OIL

107

O I L

mixture ought to stand for several days, when the brine will sepa-fater rom the oil, which will be greatly improved both in smelland colour. Should a greater degree of purity be required, theproportion of pearl-ashes ought to be increased ; and the periodintervening between the addition of the salt and water prolonged :lastly, if the same operation be repeated, and the quantity of in-gredients be reduced one-half each time, the oil may be broughtto a very light colour, and its smell rendered equally sweet as thecommon spermaceti. By this treatment, the coarsest cod, orseal-oil, may be made to burn ; and, though it be too putrid for 1use, it may be so far corrected by the first process, as to be in allrespects equal to that commonly sold. Beside its utility forlamps, animal-oil possesses a valuable property which deserves at-tention. If one drop be laid on a bug, fly, wasp, or earwig, itwill cause the immediate death of those troublesome vermin; and,even when it is damaged, it may, according to Mr. Bucknall, beadvantageously applied to fruit-trees, about a month afterthey have been washed with soap-suds, in order to eradicatemoss.

3. Oils, Vegetable, are procured either by expression, infu-sion, or distillation. 1. Those by expression are obtained fromthe seed, leaves, fruit, and bark of plants; which, being pound-ed in a mortar, the oil is forced out by means of a press, withoutthe aid of heat. Such are the oils of olives, almonds, beech-mast,rape, and linseed. 2. In essential oils procured by infusion ordecoction, the virtues of some particular plant are extracted. Tothis kind belong the oils of roses, chamomile, Sec. which, how-ever, ought to be boiled only so long as there remains any aque-ous moisture; lor otherwise they will become black. 3. Otheressential oils of vegetables are prepared by distillation, only fromthose plants, or parts of plants, that possess a considerable odour.They contain the fragranc e, warmth, pungency, and often the ac-tive powers of the substance, from which they are drawn ; whencethey have received the name of essences, or essential oils; for in-stance, those of cloves, cinnamon, &c. Oils in general, fromtheir peculiar properties, are distinguished in two kinds, fixed orfat oils, and volatilfc or essential oils.

Oil, which is of such extensive utility in the arts, was known at avery remote period. It is mentioned in Genesis, and during thetime of Abraham was even used in lamps. The olive was veryearly cultivated, and oil extracted from it in Egypt. Cecrops brought it from Sais, a town in Lower Egypt , where it had beencultivated from time immemorial, and taught the Athenians toextract oil from it. In this manner the use of oil became knownin Europe . But the Greeks seem to' have been ignorant of themethod of procuring light by means of lamps till, after the siege ofTroy ; at least Iiomer never mentions them, and constantly de-scribes his heroes as lighted by torches of wood. There are twoclasses of oils exceedingly different from each other; namely, fix-ed oils and volatile oils. Fixed or fat oils are distinguishedby the following characters: 1. Liquid, or easily becoming sowhen exposed to a gentle heat. 2. An unctuous feel. 3. Verycombustible. 4. A mild taste. 5. Boiling point not under 600°.6. Insoluble in water and alcohol. 7. Leave a greasy stain uponpaper. Those oils are numerous; and are obtained partly fromanimals and partly from vegetables, by simple expression. As thewhale-oil or train-oil obtained from the blubber of the whale ;olivc-oil, obtained from the fruit of the olive; linseed-oil and al-mond-oil, obtained from linseed and almond-kernels. Fixed oilsmay also be extracted from poppy-seeds, hemp-seeds, beech-mast,and many other vegetable substances. It deserves attention, thatthe only part of vegetables in which fixed oils are found is theseeds of bieotyledinous plants. All these oils differ from each•other in several particulars, but they also possess many particularsin common. Whether the oily principle in all the fixed oils isthe same, and whether they owe their differences to accidental in-gredients, is not yet completely ascertained, as no proper analysishas hitherto been made; but it is not improbable, as all the oilshitherto tried have been found to yield the same products. Fixedoils are considered at present as composed of hydrogen and car-bon. Lavoisier analysed olive-oil by burning a given portion ofit in oxygen-gas, by means of a particular apparatus. The quan-tity of charcoal in 44.50 grains ot' carbonic acid gas is 12.47 grains;and the quantity of hydrogen ill 22.15 grains of water is 3.32trains; both of which, when taken together, amount to 15.79

grains, and found from the analysis, that 15.79 grains of olive-offare composed of

12.47 carbon3.32 hydrogen,

79 carbon21 hydrogen

100 .

This however can only be considered as a very imperfect approxi-mation towards the truth. Fixed oil, when in the state of vapour,takes fire on the approach of an ignited body, and burns with ayellowish-white flame. It is upon this principle that candles andlamps burn. The tallow or oil is first converted into the state ofvapour in the wick; it then takes fire, and supplies a sufficient 'quantity of heat to convert more oil into vapour ; and this processgoes on while any oil remains. When exposed to the action ofcold, fixed oils lose their fluidity, and are converted into ice ; butthis change varies exceedingly in different oils. When fixed oilsare exposed to the open air or to oxygen gas, they undergo dif-ferent changes according to the nature of the oil: 1. Some of themdry altogether, without losing their transparency, when thin layersof them are exposed to the atmosphere. These are distinguishedby the name of drying oils, and are employed by painters. Lin-seed-oil, nut-oil, poppy-oil, and hempseed-oil, possess this pro-perty ; but linseed-oil is almost the only one of these liquids em-ployed in this country as a drying oil. The cause of this pecu-liarity has not been completely investigated ; but it is well knownthat these oils possess the drying quality at first but imperfectly.Before they can be employed by painters, they must be boiledwith a little litharge. During this operation the litharge is partlyreduced to the metallic state. Hence it has been conjecturedthat drying oils owe their peculiar properties to the action ofoxygen ; which is supposed either to constitute one of their com-ponent parts, or to convert them into drying oils by diminishingtheir hydrogen. 2. Other fixed oils, when exposed to theatmosphere, gradually become thick, opaque, and white, andassumes an appearance very much resembling wax or tallow.These have been distinguished by the term fat oils. Olive oil, oilof sweet almonds, of rape-seed, and of ben, belong to this class.3. Both these classes of oils, when exposed in considerable quan-tity to the action of the atmosphere, undergo another change, wellknown under the name of rancidity. But the fat oils become ran-cid much more readily than the drying oils. Rancid oils are thick,have usually a brown colour, convert vegetable blues to red, andhave the smell and taste of sebacic acid. During the changewhich they undergo, some drops of water also appear on their sur-face. The rancidity of oils then is owing to the formation of aquantity of acid in them. This, together with the water, is evi-dently the consequence of a partial decomposition. Fixed oilsreadily dissolve sulphur when assisted by heat. The solution as-sumes a reddish colour. When distilled, there comes over a great;quantity of sulphurated-hydrogen gas. When the solution is al-lowed to cool, the sulphur is deposited in crystals. By this pro-cess Pelletier obtained sulphur in regular octahedrons. Charcoalhas no sensible action on fixed oils; but when they are filteredthrough charcoal-powder, they are rendered purer, the char-coal retaining their impurities. Neither hvdogen nor azotic gashas any action on fixed oils. Fixed oils have scarcely any actionupon metals ; but they combine with several metallic oxides, andform compounds known by the name of plasters. See Plaster .They combine likewise with alkalies and earths, and form withthem compounds called soaps. The fat oils enter into these com-binations much more readily than the drying oils. See Soap.Fixed oils absorb nitrous gas in considerable quantities, and at thesame time become much thicker and specifically heavier than be-fore.

The affinities of fixed oils are as follow:

Lime,

Barytes,

Fixed alkalies.Magnesia,

Ammonia,

Oxide of mercury,

Other metallic dxidet,

Alumina.

The