ELECTRICITY. 



459 



Descriptive it did not receive any new electricity from the heat 

 Electricity. Hence he was satisfied of the truth of Mr Canton's 

 S ~^Y" ^ remark, that the electricity of the tourmalin derives its 

 electricity, not from heat, but from the circumstance of 

 its changing its temperature. In the same way Dr 

 Priestley confirmed another observation of Mr Canton, 

 that when the tourmalin was heated and allowed to 

 cool without either of its sides being touched, the two 

 sides will retain the same kind of electricity during the 

 whole time of the heating and the cooling of the tour- 

 malin. On some occasions the stone often changes its 

 electricity very slowly, and the electricity which it ac- 

 quires from diminution of temperature continues many 

 hours without any very sensible decrease. In some 

 cases, the electricity acquired by heating may be so 

 strong as to overpower the virtues acquired by cooling, 

 so that both sides of it may exhibit the same kind of elec- 

 tricity during the whole operation. Hence Dr Priestley 

 accounts for the mistake of the Duke De Noya, who 

 asserted, in opposition to ./Epinus, that both sides of the 

 tourmalin acquired in every case a positive electricity. 

 Properties Although the property which the tourmalin possesses 

 of different o f becoming electrical by heat, is strongest in trampa- 

 tourmaliiu. rent cr y sta [ 8) yet t h e black and opaque crystals often 

 possess the same property in a high degree. In some 

 crystals, however, which are rendered impure, by the 

 admixture of particles of iron and other bodies, the 

 electricity is often very feeble, and sometimes impercep- 

 tible. In such cases, the Abbe Hauy frequently found 

 that a fragment detached from the mass possessed po- 

 larity when it was not exhibited by the entire crystal, 

 and when the fragment was taken from the part which 

 approached most to the vitreous state. Hence mine- 

 ralogists have committed a great mistake, in dividing 

 the tourmalin into those which are electrical and those 

 which are opaque. M. Emmerling, in his Treatise on 

 Mineralogy, has attempted to make a different distinc- 

 tion, by maintaining that the electrical schorl or tour- 

 malin, is the only one which loses its electrical property 

 by being too much heated. M. Hauy has carefully 

 examined this supposition, and has found that green as 

 well k as black tourmalin still retain their virtue after 

 being brought to a red heat. He found also, that when 

 the heat was very great, both the electrical and the 

 black schorl were deprived of tht-ir polarity. 



For farther information on the subject of the tourmalin, 

 see Due de Noya Caraffa, stir le Tourmaline, Paris, 1759. 

 Benjamin Wilson, Phil. Trans. 175P, vol. li. p. 308. 

 Id. 1762, vol. lii. p. 4 13. Watson on the Lyncurium 

 of the Ancient*, Phil. Trans. 1759. vol. xlix. p. 397. 

 Jtpinus, Mem. Acad. Berl. 1755, and ^Epinus, Recueil 

 de Memoiret tttr la Tourmaline, 8vo. Petersburg, 1762. 

 Bergman, Phil. Trans. 1760', voLlvi. p. 236, and Berg- 

 man's Opuscula, torn. v. p. 401. Priestley's History of 

 Electricity, p. 31 1, and his Own Experiments, p. 697. 

 Wilke, Abhandlunsf-n der Koniglichen Schmedischen 

 Akademie, xxviii. <).'i, xxx. 1. 105. Zallinger, yon Tour- 

 malin, Vienna, 177J). Hauy, Mem. Acad Par. 1785, p. 

 206; Hauy, Traiti- dtPhytiquf, vol. i. and Hauy, Traite 

 de Mineralogie, torn. iii. p. 44. Napione, Siil Lincurio, 

 4to, Rome, 1 795. See also Art. 6. of the present Section. 



2. On the Electricity of the Topaz. 



Electricity The tourmalin was for a long time supposed to be 

 t'thetopaz. the only mineral which was capable of being excited 

 by heat. M. Canton, however, in the year 1760, 



found, that the Brasilian topaz had the same electrical Descriptive 

 properties as the tourmalin ; and in 1761, Mr Wilson Electricity. 

 found several other gems that had a similar property ; jjj^^j 

 but there is every reason to believe, from his own im- by Canton. 

 perfect description of them, that they were either to- 

 pazes or tourmalins. 



The Abbe Hauy, to whom this branch of electricity 

 is under peculiar obligations, has confirmed the disco- 

 very made by Canton of the excitation of the topaz by 

 heat, and he detected the same faculty in the topaz of 

 Siberia. The positive and negative poles reside in the 

 two opposite summits of the secondary form of that 

 crystal. 



The topazes from Saxony have not the property of 

 becoming electrical by heat; but the electricity which 

 they acquire by friction is so remarkable, that the 

 slightest friction with the finger is capable of exciting it. 

 The Saxon topaz often preserves its excited electricity 

 for more than half an hour when the weather is favour- 

 able. The same property belongs to the blue topaz 

 from Aberdeenshire. For farther information on this 

 subject, see Priestley's History of Electricity, p. 324 ; 

 and Wilson, Phil. Trans. 1762, p. 44. 



In the other variety, or the perioctaedral aluminous 

 fiuate of silica with a sex decimal summit, M. Hauy has 

 observed another curious electrical phenomenon, which 

 seems to be analogous to a phenomenon exhibited by 

 magnets with consecutive points. The topaz, after be- 

 ing heated, had its two extremities in a resinous state, 

 while the inteVmediate part exhibited signs of vitreous 

 electricity. We are sorry that we cannot obtain a more 

 particular account of this electrical property, as it is 

 merely mentioned by J. A. H. Lucas, in his Tableau 

 Methodique, &c. who gives the following remark upon 

 it by Hauy. " It is,'' says he, " an additional instance 

 of resemblance 'between the appearances produced by 

 magnetism, and those which are especially exhibited by 

 bodies susceptible of electricity by heat, and in which 

 the law of electrical densities is so completely analogous 

 to that observed by the magnetic densities in the arti- 

 ficial magnet." See Hauy, Annales de Museum, Hist. 

 Nat. No. v. p. 350. Id. xv. p. 1 . Hauy, Journal det 

 Mines. Hauy, Trait e de Mineralogie, vol. ii. p. 514, 

 515; and Lucas Tableau Melhodique, &c. 



'*, 



3. On the Electricity of the Borate of Magnesia. 



In the year 1791, M. Hauy obtained two crystals of Flectririty 

 that species of the boracite or borate of magnesia, which of tlle :. ho " 

 he calls defective, (Magnesie Boratee Defective; see 

 our article CRYSTALLOGRAPHY, vol. vii. p. 474. col. 2.) 

 He exposed them to the heat of the fire, and presented 

 them to an electrometer. * He immediately perceived 

 that it was electrified, and that it had several poles endu- 

 ed with opposite electricities. He experienced at first 

 considerable difficulty in determining the precise po- 

 sition of these poles, both from the delicacy of the ex- 

 periment, and from the extreme smallness of the crystals. 

 By comparing, however, the crystals with the tourma- 

 lins, he reasoned in the following manner : As in the 

 case of the tourmalin, there was only one axis which 

 coincided with that of the nucleus, there ought only to 

 be two electrical poles situated at the extremity of this 

 axis. In the cube, on the contrary, which was the nu- 

 cleus of borate of magnesia, there were four axes, each 

 of which passed through the solid angles, and conse- 

 quently there ought to be 8 electrical poles, one at the 



. 

 " ias 



This electrometer will be described in Pan II. of the present article. 



