and other Minerals when exposed to Heat. 3\ 



to ; I have always, however, avoided it in practice, by repeating 

 every series of experiments in an inverted order, by which we 

 obtain two observations at equal distances from a mean state of 

 electric tension, the mean of which will give strictly comparable 

 results. 



The principal application which I made of this method of ob- 

 serving, was to attempt to discover some relation between the 

 form and dimensions of crystals of tourmaline, and their electric 

 power. 



M. Becquerel, in a Second Memoir on the Tourmaline, pub- 

 lished in 1828,* announced the rather extraordinary circum- 

 stance, that long Tourmalines did not become at all electric by 

 heat, and that their facility of being excited was generally in- 

 versely proportional to their length. Dr Thomson, in his work 

 on Heat,f 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 Tourmalines 

 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 at- 

 tention in this country, and as of late nothing whatever has been 

 done upon them, these observations may prove the more inte- 

 resting. 



The longest Tourmaline employed by M. Becquerel, was 

 six centimetres, or 3.2 English inches in length, with a diameter 

 of about .08 inches. My largest Tourmaline is 3. c 25 inches, or al- 

 most precisely the same, with a diameter little different. Instead of 

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

 describes his, it proved uniformly susceptible of powerful excite- 

 ment, under the very same treatment which I was accustomed to 



* Ann. de Chimie. f F. 477. 



