[ 336 ] 
E xp. 45. Upon examining a 'tourmalin which was 
flat on both fldes , and polifhed , except upon its edges, 
part of the edge appeared plus , and another part op- 
poflte to it minus ; io that a line, drawn from the plus 
part through the centre of the (lone to the other fide, 
would pafs through the minus part. 
Exp. 44. Two fmaller tourmalins , that were flat 
alfo, and polifhed, like the other, exhibited the fame 
appearances. 
Exp. 4f. Another tourmalin , which was alfo flat, 
but itnpolijhed , afforded a fourth inftance of this 
kind. 
Exp. 46. The flrA of thefe tourmalins was after- 
wards polifhed , as well at the edges as the furface; in 
order to fee, whether that would make any alteration; 
but I found it ftill retained its former electrical ftate. 
Evp. 47. I experienced the fame with another 
tourmalin , which had been rough likewife. 
Exp. 48. I then, with a little emery, made that 
edge, which was plus , rough again , preserving all the 
reft fmooth ; but I could not perceive, that any altera- 
tion was made by it. I did the like with the edge of 
the other poliihed tourmalin , without being able to 
obferve any difference. 
Exp. 49. As to the fmall tourmalin , that is plain 
on one fide, and a little convex on the other, which 
you oblerved to be a very good one, it is plus on the 
plain fide, and minus on the convex ; which is con- 
trary to the large tourmalin defcribed in the begin- 
ning of this letter. 
Exp. 50. I had an opportunity of trying another 
tourmalin , now in the Britijh Mufeum , which af- 
forded another inftance of the Angular difpofition of 
this 
