Tourmaline and other* Minerals when exposed to Heat, 135 



results with great elegance and simplicity, without attempting 

 to indicate the precise temperature of the stone at any period, 

 which, whilst the heat of the medium in which it is placed 

 changes, can only be by M. Becquerel's experiment an ap- 

 proximation, since the interior of the crystal must at any mo- 

 ment have a different temperature from its surface. 



I employed a simple form of Coulomb's electrometer, 

 which I constructed for 

 the purpose with little 

 difficulty. A flat-shap- 

 ed bottle A B, having 

 a wide tubulature at C, 

 was provided. Fitted 

 to the neck is a tube 

 D, plugged at top by a 

 cork F, through which 

 passes a crooked wire 

 f> for the purpose of 

 regulating a fibre of 

 raw silk supporting the 

 needle of gum-lac e, one 

 end of which is termi- 

 nated by the disk g, of 

 gilt paper. The object 

 to be examined is intro- 

 duced through the tu- 

 bulature C, the disk 

 having previously been 

 charged with vitreous 

 or resinous electricity ; 

 and the repulsions oc- 

 casioned by the presence of the body under experiment are 

 measured with sufficient exactness by the deviation of the 

 needle reckoned on the divided circle of paper ik, I may 

 add, that the graduated circle H at the top of the glass tube 

 is employed to measure any required torsion of the silk fibre 

 produced by causing the cork F to revolve. By means of 

 this simple apparatus, I obtained results of greater accuracy 

 than my objects generally required*. 



Upon presenting to the electrometer, which is quite an in- 

 sulator, an energetic crystal of tourmaline (which should not 



* It is surprising that Coulomb's instrument has not been more employed 

 in this inquiry. Becquerel seems only to have used it once, and Dr. Brew- 

 ster had recourse to the laborious and unsatisfactory method of causing 

 pyro-electric crystals to lift fragments of the Arundo fhragmites, which can 

 give no comparative results. 



