TOURMALINE. 



IThe edges of the prifm are frequently truncated, and thus 

 [form pnfms with nine or twelve fides. The primitive form 

 of the cryllal, according to Haliy, is an obtufe rhomb. 

 The fraCfure of tourmahne is perfeftly conchoidal ; the in- 

 ternal luftre is vitreous. It is harder than hornblende, but 

 Ids hard than quartz : it fcratches glafs, and is brittle. The 

 'principal colours are dark green and brown, but it occurs 

 ;alfo red and blue, of various (hades and degrees of intenfity. 

 lit is fometimes nearly opaque, but more frequently tranf- 

 lucent or traniparent. The cryftals of tourmahne pofTefs 

 the peculiar property of being tranfp.irent when the light 

 palTes perpendicular to the axis of the prifm, and opaque 

 when it pafFes in the direction of the axis. It is always 

 iopaque when the height of the prifm is lefs than its breadth. 

 [The principal form of the cryllal has been before dated ; 

 but the moft common of the fourteen or fifteen different 

 varieties, is the prifm with nine fides terminated by fix 

 planes at one end, and by three at the other ; or by feven 

 planes at one end, and three at the other. The end which 

 [has the fmalleft number of planes exhibits negative elec- 

 tricity, and that which has the greateft number, pofitive. 

 This difference in the form of the eleftric poles is a general 

 law of all cryftalhzed minerals that are eleftric by heat. 

 Before the blowpipe, the tourmaline melts into a greyilh- 

 white porous enamel, but the red Siberian tourmaline is in- 

 fufible. The conftitucnt parts of tourmaline are given by 

 iVauquelin and Klaproth as under. 



According to Vauquelin : 



Green Tourmaline Violel Tourmaline 



from Brazil. from Siberia. 



Silex - - 40 42 



Alumine "39 4° 



Soda - - 10 

 Lime - - 3.84 

 Oxyd of iron - I2.r 



Oxyd of manganefe 2 7 



According to Klaproth : 



'' The tourmaline is a name given, as Dr. Watfon has 

 [I evinced, to the lyncurium of the ancients. See Lyncurius 

 I^npis. 



This ftone is very common in feveral parts of the Eaft 

 [ Indies, particularly in the ifland of Ceylon, where it is called 

 I tournamal. Pliny mentions a red or purple ftone which at- 

 i trafted light bodies. This is fuppofed to be the lourma- 

 j line. The firft account of this ftone, that occurs of late 

 I years, is in the Hiftory of the Royal Academy of Sciences 

 I at Paris for 17 17, where we are told that M. Lemery ex- 

 1 hibited an uncommon ftone brought from Ceylon, whicli 

 ! attracted and repelled fmall light bodies. Linnseus, in 

 his Flora Zeylanica, calls this ftone laph eleffricus, and takes 

 notice of Lemery's experiments. M. .Slpinus, having 

 been informed of the attraftive power of the tourmaline by 

 Mr. Lechman, and furnifhed by him with two ftones, made 

 many experiments, the refult of which was pubhftied in the 

 Hiftory of the Academy of Sciences and Belles Lettres 

 at Berhn, for 1 756. The fubftance of his memoir is as fol- 

 lows : the tourmaline has always, at the fame time, a pofitive 



and a negative eleclricity ; the one of its fides being in one 

 ftate, and the other in the oppofite ; and this does not de- 

 pend on the external form of the ftone. Thefe eleftricities 

 he could excite in the ftrongeft degree by plunging the 

 ftone in boiling water. 



The proper degree of heat for manifeftmg the eleftricity 

 of the tourmahne, is from 100° Fahrenheit to 212°. If we 

 raife the temperature above 2 1 2^, it lofes its eleftricity, but 

 regains it in cooling ; but if we continue to raife the tempe- 

 rature, we arrive at a point in which the mineral again be- 

 comes eleflric, but has its poles reverfed to what they were 

 before. Haliy obferved, that we may alfo change the pofi- 

 tion of the poles, by heating it unequally in the focus of a 

 lens or mirror. If an eleftrified tourmaline be broken, the 

 fragments immediately prefent eleftric poles, fituated in the 

 fame direftion as that of the large cryftal. 



If one fide of the tourmaline be heated more than the 

 other (as if it had lain upon a hot cake of metal), each of 

 the fides acquires an eleAricity oppofite to that which is 

 natural to it ; but if left to itfelf, it will return to its na- 

 tural ftate. 



If one of the fides of the tourmaline be rubbed, while 

 the other is in contaft with fome conduftor communicating 

 with the ground ; the rubbed fide is always pofitive, and 

 the other negative. If neither fide be in contaft with a 

 conduftor, both become pofitive. If, in the former of 

 thefe cafes, the tourmaline be rubbed fo as to acquire a 

 fenfible heat, and the fide which is naturally pofitive be 

 made negative, it will, upon Handing to cool, return to 

 its natural ftate ; but if it have acquired no fenfible heat, 

 it will not return to its natural ftate while any kind of 

 eleftricity remains. If it be heated, even when it is 

 rubbed and infulated (in which cafe both fides become 

 pofitive), it will ftill return to its natural ftate upon 

 cooling. 



The Due de Noya, who, in 1758, procured two of thefe 

 ftones in Holland, and made a number of experiments with 

 them, an account of which he pubhflied, mentions the ex- 

 periments of M. ^pinus, but does not admit of a plus and 

 minus eleftricity belonging to the tourmaline when heated. 

 On the contrary, he fays ' that both the fides are electrified 

 plus, but one of them more than the other. The tourmaline 

 was introduced to the notice of the Englifh philofophers 

 by Dr. Heberden, who procured one (and, indeed, the only 

 one known in England at that time) for Mr. Wilfon ; whofe 

 experiments confirmed the opinion of its pofitive and nega- 

 tive power, advanced by M. ^plnus. Dr. Franklin, like- 

 wife, in a letter to Dr. Heberden, in 1 759, informs him of 

 experiments on one of thefe ftones, fufficient for eftablifti- 

 ing the fame theoi-y. About this time Dr. Heberden pro- 

 cured fome of thefe ftones from Holland, and put them into 

 the hands of Mr. Wilfon and Mr. Canton, gentlemen par- 

 ticularly converfant with the fubjeft of eleftricity, and ca- 

 pable of ufing them with the greateft advantage to this 

 branch of fcience. Mr. Wilfon's experiments and obferv- 

 ations are very numerous ; the refult of which was, in the 

 main, the fam.e with that of M. iEpinus, eftablifhing the 

 opinion of the two different powers of this ftone ; but he 

 differs from him in afferting, that when the fides of the tour- 

 maline are unequally heated, it exhibits that fpecies of elec- 

 tricity which is natural to the hotter fide ; i.e. the tourmaline 

 is plus on both fides, when the plus fide is the hotteft ; and 

 minus on both fides, when the minus fide is the hotteft. Both 

 thefe gentlemen repeated their experiments, and each re- 

 tained his own opinion ; but Mr. Wilfon's apparatus was 

 better calculated for the purpofe of accurate experiments 

 than that of M. .£pinus, and he ufed a greater variety of 



methods 



