T U R 



TURCOIN, a town of France, in the department of the 

 North, and chief place of a canton, in the diftrift of 

 Lille. The unfortunate conflift between the French and 

 the duke of York, who commanded the allies, may be faid 

 to Itave decided the fate of the Netherlands ; 6 miles N N F 

 of Lille. ■ 



TURCOIS, or TuRQUOIS, in Natural Hijlory and Mi- 

 neralogy, a fubftance found in Perfia and other parts of 

 Afia, and formerly claffed with ftones. It has a beautiful 

 light-green colour, and is fufceptible of a high polifh. 

 The furface is fmooth and polifhed ; it has alfo a fmooth 

 ihining frafture, and is fo hard as to fcratch glafs flightly : 

 the fpecific gravity is 3.127. It has for a long time been 

 confidered as the tooth of an unknown animal impregnated 

 with copper ; but by a feries of analytical experiments. La 

 Grange has proved -that it does not contain a particle of 

 copper, but is in reahty bone coloured by phofphate of 

 iron. The conftituent parts are as under : 



Guyton Morveau fufpefted that the turcois contained 

 filex, but this is fuppofcd by other chemifts to have been 

 accidental. This celebrated chemift made fome experi- 

 ments on foflile bones, and found that they affume in the 

 fire the colour of turcois ; and when digefted in a weak 

 folution of pota(h, they turn blue, varying from a greenifh 

 to deep blue. Meflrs. Fourcroy and Vauquehn have alfo 

 ohferved that bones ftrongly calcined often affume a blueifh 

 tinge, which they confidered to be caufed by a fmall portion 

 of phofphate of iron. 



Turcois is employed in jewellery. See Gems. 



The Greeks and Latins feem to have known it under the 

 name callais : and it appears to have had a place in the ra- 

 tionale of the high-prieft of the Jews. 



Some writers mention turcoifes both oriental and occi- 

 dental, of the new rock and of the old. The oriental par- 

 takes more of the blue tinfture than the green ; and the 

 occidental, more of the green than the blue. Thofe of the 

 old rock are of a finer blue, and thofe of the new rock are 

 often whitifh, and do not keep their colour. 



The oriental ones come from Perfia, the Indies, and 

 fome parts of Turkey ; and fome even fuppofe, that it is 

 hence they derive their modern name turcois. The occi- 

 dental are found in various parts of Europe, particularly in 

 Germany, Bohemia, Silefia, Spain, and France. 



Turcoifes are found of a round or oval figure ; they cut 

 eafily, and befides feals, which are frequently engraved on 

 them, fome are formed into crucifixes, or other figures, near 

 two inches high : though De Boodt miilakenly affirms, 

 that none have been known to exceed the bignefs of a 

 walnut ; for the fpecimen exhibited by Mr. Mortimer to 

 the Royal Society was twelve inches long, five inches broad, 

 and in fome places near two inches thick. 



The turcois is eafily counterfeited ; and that often is 

 done fo perfeftly, that it is impoflible to difcover the deceit, 

 without taking it out of thecoUet. 



In the Memoirs of the Academy of Sciences for the 

 year 17 15, we have a very curious account of the formation 

 of the turcois, and the manner of managing its naturally irre- 

 gular colour, by M. Reaumur. The turcois, he obfervcs, 

 ■js one of the fofteft of precious ftones, its hardnefs ufually 

 •jot coming up to that of a cryftal, or a tranfpafent pebble ; 



Vol. XXXVI. 



T U R 



though fome are much harder than others : and ftiU the 

 harder, ceteris paribus, the more valuable, by reafon of the 

 ^l^^'^^^^y °f the polilh, which is always proportionable to 



ifj^AT ' ^ J^^^'^^'"' 3"'* the author of a fcarce treatifc, 

 caUed Mercure Indien, eftimating the feveral precious ftones, 

 lets a hard turcois, whofe blue is neither bright nor deep, on 

 the foot of the moft perfeft emeralds, that is, nearly on a 

 r5,r r^"^" ^ diamond ; but fuch are fcarcely ever met with. 

 1 hofe with any defed, he only values at a French crown 

 the carat. 



Tavernier afRrms, but erroneoufly, that there are but two 

 mines of turcoifes known in all the earth, and thofe are both 

 in Perfia ; the one called the old rock, near a town called 

 Necabourg, three days' journey to the north-eail of Meched ; 

 the other, called the netu rod, is five days' journey from it. 

 The ftones from the latter, he adds, are but little valued ; 

 and the king of Perfia hath for many years prohibited the 

 digging in the former for any but himfelf. M. Reaumur 

 takes the old rock to be now exhaufted ; ineffeft, the com- 

 mon divifion of turcoifes into the old rock or oriental, and 

 new rock or occidental, is very arbitrary and precarious. 

 All the beft, and moft perfed, whenever they occur, in 

 India or Europe, are reckoned among the former, and the 

 reft among the latter. Near Simore, in the Lower Lan- 

 guedoc, there are feveral confiderable mines of turcoifes ; 

 but that fine blue colour, admired in the turcois, is not 

 natural to thofe of thefe rocks ; the prevailing colour bein^ 

 fometimes white, and fometimcs mucli like that of tripoli of 

 Venice. Other precious ftones are dug out of the mine 

 with all their colour, to the force of which nothing can be 

 added, though it may frequently be diminiftied, as we fee 

 fire bring down the too deep colour of the fapphire, and 

 quite take away that of a pale fapphire : thefe turcoifes, on 

 the contrary, are naturally whitifti or yellowifti, of a colour 

 as ordinary as that of a free-ftone ; and by oppofing them 

 for fome time to the aftion of the fire they aflume <i blue 

 colour. 



It fecms a paradox, and yet M. Reaumur has attemptad 

 to prove, that turcoifes are originally the bones of animals. 

 In the mines in France, pieces have feveral times been found 

 in the figure of teeth, bones of the legs, &c. And tur- 

 coifes which are yet imperfeft, or half-formed, are appa- 

 rently compofed of lamini, or leaves, like thofe of bone?, 

 between which fome petrifying juice, infinuatingitfelf, binds 

 them clofe together ; and ftill, the fofter, the mure imperfeft 

 the ftones are, the more diftinguifhable are the different direc- 

 tions of the fibres and laminae, with their interfeftions, and 

 the great refemblance they bear to fra<Etured bones, and the 

 lefs to any kind of ftones known. 



To give them a blue colour, they dry them awhile in 

 the air, then heat them gradually in a furnace made after a 

 particular manner. If they be heated too haftily, the 

 humidity between the laminae wanting time to evaporate, 

 the whole will feparate into fcales or flaws. Some of the 

 ftones require a greater degree of heat to bring them to their 

 colour than others ; and even in large pieces, the feveral 

 parts ordinarily require feveral degrees of heat. 



On this account a great deal of care is to be taken in the 

 heating of them ; for the fire, which gives them their blue 

 by degrees, if they be expofcd beyond a certain degree, takes 

 it away again. 



M. Reaumur accounts for their taking a blue colour by 

 heat very well ; when frefh cut out of the rock, it fecms their 

 fubftance is found fprinkled and llreaked all over with fpots, 

 veins, little circles, &c. of a dark blue colour; thefe he 

 takes to be fources of a deep blucifli matter, which the fire 

 3 G rarefying. 



