and other Minerals when exposed to Heat. 29 
complish by heating and handling the crystal in glass test- 
tubes. 
With regard to Dr Brewsrer’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 farthest 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- 
3 
