[ 3I 5 ] 
forbear mentioning, till I have brought you better ac- 
quainted with the Tourmalin. 
The firft account I met with of the Tourmalin , and 
the remarkable properties belonging to it, was from 
a memoir in the Berlin ablsy printed in the year 1758; 
wherein it appears, that F. V. T. /. Epinus , profeftor of 
natural philofophy, made feveral very curious and 
judicious experiments upon it ; the moft material of 
which, prove a plus electricity on one fide thereof, 
and, at the fame time, a minus electricity on the other 
fide j provided the Tourmalin is moderately warmed, 
and even by hot water. Thefe appearances are the 
more extraordinary, as the like means employed in 
the fame manner upon diamonds, glafs, and all other 
eleCtric bodies hitherto tried, produce no fuch ap- 
pearances. 
The Duke de Noya , who viftted this kingdom in 
1758, wrote a fmall treatife on the fubjeCt, and pub- 
lifhed it at Paris on his way to Italy : in this work he 
mentions /Epinus' s experiments, but does not admit 
of a plus and minus electricity belonging to the Tour- 
malin when heated. On the contrary, he fays that 
the two fides are electrified plus , but one of them is 
more fo than the other ; and that it is the difference 
between thofe degrees which has led /Epinus into the 
miffake. 
I remember to have repeated moft of the experi- 
ments mentioned in the Berlin jnemoir , foon after it 
appeared in England, with the Tourmalin belonging 
to our friend Dr. Sharpy which you fortunately re- 
collected to have feen in his pofteftion many years ago 
at Cambridge ; for it was the only one known of here 
at that time : and though but a fmall one, compared 
S s 2 with 
