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Kt of fait in this procefs hot only encrcafed the natural fer- ment in the liquor, which is neceflary to the reparation of the oil, but that it alfo attenuated the particles of the oil in the feed, and made them much more eafily feparate them- felves, and rife with the heat, than they would _ otherwise have done. What not a little confirms this opinion alio, is the known experiment of rendering camphor liquid by means of acid fpirits. Mem. Acad. Par. 1 700. The modern chemifts have not looked upon the oil as one of the genuine principles of the plant it is drawn from, as has been the cuftom among writers of earlier date. They know that thefe oils are themfelvcs mixed bodies, and ca- pable of another analyfis, by which they are feparated into fait phlegm, and earth. Thefe three principles conftitute 3.}] oils ; but thefe differ greatly in different plants, and that perhaps, more from their manner of admixture than from their different quantities. That bodies of the fame kinds, variously mixed, may have thefe different appearances, is in nothing more evident, than in the mixture of quickfilvcr, and fulphur; if this be done only by grinding, the refult is a black powder; but, if they are fublimed together, they form a red, bright, compact body, formed of feveral con- geries of ftriie, called cinnabar. And, in the fame manner, the fame principles, phlegm, fait, and earth, may, accord- ing to the different manner of their mixture, form oils of very different natures one from another : nor is this all the fource of their difference, for, by our own management of them, we make them appear in very different forms, in the fame fpecies. Thus the oil naturally contained in a feed is produced from it, according to our different management, in three different forms. To inftance this : in the oil of anifeed if we heat and prefs this feed, we draw what is called an oil by expreffion, a fattifti fubftance, of the nature of oil of olives, almonds, and the like, and with only a mo- derately acrid tafte ; if we diftil it with water in an alem- fcic, we obtain the ejjential oil) or common oil of anifeed ; and if we diftil it dry by the retort, we obtain a fetid, thick, and empyrcumatic oil. The oil in the feed is evidently the fame, but it puts on thefe forms according to our pleafurc. Mem. Acad. Par. 1707.

Exprejfed oils, as well as balfams, and refins, when applied to the animal bodies, increafe putrefaction, and are the moft effectual fuppurants and mcarners. Monro, in Med. Eff. Ed. Vol. 5. Art. 24.

It may be eafily fecn from this, that little knowledge of the nature of the oils of plants is to be obtained from the com- mon analyfes; the way to truly underftand them, is to mix them with different fubftances, and to digeft them alone, and, in mixture with thofe fubftances, to try the different events. Mr. Geoffroy made a multitude of experiments on the oil of thyme with this view, and, from the events of thefe, has made feveral very fair deductions as to the caufes of the different colours of the flowers of plants.

A large quantity of dried thyme being diftilled with water, in earthen cucurbits, yielded him a pure deep yellow oil ; this he reclined by diftilling it again, with a great quantity of water ; and, by this means, obtained a pale yellow oil, which he ufed for all his experiments.

He divided the oil into feveral parcels, and with one he mixed diftilled vinegar; with others the acid fpirits of nitre, vitriol, and fea fait, fcverally reduced by a mixture of water to the ftrength of common vinegar, which is about the ftandard of the acidity found in the juices of plants. All thefe mixtures were fet in digeftion, and the oil became of a deep faffron co- lour. If the acid fpirits had been ufed in their native ftrength, they would have immediately burnt up the oil. Another par- cel of the c;7 was digefted with fpirit of fal armoniac made with lime. This pafled all the degrees of yellow, of red, and of pur- ple ; and finally became of a true violet-blue. Spirit of urine, and that of fal armoniac with fait of tartar, produced alfo the fame changes, except that the degrees of colour differed. On the contrary, the fixed alkali of oil of tartar digefted with the fame oil, only changed it to a deep brown. OH of tartar added to the oil with fpiris of fal armoniac, when it is yet only of a fine purple, turns it to a deep blue ; and diftilled vinegar mixed with it in the fame ftatc, turns it to a deep blackifh colour ; this mixed with fpirit of wine, tinges it to a greyifh colour ; and a little oil of tartar being then added, it becomes green, and that colour does not go off. If diftilled vinegar be added to this mixture, it deftroys at once the alkali, and the green colour which was owing to it, and gives the oil its ori- ginal yellowtfh hue.

OH of thyme which has been made blue by means of oil of tartar, being mixed with fpirit of wine, the whole appears grey, and, on adding more oil of tartar, it becomes blue ; di- ftilled vinegar takes off this blue colour, leaving the liquor redifh ; and more oil of tartar makes it blue again. It appears from thefe laft experiments, that oil of tartar aefs differently on the oil of thyme, according to its different ftate ; turning it blue or green, according as it is rarified, or concen- trated. It Ihould feem alfo, that the fpirit of wine contained a latent acid, fincc all its effects on the oil of thyme ate over- powered by a mixture of oil of tartar.

Mr. Geoffroy was at the pairts of trying this whole courfe of experiments on feveral other ejjential oils, as thofe of la- vender, juniper, turpentine, and others', but they did not pro- duce the fame effect on all. Hence it appears that there is' great difference between the ejjential oils of different plants, hi regard to the effects of other bodies on them. The fame courfe of experiments he tried On other oils, not of the vegetable kingdom, and, among thefe, he found hone; except that of yellow amber, which at all approached to the nature of oil of thyme.

This oil, mixed and digefted with fpirit of fal armoniac, ac- quires a purple colour ; oil of tartar mixed with this, does not change its colour, but when fpirit of wine is added to this mixture, the oil of tartar becomes blue, while the oil of am- ber continues of its purple colour.

There feems to be a certain degree of denfity neceflary in ah oil to render it capable of all the changes of colours ; this deh- fity feems perfectly hit in the oils of tin me and amber, and all the changes of the colours that arc given to them, feem only the effect of rarifying, orcondenfing them. If they are rarified to the utmoft, by being mixed with fpirit of wine, they be- come colourlefs and tranfparent ; and, if condenfed in the higheft degree, as in the experiment with the diftilled vinegar, they become blackifh. Thefe are the two extremes, and all the other colours are but decrees and fliades approaching to, or receding from, one or the other of thefe. The oils naturally niore rarified, fuch as oil of turpentine, and the like, have na- turally no colour, nur can receive any from thefe procefles ; only if the mineral acids be mixed with them in their full ftrength, they burn them up to a fort of refin, and, finally, to a black mafs, rcfcmbling charcoal. But, perhaps, future attempts may teach us an art of fo modifying the particles of thefe oils as to render them fufceptible of all the changes that oil of thyme is.

The colours of the leaves and flowers of plants are of the fame nature with thofe produced in the oil of thyme, in thofe pro- ceffes ; and, as chemifts, in general, agree that al! colours are the refult of fulphurs differently acted upon by falts, perhaps, great light may be given into their nature and origin from thefe experiments. Mem. Acad. Par. 1707. See the article Colours o/Tlowers.

The efjential oils of vegetables may be divided into two claffes, according to their different fpecific gravities, fome floating upoji water, and others readily finking to the bottom thereof. Thus the ejfent'ial oils of cloves, cinnamon; and faflitfras, rea- dily fink, but the oils of lavender, marjoram, mint, &c. fvvimi upon the water. The lighteft of all the e£ential oils, is, per- haps, that of citron peels, which even floats on fpirit of wine ; and the heavieft of them feems the oil of falfafras. For the obtaining the full quantity of the more ponderous oils from cinnamon, cloves, falfafras, &c. it is proper, 1. To reduce the fubjects to fine powder. 2. To digeft this pow- der for fome days in a warm place, with thrice its quantity of foft river water made very faline with the addition of fea fait, or fharp with oil of vitriol. 3. To ufe the deco£tion left in the ftill, inftead of common water, for a frefh digeftion. 4. To ufe alfo, for the fame purpofe, the water of the fecond running, after it has been cleared of its oil. 5. Not to di- ftil from too large a quantity of the fubject. at once. 6. To leave a confidcrable part of the ftill empty. 7. To ufe a brifk fire, or a ftrong boiling heat at firft, but to flacken it a little afterwards. 8. To have a low ftill-head, with a pro- per internal lodge, and current, leading to the n'ofe of the worm. And, 9. To cohobate the water,- or pour it back up- on the matter in the ftill, after feparating its oil, and repeating this once or twice more. Shaw's Lectures, p. 141. Ejjential oils, when applied to the human body, ftimnlate, e- lode, refift putrefact ion ; and, mixed with the blood, raife fome degree of fever. Med. Eff. Edinb. Vol. 5. Art. 24. The vegetable world affords vaft variety of 'ejjential oils, moft of them very odorous, and of great virtues. The ejjential tils of many of the plants ufuajly diftilled have their peculiar refervoirs in the plant. Mr. Geoffroy per- ceiving this, determined to fearch for then! through the diffe- lent parts of the plants, and enquire into the rcafon of feveral precautions which are always found neceflary, in order to fuc- ceed inthediftillation of them. See QiL-bladders, infra. From what has been laid above, it appears, that the oils in common ufe are of three different kinds. 7'he firft are mere oily, or fatty bodies, extracted cither by expreffion, or by de- coction : of the firft kind are thofe of almonds,- linked* nuts, olives, and the like ; and of the other, that of bay berries, arid the like, which are procured by boiling the fubftance, on which the oil collects itfelf on the top of the water. The fecond kind of oils are thofe extracted from vegetables, by common diftilbtion by the alembic, with the help of wa- ter ; thefe contain the oily and volatile part of the plant, and carry, by way of pre-eminence the name of ejjential, or tiherial oil

THe third fort of oils are the fetid ones, which are, in like manner, produced by diftillatioii, but by means of a different diftillation in a naked fire* and without the aftifhnce of water.