MINERALOGY. 



untaraiflied. The crofs frafture is even paffing into flat 

 conchoidal ; it is eafily frangible, and rather hard ; it is 

 more or lefs flexible and elaftic. Before the blow-pipe, on 

 chaicoal it melts into a metallic globule, without any fmell 

 of arfeuic or fulphur, which ferves to diftinguifli it from 

 capillary pyrites. Native nickel contains the metal nickel 

 with a minute portion of cobalt. See Nickel. 



Copper Nickel, Nkkel arftnical, Haiiy , has a copper or crim- 

 fon colour, of diflTerent degrees of intenfity, and often tarnilh- 

 ed grey or black. It occurs mafilve and differainated, and 

 ibmetimes reticulated, botryoidal,ordendritical,and very rare- 

 ly cryllallized infix-lided tables; it hasa ihining metallic luilre. 

 The frafture is imperfetlly conchoidal, pafling into granular 

 and uneven. It is ufually compaft, but fometimes occurs 

 in granular difl.in(3; concretions. It yields with difficulty 

 to the knife, and is difficultly frangible. Its fpecitic gravity 

 is from 6.6 to 7.5. Before the blow-pipe it gives out an 

 arfenical vapour, and then fufes with difficulty into a dark 

 fcoria, mixed with metallic grains : it is foluble in nitro- 

 muriatic acid, forming a dark green liquor, from which 

 caullic alkali throws down a pale green precipitate, which 

 diilinguifhes it from copper ; the precipitate of the latter is 

 a dark brown. This ore is a compound of nickel and 

 arfenic ; it occurs m fmall quantities in the lead-mines of 

 X.ead-hills and Wanlockhead, in Scotland, and in various 

 parts of the continent of Europe, generally in veins in pri- 

 mitive mountains : it nearly refembles native copper, but 

 may be difliinguiftied by its brittlenefs. 



Black Nickel-Ore has a greyifli or brownifli-black colour ; 

 it occurs maffive, diffeminated, and in cruflis ; it is duD, 

 has an earthy fracture, but becomes refinous and fhining 

 in the ftreak ; it foils flightly. In nitric acid it forms 

 an apple-green folution, which lets fall a precipitate of white 

 arfenic. It is rather a rare mineral, and is found in veins 

 with other ores of nickel, and is fuppofed to be formed by 

 the decompotition of copper nickel. 



Niciie Ochre, A'ickel oxyde, Haiiy, has an apple-green 

 colour, but on expofure to the air becomes greenilh-whitc : 

 it is generally found as a thin coat or cruft, and feldom maf- 

 five or diffeminated ; it is in loofe powder or friable, meagre 

 to the touch, and light. It is infufible before the blow- 

 pipe, but with borax it fonns a red colour ; in cold nitric 

 acid it remains infoluble. This ore occurs at Lead-hills and 

 Wanlockhead, in Scotland, and in Saxony and France. 

 NiGRixE. See Titanium. 



Nitre, A'rt/k'c, Potajfe nitratee, Fr. occurs in flakes, crufl:s, 

 and capillary cryftals on the furface of the ground. In 

 jnany countries, it appears to form at certain feafons of the 

 year. It abounds in many of the plains of Spain, Hungary, 

 the Ukraine and Podalia, and on the walls and floors of 

 chalk caves in France. It is very abundant in India, Egypt, 

 and in fome parts of North and South America. See 

 Potash, Nitrate of. 



Obsidiax, or Volcanic Glafs ; Lava vitreufe obfiJlenne, 

 JHaiiy. ( See Obsidi anus Lapis. ) This mineral bears fo 

 clofe a refemblance to dark-green bottle-glafs as fcarcely 

 Xo be difl;inguiflied from it. Its moil common colours are 

 <dark-grey or greenifli, or brownifli-black ; it has a vitreous 

 and Ihining or fplendent lullre ; the fratlure is large and 

 perfectly conchoidal. It pafles from tranfparent to nearly 

 opaque, according to the intenfity of the colour ; it fcratches 

 glafs, but is eafily frangible. The fpecific gravity varies 

 from 2.34 to 2.38. The black obfidian of Iceland is faid to 

 melt into a pale a(h-grey veficular glafs on charcoal ; that of 

 Spanifli America loil its black colour when expofed to heat, 

 became white, fpongy, and fibrous, and increafed to feven 

 ox eight times its original bulk ; hence it appears that fonie 



gafeous fubftance efcapes. Humboldt conjectures, that the 

 gas evolved during the volcanic fufion of obiidian in the 

 interior of the earth, may give rife to the earthquakes that 

 agitate the Cordilleras. According to AbUgaard, the con- 

 ftituent parts of obfidian are given as under : 



Obfidian of Iceland. 



Silex - - . . . ^^ 



Alumine - - . . j 



Oxyd of iron - - . j , 



Lofs, fuppofed to be potafh or foda 10 



100 



Some analyfes give 1.6 lime, and only 5 of potafh and foda. 

 Obfidian is found in a great many volcanic countries, and 

 alfo in various parts of the world, where no volcanic fires 

 have exifted fmce the hifl:ory of man. For a long period 

 it was contended by Werner and his followers, that this 

 mineral was of aqueous origin, but the appearance of obfi- 

 dian, and the fituations in which it occurs, offer decifive 

 proofs of its formation by fire. The mountain de la Caf- 

 tagna, in Lipari, according to Spallanzani, is wholly com- 

 pofed of it, which appears to have flowed in fucceffive cur- 

 rents, like ftreanis of water falling with a rapid defcent and 

 fuddenly frozen. The obfidian is fometimes compact and 

 fometimes porous and fpongy. On the fouth fide of the Peak 

 of Teneriffe, there is a ilream of obfidian feveral miles in length. 

 In the illand of Felicuda, a cuiTent of lava intermixed with 

 obfidian may be traced to the very crater of a volcano. 

 Obfidian appears to be lava fuddenly cooled, as it is well 

 known, fince the experiments of Mr. Keir, Mr. Watt, and 

 fir James Hall, that lava or bafalt cooled fuddenly forms 

 black glafs ; and we have a fpecimen of lava from Vefuvius, 

 which flowed in the year 18 18, one part of which is com- 

 paft black lava, the other is a vitreous fubftance paffing from 

 pitch-ilone to obfidian. See Volcanic Product. 

 OcTAHEDRiTE, Aiiatafe. See Titanium. 

 Olivine. (See Olivine. ) This mineral forms a con- 

 ftituent part of many lavas, and is of frequent occurrence 

 in bafaltic rocks. When cryitallized, it is in broad reftan- 

 gular four-fided prifms, which are imbedded, and fo eafily 

 broken, that it is difficult to afcertain their form ; the ilruc- 

 ture of tiie cryftals is imperfectly lameUar, in a direction 

 parallel with the planes of the prifm. Werner coi'.fiders 

 olivine as a diftinft fpccies from chryfolite (fee Chryso- 

 lite), with which it isclafled by many mineralogifts. The 

 colour, luftre, and frafture of olivine and chryfolite, are not 

 precifely the fame ; olivine is alfo fofter and more frangible 

 than chryfoUte. Chryfolite is more tranfparent, and has 2 

 greater fpecific gravity, being from 3.4 to 3.34 ; that of 

 olivine is 3.23 to 3.26. Ohvine is often much intermixed 

 with augit ; it has generally a paler colour and greater 

 tranfparency than the latter mineral. Olivine is fubjeft 

 to decompofition, and when this commences it exhibits on 

 the furface irridefcent colours, but afterwards falls into an 

 earth refembling iron oclire. 



OlIVEN- 



