C 3I 6 ] 
with JEpinus's, yet it was large enough to fatisfy me 
that his opinion was well founded. Befides, the trials 
you made with the fame ftone before I had it, were 
a farther confirmation of the truth. However, the 
following experiments, which I made to procure more 
data towards attaining fome fimple explanation of 
thefe curious phenomena, will lufficiently prove, that 
one fide of the ftone is really electrified plus , and the 
other fide minus. And had the Duke de Noya made 
farther experiments, and purfued the fame method I 
have done, I think he would have been of the fame 
opinion. 
The largeft Tourmalin I had from you, and with 
which I made the following experiments, weighs 
above 120 grains. It is of an oval form, and poliffied: 
the greateft: diameter meafures an inch and a quarter, 
and the leafl one inch. One fide is plain , the other 
is convex, but cut into feveral finall planes or facettes, 
fomething like a rofe diamond j the thickeft part of 
which is near one third of an inch. This lhape does 
not leem to me to be the moll convenient for make- 
ing experiments, but I would not alter it, left the 
ftone fhould break; for there are feveral cracks in it; 
and I fear it will be difficult to meet with another of 
the fame fize. 
To make experiments with the Tourmalin requires 
the greateft attention, as the appearances fometimes 
are fcarcely fenfible, infomuch that I have been obliged 
to employ the tendered kind of apparatus, and even 
interpofe a fort of fkreen to prevent my breath, or 
other like motions of the air, from difturbing the ex- 
periment. 
My apparatus for making many of thefe experi- 
ments, confifts of two very fmall balls made of the 
pith 
