OIL
OOL
( 137 )
of tills oil is rectified by three fucceffive diflilla-tious from the furface of water, or by a greaternumber without water, it becomes at lengthquite colourlefs and tranfparent; it has a power-ful but fomewhat aromatic odour, and is nearlyas light and volatile as ether. It contains alittle ammonia, and hence changes the colourof fyrup of violets green, it is fparingly folublein water, and largely fo in oils, ether, and alco-hol. It combines both with acids and alkaliesinto imperfect foaps; it is very inflammable,and like the vegetable effential oils, may be feton fire by ftrong nitrous acid. If expofed evento the light it is partly decompofed, lofing itstranfparency, and becoming of a brown colour.It was formerly employed in medicine, but isnow wholly difufed.
Oil Mineral , or Petroleum. See Bitumen.
Oil of Wine. See Ether.
Oil of Vitriol. See Sulphuric Acid .
Oil of Tartar , per deliquium. See Car-bonat of Potash.
Oisanite. See Anatase.
Olefiant Gas. See Carburetted Hy-drogen.
Olive Oil . See Oils Vegetable Fixed.
OLIVIN . Volcanic Chryfolite. Peridot granu-liforme. Hauy.
Its colour is between afparagus and olive-green, but by expofure to the weather it be-comes yellowifh-brown. It occurs imbeddedor difieminated in bafalt, and fometimes inrounded pieces, alfo, though rarely, in fmallrectangular prifms. Internally its luftre variesbetween glimmering and fhining, and is vitreouspaflmg into refinous. Its fracture is more orlefs imperfectly conchoidal, palling into fplinteryand uneven. Its fragments are indeterminately(harp-edged. When in large pieces it exhibitsgranular diftindb concretions. It varies fromfemi-tranfparent to tranflucent. It is hard,but lefs fo than quartz ; it is brittle and eafilyfrangible. Sp. gr. 3.22 to 3.26. It is nearlyinfufible before the blow-pipe without addition ;and lofes its colour by digeltion in nitric acid.It has been analyfed by Klaproth with the fol-lowing refult:
48. to 52 Silex
37. — 38.5 Magnefia
0.25 — 0.20 Lime
12.5 — 12. Oxyd of iron
Lofs
97-75
2.25
100.
It occurs only in proper bafalt, and not inany other rock of trap formation. It > s ver yeafy of decompofition, to which circumftancemany bafalts owe their porous texture- It isfound very abundantly in Bohemia and otherparts of Germany, alfo in France, Britain ,Norway, and Sweden .
Of this mineral there are the four followingfubfpecies.
1. Subf. Noble Opal. Edler Opal, Wern.L’Opale noble. Broch.
Its colour is milk-white, or pearl-grey, andwhen held between the eye and the light is palerofe-red and wine-yellow: by reflected light itexhibits, as its pofition is flightly varied, theclearelt and molt beautiful iridefeent colours,particularly verdegris and emerald-green, gold-yellow, fire-red, bright-blue, and pearl-grey ; allthefe tints are often exhibited in the fame fpeci-men, but when only one colour is prefent thegreen is the molt efteemed. It occurs in mafs,dilfeminatedand in fmall veins. Its internal luftreis vitreous and brightly fhining. Its fractureis conchoidal. Its fragments are indeterminate(harp-edged. It is moderately tranflucent paf-fing to femi-tranfparent, and even tranfparent.It is moderately hard and very brittle. Sp. gr.2.114..
Before the blow-pipe it crackles and flies,and becomes of an opake white, but does notmelt. It is compofed, according to Klaproth ,of
90 Silex10 Water .
ioo
It occurs difieminated in clay porphyry. Itwas formerly found near Freyberg in Saxony ,but is at prefent met with only at Cfcherwen-itza, near Kafchau, in Upper Hungary.
It is the molt beautiful of the gems, and! among the Eaftern nations is in particularlyhigh eflimation. In Europe its value thoughconfiderable, is inferior to that of the other gemsfrom its foftnefs and liability to fplit on a fuddenchange of temperature. It is chiefly employed innecklaces, and ear-rings, and finger-rings.
The moll beautiful varieties are called orientalopal, but there is no fatisfabtory evidence tofliew that this fubftance has ever really beenfound in the Eafl,
Some varieties of opal, from expofur 3 to the2 A 2