O I L
O I L
It feems to follow from this account, that the ethereal, or ejjential oils are the volatile exalted ftdphurs of thofe plants ; and as fulphurs are the bafis of all fmells, it is eafy to be pre- fumed, that ail fweet-fcented plants have an ejfetttial «7in fome of their parts The balfams, and refills, which we find in many vegetables, feem to have their origin from the ejjential oils of the trees or plants they belong to, and which are ufu- ally found greatly to abound with them. Indeed, in the bal- famiferous trees, the liquor which flows out on wounding the trunk, may very properly be d'eemed a fon of ejjential oil of a harder confidence than the common ones: Of this nature are the balfam of Capivi, ofGilead,and the reft; and thefe very oils, farther indurated, furnifh us alfo with the fcented refins, fuch as benjamin, myrrh, and the like.
All ejjential oils become thick and refinous on being mixed with any foreign acid ; they alfo acquire the fame confidence of thcmfelves in long keeping, and that partly from the fame principle; their fubtler parts evaporating give room for the acid fait, which they naturally contain, to difplay its power; and being then in an over-proportion to the reft, it does not fail to act upon the whole in the fame manner that any other acid would. Thus the oil remains no longer what it was, but becomes a refinous fubftance, of a fmell refembling turpen- tine.
Chemiftry is not, however, without its refource on this occa- fion ; for this thickened oil being mixed with a large quantity of water, and diftilled in a balneum marise, a great part of it will come over with all its wonted fluidity, limpidity, and odour; but even this new diftilled oil is not exempted from the fame change again. This difadvantage in the keeping the ejjential oils diftilled in the common way, has occafioned Mr. Geoftroy to contrive another method of making them. This renders them more limpid and fluid than in the common way, and they are never fubject to grow thick; but the medium not being water, fo much of the oil cannot be procured from the fubject, becaufc fome is abforbed and ftained in the me- dium. The inftance this gentleman gives of his new method is in the eflence of citron, and is this :
A common alembic is to be filled with the thin pared, exter- nal rind3 of the fruit ; a fufficient quantity of fpirit of wine is to be poured on thefe, and the veiTel being well clofed, the whole is to Hand fome time in digeftion ; during this time, the fpirit charges itfelf with a vaft quantity of the ejjential oil of the bark, and being carefully diftilled in balneo maris, it brings over with it a great deal incorporated in itfelf, but, be- fide that, there is a large portion fwimming on the furface of the fpirit, and eafily feparable from it in the common way : this is as clear, pellucid, and thin, as the fpirit itfelf, and is, in all refpects, fuperior to the eftence made by any other means. The fpirit of wine may be faved for afecond opera- tion of the fame kind, and there will then be no lofs, from it; for being already fated with as much of the oil as it na- turally abforbs, it will take in no more by the fame means, and therefore may ferve for ever fo many new diftillations, without lofs. If all the oil be finally defired to be feparated from the fpirit, it may eafily be obtained by mixing the fpirit with a large quantity of water, as is done in making the oil of fpike. The oil will then fwim at top, and the fpirit may be feparated again from the water by diftillation. Some of the ejjential oils are fo abundantly charged with falts, that, in long keeping, they will actually depofit plain concretions of the faline kind. This is frequently ob- ferved in oil of turpentine, which, though ever fo well rectified, will yet affix long cryftals like needles to the fides of any veflel in which it has been long kept ; the oils of marjoram, and feverfew, do the fame; as alfo fcveral ■ of the others. AH the other oils alfo fufTer great changes in .keeping ; that of juniper becomes thick like turpentine, in a Ihort time ; and thofe of fage, and of rofemary, after fome keeping, are not diftingu tillable from one another. The age and ftate of the plant may alfo make great difference in the fmell of the oil-, and there have been many inftances of the oil of fage becoming wholly like camphor in keeping. The di- ftilled oils of fome plants vary alfo at times in their colour. The oil of rue, and that of wormwood, fometimes come over green, and fometimes brown ; the oil of chamomile in Italy is always blue, but Mr. Geoftroy fays, that he could never get a blue oil from the chamomile of France by a ample di- ftillation ; and as to thofe oils which fometimes are green, they all become of the common clear ycllowifli colour of the other oils, if rectified by difti.ling them over again in a bal- neum marine.
In dry feafons wormwood affords but little oil, accompanied with a bituminous matter, or a genuine refin ; and, in thefe feafons, if the young moots only are diftilled, the oil is almoft conftantly found to come over green ; but in moift years the wormwood yields a much larger quantity of oil, and it is then always brown. Wormwood which has grpwn on dry fandy foils is always more white, and woolly, on the leaves, and always affords an oii of fome tendency to a greeniih colour ;. while, on the contrary, fuch as grows in rich moift ground, has larger, and lefs woolly leaves, and yields a brown oil. The feafon, and degree of heat, added to this difference of foil, may alfo make greater changes than could be imagined
in the oils of plants. Mr. Geoffroy affures us, that he once made, in a dry feafon, from a quantity of thyme which had grown in a dry foil, an oil as pure, as limpid, and as fweet and high-fcented, as that of the hotter countries, whereas the oil of thyme made in France is ufually much poorer, and of an obfeure colour, and coarfe earthy icent. The oils of plants have not always the fame taftes with the plant they are diftilled from, or, at leaft, not in the fame de- gree. Nothing is more bitter than wormwood, yet the cil of wormwood has no remarkable bitternefs. Anile, which is of a fweet tafte, yields, on the contrary, an oil infinitely more fweet than the feed ; and pepper, which is fo remarkably hot and acrid, affords an oil no way remarkable for its pungency. Thyme, which is in itfelf very acrid and pungent, conveys that property, in a yet greater degree, to its oil; there is, indeed, no ejjential oil fo acrid and firey as that of this plant. The fetid oils drawn in dry diftdlation by the retort, in an open fire, no way differ from thefe but by the damage the fire has done them, and may always be converted into thefe by repeated diftillations. N T ay, the very fat oils, fuch as that of almonds, may be attenuated fo far as to become as fine, and as fubtile, as the ejjential oils. This is to be done by means of quick lime; and fcveral repeated diftillations of this oil, or any other of a like kind, with frefh lime to every di- ftillation, will reduce it to be volatile, penetrating, and capable of being raifed, and diftilled, by means of water, which is the great reft of the ejjential oils. The bituminous, and fetid sils, may alfo, in the fame manner, be reduced by repeated diftil- lations, to the ftate of the ejjential ones, and to be equally fluid, and limpid, and equally penetrating. Memoirs Acad. Par. 172 1.
Adulterations of ejjential Oils. Many of the ejjential oils being dear, it is a very common practice to adulterate, or debafe them, feveral ways, fo as to render them cheaper both to the feller and the buyer. Thefe feveral ways feem reducible to three general kinds, each of which has its proper method of detection. Thefe three ways are, 1. To adulterate them with expreffed oils. 2. With alcohol. And, 3. With cheaper ej- jential oils.
If any ejjential oil be adulterated with an expreffed cil, it is eafy to difcover the fraud by adding a little (pint of wine to a few drops of the fufpected ejjential oil, and making them together ; for the fpirit will diftblve all the oil that is effential, or procured by diftillation, and leave all the expreffed oil that was mixed with it untouched.
If an ejjential oil be adulterated with alcohol, or rectified fpirit of wine, it may be done in any proportion, up to that of aa equal quantity, without being eafily difeoverable either by the fmell, or tafte. The way to difcover this fraud is to drop a few drops of the oil into a glafs of fair water, and if the oil be adulterated with fpirit, the water will immediately turn milky, and .by continuing to fhake the glafs, the whole quan- tity of fpirit will be abforbed by the water, and leave the oil pure at top. Shaw's Lectures, p. 1 45.
Finally if an ejjential oil be adulterated by a cheaper ejjential oil, this is commonly done very artfully : the method is to put fir-wood, turpentine, or oil of turpentine, into the ft ill, alon^ with the herbs to be diftilled for their oil, fuch as rofemary, lavender, origanum, &c. and, by this means, the oil of tur- pentine diftilled from thefe ingredients comes over in great quantity, and is intimately blended with the cil of the genuine ingredient. The oils thus adulterated always difcover thcm- felves in time, by their own flavour being over-powered by the turpentine fmell ; but the ready way to detect the fraud, is to drench a piece of rag, or paper, in the oil, and hold it before the fire ; for thus the grateful flavour of die plant will fly off, and leave the naked turpentine fcent behind.
FernOiL. Seethe article Fern.
Green Oil, oleum viride, a form of medicine prefcribed in the lateLondon pharmacopoeia, and made in the following manner. Take leaves of bay, rue, marjoram, fea wormwood, and' chamomile, each three ounces ; oil of olives a quart. Boil the herbs in the oil till they are crifp, and then ftrain off the oil, and when it has ftood for the faeces to i'ubfide, put it up- for ufe. Pemberto?i'$ London Difp. p 350.
Red Oil, in the porcelain manufacture, a name given to a pecu- liar colour ufed on the China ware, or to thofe pieces of the ware which are coloured with it. It is a very elegant orna- ment, and would be worthy our attempting to imitate in Eng- land, on our better forts- of wares. They do it in the follow- ing manner: they mix the red colour called tam-'an-hum, or' the copperas red,before defcribed under the article of Cclour- ing of china ; this they mix with oil of ftone, (Tee Oil cf Jlone. ) and with another cil, as they exprefs it, of the fame kind, made of a whitifh fort of pebble- or agate, found on the fhores of their rivers, and the place of which might pro- bably be fupplied with us by common chryftal. The powder is to be thoroughly mixed with thefe liquors, and the veflel dipt carefully into the mixture, or fome parts of it only co- vered with it in figures : after this, it is to be fez by to dry, and, when thoroughly dried, it is to be baked in the commoa way. The general method is that of covering the veffel all over, both infide and out with this red ; and it comes out of the molt bright and brilliant colour imaginable, but it will
