OBS C 175 ) 0IL
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OBSIDIAN. Iceland Agate.
Its ufual colour is pure black, whence itpaffes into greenilh and greyilh-black, black-ifh, bluilh, and greeaiin-grey, and fmoke-grey. It occurs in mafs, and in rough, roundifh,detached pieces. Internally its luftre is moreor lefs fliining and vitreous. Its frafture isperfect and large conchoidal. Its fragmentsare indeterminately angular and very fharp-edged. It is more or lefs tranfparent on theedges. It is hard, but eafily frangible. Sp. gr.2.34. Before the blow-pipe it melts withoutaddition into a greyifh-white opake fomewhatporous enamel. The Obfidian of Iceland hasbeen decompofed by Bergman, and more re-cently by Abildgaard, with the following refuits:
Berg. Abild.
69 — 74 Silex
22 — 2 Alumine
9 — 14 Oxyd of iron
100 90
The hardnefs and opake blacknefs of thismineral added to the high poliAi of which it iscapable, have caufed it to be employed forvarious kinds of ornaments. In Peru at thetime of its conqueft by the Spaniards, it wasufed for mirrors, and has been faAiioned inEurope into reflectors for telefcopes.
It occurs in large beds in the Lipari iflands,near Sicily , where it evidently paffes into pu-mice : it is met with near Tokay in Hungary ,in fmoke-coloured nodules in decompofinggranite and porphyry (thefe nodules fometimesgo by the name of Luxfapphire). It aboundsin Iceland , not only in the vicinity of Heclabut elfewere; alfo in the ifland Melos in theArchipelago ; near Grantola in the north ofItaly: in the ifland of Madagafcar, in Peru andSiberia .
It is one of the minerals whole origin is■warmly contefted by the Neptunians and Vol-canifts, many of the latter denominating itVitreous lava.
Various fubftances are included under thegeneral appellation oil, which however differ foeffentially in many particulars from each other,as to render it impoffible to comprehend themunder any proper fpecific definition: we havetherefore divided this clafs of bodies among thefour following articles..
Oils Vegetable TJnBuous .
Unftuous or fixed vegetable oil occurs prin-cipally in the cotyledons of certain feeds, andfometimes though rarely in the pulp or flelh of-fruits. In the former it is always mixed witha greater or fmaller proportion of fecula andmucilage, on which account thefe feeds when-rubbed in a mortar with cold water, form withit a white opake milk-like fluid called an emul--ficn; from which by long handing (or morefpeedily by the addition of an acid) the oil iepa-rates, riling to the furface in the form of athick cream.
There are two ways of extracting oil: thefirft and mod ufual is by means of preffure;the fecond is by immerfion in hot water; inthis latter cafe the oil feparates from the otheringredients with which it is naturally mixed,and rifes by the force of gravity to the.furface of the water, from which it is fkimmedoff.
Recently drawn oil however is more or lefs'impure on account of its containing a variableproportion of mucilage, fecula, and perhapsother fubftances: of thefe a part is alwaysdepofited by reft, efpecially if the contaft ofthe air is not entirely excluded; but anotherportion remains in permanent folution, and tothis that partial fpontaneous decompofition inoils called rancidity is principally owing.
Fixed oil at the common atmofpherical tem-perature is ufually in the ftate of a moderatelythick, but not ; properly fpeaking, vifeid fluid ;to the touch it is remarkably frnooth and unftu-ous; it has a mild fub-naufeous tafte, and apeculiar flavour, according to the vegetable fromwhich it is procured. Its colour when recenthas more or lefs of a greeni 111 tinge, which bykeeping becomes yellow, and in fome- inftancesorange-coloured, verging on red. Its fpecificgravity is ufually between that of alcohol and ■of water, hence it finks in the former and floats ;on the furface of the latter of thefe fluids.There is nothing in which oils differ from each -other fo much, as in the temperature at whichthey congeal % fome continue folid at the higheft'atmofpherical temperature, as palm oil, and thereft of the vegetable butters (as they are calledfrom this very circumftance) others requirebeing cooled down to . the freezing point ofwater, and others again are capable of en--during a much greater cold without folidifying,
Water feems to be incapable of combining.