Dr Priestley made a tourmalin very hot, and having covered it all over to the thickness of a crown piece with melted sealing-wax, he found that its electrical %little was as powerful when enveloped in this coat ing as when exposed to the open air. Having tied the tourmalin ri a silk thread, which only touched the edge of it on both sides, he suspended it before a fire, so that it might be heated equally in every part; when it was so hot that he could hardly bear to handle it, he al lowed it to remain in the same situation a quarter of an hour, in order to be certain that the heat was equally diffused. By means of a bundle of fine thread which had for sonic time been held in the same degree of heat, he took off the electricity which the stone had acquired, and continuing it in the same situation, he found that it did not receive any new electricity from the heat. Hence lie was satisfied of the truth of Mr Canton's 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 electricity during the whole operation. Hence Dr Priestley accounts for the mistake of the Duke de Noya, who asserted, in opposition to ii:pinus, that both sides of the tourmalin acquired in every case a positive elec tricity.
Although the property which the tourmalin possesses of becoming electrical by heat, is strongest in transpa rent crystals, yet the 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. Al. 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 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 their polarit}.
For farther information on the subject of the tonrinalin, see Duc the Noya Caraffa, 8111' le Tourmaline, Paris, 1759.
Benjamin Wilson, Phil. Trans. 175), vol. Ii. J. 308. Id. 1762, vol. lii. p. 443. Watson on the Lyncuriunz of the Ancients, Phil. Trans. 1759, vol. xlix. p. 3)7. X.pinns, Menz. ?ad. Berl. 1756, and .Epinus, Rec tell de tlIenzoires sur la Tourmaline, 8vo. Petursburg, 1762. Bergman, Phil. Trans. 1766. vol. lvi. p. 236, and Berg man's Olzuscula, tom. v. p. 4u1. Priestley's History of Electricity, p. 314, and his Own Experiments, p. 697. Wilke, Abhandlungen der Koniglichen Sclzwedischen ilkadcznie, xxviii. 95, xxx. 1. 105. Z.dlinger, Von Tour nzalin, Vienna, 1779. Hauy, Mem. Acad. Par. 1785. p. 206 ; Hauy, Traits de Physique, vol. i. and Flatly, Traits de ifineralogie, torn. iii. p. 44. Napione, Sul Lincurio, 4to, Rome, 1795. See also Art. 6. of the present Sec tion.
2. On the Electricity of the Tape:.
The tourmalin was for a long time supposed to he 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 properties as the tourmalin ; and in 1761, Mr Wilson found several other gems that had a similar property ; but there is every reason to believe, from his own im perfect description of them, that they were either topazes 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 crys tal.
The topazes from Saxony have not the property of becoming electrical by heat ; but the electricity wltch 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 Pricstley's History of Electricity, p. 324 ; and Wilson Phil. Trans 1762, p. 44.
In the other variety, or the perioctaFdral aluminous fluate of silica with a sex-decimal summit, DI. 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 resiaous state, while the intermediate 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 artificial magnet." See Hauy, Annales de Museum, Hist. ..IVat No. v. p. 350. Id. xv. p. 1. Hauy, Journal des Mines. Hauy, Traite de Mineralogie, vol. ii. p. 514, 515 ; and Lucas, Tableau Methodique, &c.