems are cut brilliant fafhion, and three 
retain their natural fhape. Of the fix, when heated 
properly, and whilft they are cooling, three are electri- 
fied plus, and two minus , at the table furface, and at 
the collet, or oppofite furface, the three are eleCtrified 
minus , and the two plus. The fixth, which is the 
larged, and of an oblong fhape, appears to be electri- 
fied plus near one end, and at the other end, in the 
oppofite part, minus. 
Thefe inftances are farther proofs of the law ob- 
ferved in the Tourmalin, viz. that the eleCtric fluid 
flows to or fro, in one invariable line , according to 
the circumftances attending the experiment. See the 
Letter to Dr. Heberden, Philofophical TranfaCtions, 
Vol. LI. 
From the contrary appearances happening with 
gems of the fame fhape, it is now abundantly evi- 
dent, that the direction of the fluid does not depend 
upon the external figure of the gem, but upon fome 
particular internal make or conftitution thereof. And 
that there is fome fuch natural difpofition in all gems 
affording thefe appearances, may be collected from 
another curious fpecimen of the Tourmalin kind ; that 
is green, and formed in long flender cryflals with 
fever al fides, many of which are found flicking to- 
gether, and are brought from South America. 
Mr. Emanuel Mendez da Cofta, member of this 
Society, and well known for his fkill in natural hi- 
ftory, furnifhed me with a parcel of thele uncommon 
2 cryftals 
