Tourmaline and other Minerals when exposed to Heat. 141 



(the terminations, however, of the crystal are not preserved,) 

 which has the singular property of presenting in cooling a 

 vitreous pole at both ends. Having ascertained this point, I 

 proceeded to examine the electricity of its parts by means of 

 Coulomb's Proof-plane, by which the electricity of any portion 

 is insulated and examined. As I expected, I found the cen- 

 tral portion of the crystal resinously electrified. This remark- 

 able fact is not unexampled. Haiiy has recorded the case 

 of a crystal of topaz which had a similar property, which as it 

 is analogous to known facts in the phaenomena of magnetism 

 and of double refraction, Dr. Brewster conceived that the 

 crystal of topaz was composed of two with the vitreous poles 

 in contact, as in that case resinous electricity was developed 

 at both ends. Be this as it may, the example of tourmaline 

 which I have cited proves that the junction of the separate 

 crystals, if such exist, may be imperceptible, and as the pro- 

 bability that such irregularity should exist, however caused, is 

 in proportion to the length of the specimen, this may perhaps 

 explain the want of excitability observed by Becquerel in very 

 long crystals. 



The phaenomena of tourmaline, though entirely electric, 

 bear so strong an affinity to those of magnetism, that the study 

 of their relations must be considered extremely important. I 

 have therefore one remark to make upon an experiment which 

 Dr. Brewster thinks indicative of a "singular breach of analogy 

 between the distribution of the pyro- electrical and magnetical 

 forces." After observing that in the process of reducing a 

 magnet to powder, the coercive force employed effectually de- 

 stroys all trace of magnetism, he adds, that powder of tour- 

 maline is highly electric when placed on a glass and heated, 

 which is shown by its adhering in conglomerated masses, ex- 

 hibiting the appearance of viscidity when stirred. It appears 

 to me that this experiment does not go to show that tourma- 

 line in a state of excitation does not lose its electricity when 

 bruised in a mortar ; indeed, such an experiment it would be 

 impossible to perform. A tourmaline, when it is not changing 

 its temperature, is as inert as a bar of iron before it is mag- 

 netized ; the process of heating or cooling the one is precisely 

 equivalent to that of conveying magnetism by induction or 

 otherwise to the other. The powder of tourmaline is there- 

 fore analogous to the filings of iron, both being equally inert, 

 till the native electricity of the former, and the native magnet- 

 ism of the latter, is decomposed, when the result in both is per- 

 fectly identical. 



I shall now only very briefly allude to the conclusions to 

 which some experiments on the electricity of other minerals 



