1 38 Prof. Forbes' s Experiments on the Electricity of 



M. Becquerel, in a second memoir on the Tourmaline, 

 published in 1828 # , announced the rather extraordinary cir- 

 cumstance, that long tourmalines did not become at all electric 

 by heat, and that their facility of being excited was generally 

 inversely proportional to their length. Dr. Thomson, in his 

 work on Heatf, mentions the former assertion, and observes 

 that it is one which he has never had an opportunity of trying. 

 As my experiments have been generally made with black tour- 

 malines from Van Diemen's Land, some of which are of great 

 length, this point early occurred to me as one deserving of 

 investigation. As these inquiries seem at no period to have 

 excited much attention in this country, and as of late nothing 

 whatever has been done upon them, these observations may 

 prove the more interesting. 



The longest tourmaline employed by M. Becquerel was'six 

 centimetres, or 3'2 English inches, in length, with a diameter 

 of about *08 inch. My largest tourmaline is 3*25 inches, or 

 almost precisely the same, with a diameter little different. In- 

 stead of finding this crystal " tout a fait refractoire," as M. 

 Becquerel describes his, it proved uniformly susceptible of 

 powerful excitement, under the very same treatment which I 

 was accustomed to use towards those of smaller dimensions. 

 The intensity too was very great, though more slowly attained 

 than in shorter ones. Various tourmalines, between two and 

 three inches in length, uniformly show great activity on being 

 removed from the heat to which they have been exposed, and 

 left to cool, when applied to the electroscope. 



This discovery led me to some inquiry into the effect of di- 

 mension in modifying electric action. Here it is necessary to 

 draw a distinction between the case of excitation and the in- 

 tensity of the effect produced. M. Becquerel generally men- 

 tions the temperature at which electricity appeared : my in- 

 quiries have been directed to the maximum intensity of that 

 electricity when excited, which is in some respects the more 

 satisfactory information of the two. The determination of the 

 temperature, we have already seen, is a point of great uncer- 

 tainty, since every range of atoms, from the centre to the sur- 

 face, must have a different temperature. Of course, for the 

 reason, the maximum effect is the integral of an infinity of 

 variable forces. 



Amongst many experiments on different groups of crystals, 

 I may mention the following as the best determined. Six tour- 

 malines, all 1*3 inch long, whose thicknesses, or areas of sec- 

 tion, were represented by the numbers 14-, 11, 7, 6, and 4, had 



* Ann. de Chimie. t p. 477. 



