Tourmaline and other Minerals when exposed to Heat. 1 37 



electric during cooling uppermost; the adhesion was presently 

 complete, so that the glass could be held with the stone sus- 

 pended from its under surface. The other surface of the glass, 

 behind the tourmaline, being then touched with a minute disk 

 of gilt paper, insulated on a thin stick of gum-lac, and then 

 presented to the electrometer, resinous electricity was found 

 of considerable intensity, showing that a decomposition of 

 electricity had actually taken place at the second surface of 

 the glass, the resinously electric pole of the tourmaline form- 

 ing 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 suffi- 

 ciently long warm to prevent the recombination of the electri- 

 cities 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, recombination will be prevented, and the 

 electric state will become comparatively permanent. 



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

 already described, affords an easy and general method of 

 comparing the electric intensities of different crystals. For by 

 measuring the maximum deviation produced by any specimen, 

 we obtain, wholly independent of the exact temperature, a 

 measure of its electric power, a measure independent of time, 

 and, as experience shows, 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 ordinary attention to collateral circumstances, of consi- 

 derable accuracy, I have proved by repeating the measures 

 of the intensity of a particular specimen several times in suc- 

 cession. 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 comparative 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 to. I have al- 

 ways, however, avoided it in practice, by repeating every 

 series of experiments in an inverted order, by which we ob- 

 tain two observations at equal distances from a mean state of 

 electric tension, the mean of which will give strictly compa- 

 rable results. 



The principal application which I made of this method of 

 observing was to attempt to discover some relation between the 

 form and dimensions of crystals of tourmaline, and their elec- 

 tric power. 



Third Series. Vol. 5. No. 26. Aug. 183*. T 



