TOURMALINE. 



methods for communicating heat to liis tourmaline. Mr. 

 Wilfon imagined that this ftone, as well as glafs, was per- 

 meable to the eleftric fluid, and that the refi'tance to its en- 

 tering the fubftance of it was lefs on what he calls the nega- 

 tive than on the pofitive fide ; for rubbing the pofitive fide 

 of the ftone ftrongly, he found both fides eleftrified fliis ; 

 by rubbing the negative fide in the fame manner, both fides 

 were eleftrified />/«;, m.ore ftrongly than before. Several 

 experiments led Mr. Wilfon to conclude, that the tourma- 

 line refitted the exit and entrance of the eleftric fluid con- 

 fiderably lefs than glafs, or even than amber ; and he in- 

 fers, upon the whole, that this ftone differs in nothing from 

 other eleftric bodies but in acquu-ing electricity by heat. 

 Mr. Wilfon alfo conceived, that the tourmaline fuffered the 

 eleftric fluid to pafs through it only in one direftion, bear- 

 ing in this refpeft fome analogy to the loa.l-ftone, and hav- 

 ing as it were two eleftric poles, which arc not eafily de- 

 ftroyed or altered. He alfo apprehended, that the eleftric 

 fluid, flowing through all the Hones and gems which re- 

 ferable the tourmaline in their eleftrical properties, moves in 

 that direftion in wbJch the grain happens to lie ; the re- 

 fiftance, as he fuppofes, which the fluid meets with, being 

 lefs in that direftion than in any other. 



Notwithftanding the attention given to this fubjeft by 

 M. iEpinus and Mr. Wilfon, the moft important difcovery 

 relating to the c-leftricity of the tourmaline was referved for 

 Mr. Canton, who, in a paper read before the Royal Society 

 in December 1759, obferves, that the tourmaline emits and 

 abforbs the eleftric fluid only by the increafe or diminution 

 of its heat ; for if the tourmahne, he fays, be placed on a 

 plain piece of heated glafs or metal, fo that each fide of it, 

 by being perpendicular to the furface of the heated body, 

 may be equally heated ; it will while heating have the elec- 

 tricity of one of its fides pofitive, and that cf the other 

 negative. This \%'i!l likewife be the cafe, when it is taken 

 out of boiling water, and fuffered to cool ; but the fide 

 which was pofitive while it was heating, will be negative 

 while it is cooling ; and the fide which was negative will be 

 pofitive. In the Gentleman's Magazine for September 1 759, 

 (vol. xxix. p. 424. ) he pubhfhed the refult of fome ex- 

 periments which he had made on a tourmaline procured from 

 Holland, in a feries of propofitions, comprifing the prin- 

 cipal part of what is known on this fubjeft. They are as 

 follow: I. When the tourmaline is not eleftrical or attrac- 

 tive, heating it, without friftion, will make it fo ; and the 

 eleflricity of one fide of it (diftinguifhed by A) will be 

 pofitive, and that of the other fide (B) will be negative. 

 2. The tourmahne not being eleftrical, will become fo by 

 coohng ; but with this difference, that the fide A will be 

 negative, and the fide B pofitive. 3. If the tourmahne, in 

 a non-eleftrical ftatc, be heated, and fuffered to cool again, 

 without either of its fides being touched ; A will be pofitive 

 and B negative, during the whole time of the increafe and 

 decreafe of its heat. 4. Either fide'of the tourmaline will 

 be pofitive by friftion, and both may be fo made at the 

 fame time. He luggefts that, if air be endued with fimilar 

 properties, or be capable of becoming eleftrical by tl;e in- 

 creafe or diminution of its heat (as is probable by attending 

 to its ftate before and after a thunder-ftorm), tliunder- 

 clouds, both pofitive and negative, as well as thunder-gufts, 

 may be eafily accounted for. Mr. Canton, with the tour- 

 maline which he received from Dr. Heberden, made other 

 new and curious experiments, firtt publilbed by Dr. Prieftley. 

 He put one of them, which was of the comm.on colour, 

 into the flame from a blowpipe, and burnt it white ; when 

 he found that its eleftrical property was entirely deftroyed. 

 The eleftricity of another was only in part deftroyed by fire. 



He joined two others, made foft by fire, without deftroj-- 

 ing their eleftrical property. The virtue of another w? 

 improved by being melted at one end ; and he found (coi 

 trai-y to what Mr. Wilfon had obferved of another to... 

 maline, heated in the fame manner) that one tourmaline r^ - 

 tained its eleftrical property, after it had been frequently 

 made red-hot, and in that ftate put into cold water. But 

 the moft curious experiment was made on a large irregular 

 tourmaline, about half an inch long, which he cut into three 

 pieces ; taking one part from the pofitive and another from 

 the negative end. Trying thefe pieces feparately, he found 

 the outer fide of the piece cut from the end that was nega- 

 tive when cooling, was likewife negative when cooling ; 

 and that the outer fide of that piece which was cut from ^ 

 the end that was pofitive when cooling, was likewife pofitive 

 when cooUng ; the oppofite fides of both pii;ces being, ac- 

 cording to the general law of the eleftricity of the tour- 

 maline, in a contraiy ftate. The middle part of the fame 

 ftone was aftefted as it had been when it was entire ; 

 the pofitive end remaining pofitive, and the negative end 

 negative. 



Dr. Prieftley, about the clofe of the year 1766, direfted 

 his attention to the tourmahne : being in poffefiion ■ f Dr. , 

 Heberden's large polifhed one, which weighed one hundred 

 and twenty grains, of an oval form, plane on one fide and 

 convex on the other, and which had paffed through the 

 hands of Mr. Wilfon and Mr. Canton, he purfued his in- ■ 

 veftigation of the properties of this ftone in a variety of ' 

 experiments. Several of them were undertaken with a view 

 of determining, whether the tourmaline coUefted its eleftri- 

 city from the neighbouring air : he was led to this conjec- 

 ture from the confideration of Mr. Wilcke's experiments 

 on the produftion of fpontaneous eleftricity, by melting 

 one fubftance within another ; and his experiments feem to 

 prove that his conjefture was juft. He alfo difcovered a 

 method of reverfing all the experiments made upon the 

 tourmaline, making that fide which is pofitive in heating or 

 coohng to be negative ; and that which is negative to be 

 pofitive : fo that the kind of eleftricity Ihall be juft what 

 the operator (hall direft, by the apphcation of proper fub- 

 ftances to the ftone. Dr. Prieftley not only tried how the 

 tourmaline would be affefted by being heated or cooled in 

 contaft with various fubftances, to which only one of its 

 fides was expofed at once, but he alfo made other experi- 

 ments, in which the ftone was entirely furrounded by them. 

 Having covered the ftone to the thicknefs of about a crown- 

 piece with feahng-wax, he found it to aft nearly, if not quite 

 as well, through this coating of wax, as if it had been ex- 

 pofed to'the air : hence, if a tourmaline be concealed in a 

 ftick of fealing-wax, the wax will feem to have acquired 

 the properties of the tourmaline. See on this article Phil. 

 Tranf. vol. h. p. 308, &c. p. 394, &c. vol. liii. p. 436, 

 &c. Frankhs's Letters, &c. p. 376. Prieftley's Elec- 

 tricity, vol. i. p. 347, &c. vol. ii. p. 308, &c. 8vo. ed. 



The tourmaline, according to Mr. Kirwan, is a fihceous 

 earth, imperfeftly united with from 1.05 to 1.47 of its 

 weight of argil, from 0.3 to 0.4 of its weight of mild cal- 

 careous earth, and from 0.15 to 0.243 of its weight of 

 iron. This ftone has been found in Ceylon, Brazil, 

 and the Tyrol : a fpecimen of each has been examined 

 by Mr. Bergman ; that of Ceylon is of a dark-brown 

 or yellowifh colour; its fpecific gravity 3.065 or 3.295: 

 that of Brazil is green, blue, red, or yellow, and its 

 fpecific gravity 3.07 or 3.18 : that of the Tyrol, by re- 

 flefted hght, is of a blackifh-brown, but by refrafted light, 

 yeUowifti, or in thin pieces, green ; its fpecific gravity 3-05. 

 In fire, none of them decrepitate ; but thofe of Ceylon 



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