VOL. LI.] PHILOSOPHICAL TRANSACTIONS. . 403 



the same effect on glass ; but electrified it less than the tourmalin : and amber, 

 though like the other bodies, it suffered an increase of power by the same treat- 

 ment, was electrified the least of all. 



Exper.Al. When the tourmalin had received the same number of blasts 

 against the plain side, while the finger touched the convex side, it afforded 

 different appearances ; for the plain side was electrified plus, and the other minus. 

 After a short time, both sides were plus ; and some time after this the stone 

 recovered its natural state, the plain side being minus, and the convex plus. 



Exper. 42. The convex side was now presented to the bellows in the same 

 manner, and received an equal number of blasts. In this experiment both sides 

 were plus, but weaker than in the last experiment ; and, after a time, the stone 

 returned to its natural state, affording a plus and minus appearance. 



But it remains to be inquired into, whether the tourmalin is so disposed by 

 nature, as to suffer the electric fluid to pass through it only in one direction, 

 like magnetism through the loadstone, or is indifferent which way it flows. 



Exper. 43. On examining a tourmalin which was fiat on both sides, and 

 polished, except on its edges, part of the edge appeared plus, and another part 

 opposite to it minus ; so that a line, drawn from the plus part through the cen- 

 tre of the stone to the other side, would pass through the minus part, 



Exper. 44. Two smaller tourmalins, that were flat also and polished, like 

 the other, exhibited the same appearances. 



Exper. 45. Another tourmalin, which was also flat, but unpolished, af- 

 forded a 4th instance of this kind. 



Exper. aQ. The first of these tourmalins was afterwards polished, as well at 

 the edges as the surface ; in order to see, whether that would make any altera- 

 tion ; but it still retained its former electrical state. 



Exper. 47. He experienced the same with another tourmalin, which had 

 been rough likewise. 



Exper. 48. He then, with a little emery, made that edge, which was plus, 

 rough again, preserving all the rest smooth ; but he could not perceive that any 

 alteration was made by it. He did the like with the edge of the other polished 

 tourmalin, without being able to observe any difference. 



Exper. 49. As to the small tourmalin that was plain on one side, and a little 

 convex on the other, it was plus on the plain side, and minus on the convex ; 

 which was contrary to the large tourmalin described in the beginning of this 

 letter. . ' 



Exper. 50. He had an opportunity of trying another tourmalin, now in the 

 British Museum, which afforded another instance of the singular disposition of 

 this stone. This tourmalin is plain on one side, and concave on the other ; 



3 f2 



