1 34 Prof. Forbes's Experiments on the Electricity of 



fundamental question of whether the tourmaline must be in 

 the act of changing its temperature, in order to the develop- 

 ment of electricity, may be considered to rest on the authority 

 of Becquerel (who answered it in the affirmative), former au- 

 thorities being divided upon it. 



Dr. Thomson, in his work on Heat and Electricity, pub- 

 lished in 1830, observes, that " when the tourmaline is once 

 excited by heat, it retains its electricity for a long time, if care 

 be taken to place it upon non-conductors. iEpinus found it 

 electric after an interval of six hours*. He adds in a foot- 

 note, " These facts, as stated by JEpinus, if accurate, seem in- 

 consistent with the statement of Canton and Becquerel, that 

 the electricity is only developed whilst the stone is changing 

 its temperature." A statement of Dr. Brewster's might also 

 appear to support the views of iEpinus, and by opposing that 

 of Becquerel, leave the question still undecided. He men- 

 tionsf that a slice of tourmaline cut transversely to the axis 

 of the crystal, and placed on a plate of glass heated to 212°, 

 adhered to it for six or eight hours, even when the glass was 

 uppermost, the electricity of the tourmaline thus supporting- 

 its own weight. 



The experiment which I am about to describe will, I think, 

 set at rest the question, and is in fact capable of showing 

 within a few minutes, and in a very pleasing manner, the 

 most essential facts of the relation of the electricity to tempe- 

 rature. M. Becquerel found that when a crystal of tourma- 

 line was heated to 212°, its electricity was inappreciable so 

 long as the temperature remained stationary ; but that when 

 placed in a cooler medium, the intensity of the electricity was 

 not, as might have been expected, proportional to the rapidity 

 of the change of temperature, which of course would corre- 

 spond to the period at which the temperature was highest, 

 but, on the contrary, arose gradually to a maximum, when the 

 tourmaline was about half way cooled to the temperature of 

 the apartment; then gradually diminishing, redescended to 

 zero when it reached that point. This remarkable result 

 M. Becquerel obtained by suspending the crystal horizontally 

 by a fibre of silk under a glass cover, the temperature of the 

 air in which he had the means of regulating ; he then applied 

 to the extremities of the crystal, wires from the opposite poles 

 of a dry pile, and, counting the number of oscillations made 

 by the tourmaline, deduced the intensity. 



The form of the experiment which I have contrived, and 

 which bears out M. BecquerePs conclusions, gives the same 



• p. 478. f Edinburgh Journal of Science, vol. i. p. 211. 



