26 Prof. Forbes's Experiments on the Electricity of Tourmaline 



by Dr Bkewster, published in 1824, which we have gained on 

 this subject since the appearance of the works of Hauy. The 

 undecided state in which several points of the first importance 

 were left by the philosophers of the last century, is not a little 

 remarkable. In fact, the answer to the fundamental question, of 

 whether the tourmaline must be in the act of changing its tempe- 

 rature, in order to the development of electricity, may be con- 

 sidered to rest on the authority of Becquerel (who answered it 

 in the affirmative), former authorities being divided upon it. 

 Dr Thomson, in his work on Heat and Electricity, published 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. tEpinus found it electric after 

 an interval of six hours # ." He adds in a foot-note, " These 

 facts, as stated by ./Epinus, if accurate, seem inconsistent 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 tEpinus, and by opposing that of Becquerel, leave 

 the question still undecided He mentions f, that a slice of tour- 

 maline 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 shewing within 

 a few minutes, and in a very pleasing manner, the most essential 

 facts of the relation of the electricity to temperature. M. Bec- 

 querel found that when a crystal of tourmaline 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 ex- 



* P. 478. -f- Edinburgh Journal of Science, i. 21 1. 



