1 36 Prof. Forbes's Experiments on the Electricity of 



be too thin), heated to a considerable temperature, the gilt 

 disk being charged with the same electricity as is acquired in 

 cooling by the pole of the crystal presented, the following 

 results appear : At first the tourmaline exerts no influence 

 whatever; but after its temperature has begun to fall, the 

 repulsive power is gradually developed, and the gilt disk 

 slowly recedes as the increasing force appears : the recession 

 becomes very minute, and at length reaches a maximum, at 

 which the needle remains for a time stationary. Soon, how- 

 ever, a diminution of repulsion takes place ; the disk reap- 

 proaches its original point of rest; and, if left long enough, 

 will return to the zero point precisely opposite the crystal, 

 which then, as at first, produces no action whatever. It is un- 

 necessary to point out how completely this verifies M. Bec- 

 querel's views, and demonstrates that, as soon as the tempera- 

 ture completely ceases to change, not the minutest vestige 

 of electricity remains, though the insulation throughout should 

 be maintained as completely as possible. This I generally 

 accomplish 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 phasnomena. 

 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) com- 

 municate 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 tourmaYmefurthest 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 sur- 

 face of the glass, is powerfully attracting the opposite electri- 

 city of the side of the tourmaline 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 



