and other Minerals when exposed to Heat. 29 



complish by heating and handling the crystal in glass test- 

 tubes. 



With regard to Dr Brewster's remarkable experiment, 

 it partakes, I suspect, of a partly different class of phenomena. 

 It occurred to me that it might perhaps, if confirmed, be ex- 

 plained thus : — The slice of tourmaline may be considered, in 

 some respect, as an electrical coating to the glass. Suppose that 

 the tourmaline and the glass are heated together, and that the 

 side of the slice next the pole of the crystal, assuming vitreous 

 electricity by cooling, is next the glass. Let the other side of 

 the glass (which we shall call the second surface) communicate 

 with the table or any other conductor ; by the law of Induction, 

 then, it will assume resinous electricity, the first surface repelling 

 the vitreous. Conceive the glass plate now to be insulated, we 

 shall then have this state of things : — the surface of the tourma- 

 line far thest from the glass, by its even excitation, is resinously 

 electrified, for we have supposed the side which coats the first 

 surface of the glass to be vitreous ; the resinous electricity, which 

 is insulated at the second surface of the glass, is powerfully 

 attracting the opposite electricity of the side of the tourma- 

 line next itself, and prevents the recombination which would 

 otherwise take place with the electricity of the other side or 



pole. 



Having succeeded in repeating Dr Brewster's experiment 

 with thin slices from a large crystal of black tourmaline, I found 

 these hypothetical views confirmed. Having heated a slice cut 

 transversely to the axis of the crystal, I laid it upon a plate of 

 cold glass with the side which became vitreously electric during 

 cooling uppermost ; the adhesion was presently complete, so 

 that the glass could be held with the stone suspended from its 

 under surface. The other surface of the glass, behind the tour- 

 maline, being then touched with a minute disk of gilt paper, in- 



