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



sulated on a thin stick of gum-lac, and then presented to the 

 electrometer, resinous electricity was found of considerable in- 

 tensity, shewing that a decomposition of electricity had actually 

 taken place at the second surface of the glass, the resinously 

 electric pole of the tourmaline forming the coating of its first 

 surface, thus attracting the vitreous electricity of the second 

 surface, and disengaging the resinous. Hence it is easy to see, 

 that if the tourmaline remains sufficiently long warm to prevent 

 the recombination of the electricities of its two poles, until the 

 disengaged electricity at the second surface of the glass shall 

 have been carried off by the air or otherwise, that recombination 

 will be prevented, and the electric state will become compara- 

 tively permanent. 



The use of Coulomb's electrometer, in the manner I have al- 

 ready described, affords an easy and general method of compar- 

 ing the electric intensities of different crystals. For by measur- 

 ing the maximum deviation produced by any specimen, we ob- 

 tain, wholly independent of the exact temperature, a measure of 

 its electric power, a measure independent of time, and, as expe- 

 rience shews, little if at all affected by the precise heat to which 

 the crystal has at first been raised, at least within moderate 

 limits. That the experiment admits, even with the most ordi- 

 nary attention to collateral circumstances, of considerable accu- 

 racy, I have proved by repeating the measures of the intensity 

 of a particular specimen several times in succession. It will at 

 once occur, that a source of fallacy must be guarded against in 

 the loss of the electricity with which the disk of the electrometer 

 is charged, which, as it is constantly diminishing by the contact 

 of air, would give the intensities last measured in a series of com- 

 parative experiments too small. In favourable circumstances, 

 and by allowing the disk to remain some time charged before the 

 series is commenced, it is surprising how little this error amounts 



