and other Minerals when exposed to Heat. 35 



The unequal temperature of the portion of any section, pre- 

 vents, as I have already observed, all the parts from giving a 

 maximum intensity at once. This will diminish the total effect, 

 but as all the parts afford the same kind of electricity, the re- 

 sultant can never be null on this account. Therefore, even if 

 the irregularities of amount did not compel us to admit innate 

 varieties of structure or electric disposition in different specimens, 

 stubborn facts must force us to some such conclusion. In the 

 course of my researches, I have met with a crystal of tourmaline, * 

 possessing no external irregularities of structure, (the termina- 

 tions, 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 central portion of the crystal resi- 

 nously electrified. This remarkable fact is not unexampled. 

 Hauy has recorded the case of a crystal of topaz which had a si- 

 milar property, which, from its analogy to known facts in the phe- 

 nomena of magnetism and of double refraction, Dr Brewster 

 conceived to be owing to the union of two distinct crystals, 

 with the vitreous poles in contact, as in that case resinous elec- 

 tricity was developed at both ends. Be this as it may, the ex- 

 ample of tourmaline which I have cited, proves that the junction 

 of the separate crystals, if such exist, may be imperceptible, and 

 as the probability 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 Becquerei, 

 in very long crystals. 



The phenomena of tourmaline, though entirely electric, bear 

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



* It is No. 3. of the Series, at the foot of p. 32. 



e 2 



