Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/148

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how much the colour of bodies depends upon the mechani- cal fituation of their parts.

Oil of flone. In the manufacture of the Cliinefe porcelain, they ufe a liquid matter of a white colour, which they cull by this name, on which their great myftery of finifhing their work depends \ yet this has been lefs enquired into by the imitators of that ware in Europe, than many other ar- ticles of lefs confequence. The ftone of which this oil is made, is of the fame degree of hardnefs with that which the petunfe is prepared of. They procure it from quarries, and chufe fuch as is of a good white colour, and has many dark green fpots in it.

Thefe fpots are of the colour of the leaves of cyprefs. Sometimes a ftone is chofen which has a brown ground, variegated with fpots and blotches of a reddifti colour. They firft carefully wafti this ftone; then laying it in a clean place, they break it to pieces with iron inftru- ments, and afterwards grind thefe to a perfectly fine, and impalpable powder, by rubbing them in large mortars, with peftles of ftone faced with iron, and turned either by the labourers, or by water. When the whole is thus reduced to a fine powder, they throw it into a veflel of water, and ftirring it brifldy about, they let the coarfer part fubfide to the bottom, and there fwims a fine thick matter like cream, for two or three inches depth, on the furface. This they carefully fcum off, and putting it into another veffel of clear water, they let it throw down any coarfe matter it may yet contain ; and, finally, taking off the thick furface again, they mix this with fome frefti water in an- other veflel, and leave it to fubfide ; then pouring off the clear water, they take out the remainder at the bottom of the veflel, which is perfectly fine, • and refembles a thick cream. To every hundred pounds of this, they add one pound of a fubftance of the .nature of which we are not yet perfectly informed. It is faid to be a mineral re- fembling alum. They calcine this firft, and then beat it to a fine powder, and this being added to the cream, or oil, as it is called, fervrfs to keep it always in the fame li- quid ftate. This fubftance, when finifhed in this manner, has very little title to the name of an oil} it Js rather a

■ ■■ varnifh, and is always ufed in mixture with another var- nish* which is called at this time fern oil, and ufed to be

. called lime oil; it is prepared in the fame manner with the other after burning, See Ffrn-o//.

Oil of vitriol. See the article Vitriol;

Oil oftvax. See the article Wax.

O i l bladders, in plants. — It is a difcovcry, partly of Mr.Tour- nefort, and partly of Mr. Geoffroy, that the effential cih of all plants are contained in their perfect and pure ftate, in the plants themfelves, while growing ; they have, in different ve- getables, different parts afligned for their reception ; in fome the flower ; in others, the flower cup only, as in rofemary, and the like ; in others the bark; in fome, the parenchyma of the root ; and in others, the wood.

Wherever they are lodged, they are always contained in thefe vcficles, or capfules, which require only to be broken, or burft open, in order to fct them loofe; this is ef- fected by the boiling water in the common diftillations, and the natural fubtilty and lightnefs of the oil then makes it ea- fily rife in vapour, and its oleaginous quality rendering it in- capable of mixing with the water, it fwims at the top, and is eafily feparated.

Thefe veftcles are moft ufually placed in, or about, the flower, and it is in many plants a very eafy matter to trace them to their feveral places of principal refidence. Mr. Tournefort has obferved that thefe oil bladders in the fraxinella taking their origin at the root, thence afcended to the fruit or covering of the feeds. This plant has a very remarkable variety in its oil in the different parts. The flower is fweet-fcented; the veficles placed along the ftalk contain a very fliarp and acrid oil, fomewhat refembling the effence of lemons, wnen kept a long time. The leaves have fcarce any fenfible odour, but the root has a fcent peculiar to itfelf, and different from that of all the reft of the plant. Geoffrey, ap. Memoires Acad, Paris 1 7 2 1.

The effential oil of cinnamon has alfo a difference of the fame kind, which is owing to the different fluid contained in the veficles in different parts ; the bark of the root yields an oil, which, after keeping fome time, always yields a pure and genuine camphor; whereas the oil of the bark from other parts of the tree has nothing of this property. It is alfo eafy toobfervc, that the capfules of certain fruits have more odour than the fruits themfelves. The coat which co- vers the piftachia nut, contains much more effential oil than it? kernel ; and the bark which furrounds the amomum, and cardamoms, has much more finell than the kernel of the feed. The firil bark of the nutmeg is well known to have a more fragrant fmell than the nutmeg itfelf. being the mace ; yet there is, within this, another covering of that fruit, which has no fcent at all.

Moft of the feeds of the umbelliferous plants, which ufually pafs for aromatics of the firft and moft eminent kind, have, in reality, no fine 11 at all hi themfelves ; the oil bladders which

yield their virtues being placed in their outer covering, the kernel within the feed ufually containing a fat oil of the olive or almond kind, and wholly different from the effential. The oil of anifeed made by expreffion is fluid, of a green co- lour, and of a fatty nature, but is highly fcented with the oil which it has taken with it from the covering ; on the con- trary, the effential oil procured from the fame feed by diftil- lation, is thin, not fatty, of a much more clear and penetrat- ing fcent, of a pale colour, with no admixture of greennefs, and readily concretes into a mafs like butter on external cold. In coriander feed, the kernel carefully feparated from its mem- branes has no fmell, and contains only a fatty, oil, like that of the olive, while the veficles are all ranged in the rind, and therefore in that alone is the aromatic fmell. Thefe veficles in the flrin of fruits are the occafion of the fweet flavour of many ; as of the ftrawberry, rafpberry, and the like ; in thefe they are too fmall to be diftinguifiiable, while, in the thicker rinds of the orange and lemon kind they are eafily difcerned, and, on the leaft prefliire, burft, and throw out their contents, which make what we call the ze/l, and which is feparated in this manner in greateft quantity in hot countries, and where there are plenty of the fruit, is preferved under the name of bcrgam-4, and other effences of a like kind. There is, however, no fubject in the vegetable world in which thefe veficles are more plainly perceived than in the berries of the juniper, a fruit very well known, aud in conftant ufe with us. Thefe berries are at firft green, afterwards of a reddifh brown, and finally black, and they are two years in coming to maturity. The berry is formed at the top, in the manner of a bud of the rofe, or piony, which is juft ready to open ; and it fbmetirnes does open into four or five parts, according to the number of feeds it contains. It contains, befide thefe* an acrid, a fweet, and an aromatic juice, which leaves a fcnnble bitternefs behind it in the mouth.

In order to account for thefe fo different taftes in the' fame fruit, Mr. Geoffroy attributes its acrid quality to tie fkin, or hulk, and in fome berries, indeed, to the unripenefs of the whole, for want of a due heat of the weather ; the fweet talte is from the juice of the fruits which is properly a faccharinc or mellcous liquor, and the aromatic flavour is wholly owino- to the oil which is lodged in the veficles expanded over the whole inner fubftance of the fruit. Thefe fubftances are in- deed fo detcrminately marked in the berry, that it is pofliblc to fcparate them ; but thefe are not all the differences of tafte and matter in this little fruit, every kernel of which has in it five or fix veficles, lodged each in a peculiar, and appropriated cavity, in the external part of the feed. Thefe are eafily fepa- rated from the reft of the fruit, and, when that is dry, are found to contain an abfolute refin ; they are of a dufky co- lour, and each drop of refin in them is of an oval figure, and is very bitter to the tafte ; hence therefore is the remark- able bitternefs that affects the mouth, when the berry is tho- roughly broken by the teeth.

The knowledge of this may be of great ufe to the apothecary in his medicinal preparations of juniper berries of feveral kinds. When he wants only the extract, which is the mel- leous juice of the berries alone, they ought not to be ftamped or bruifed, to diflodge this refin, which has no bufinef's there, but ibould be only boiled, and the decoction infpiffated. But when the effential oil is required, the whole berry muft be bruifed, and that in a thorough manner, that all its kernels be broken. The cells, or veficles of oil, now indurated into a refin, burft open, and the whole difperfed in fuch a manner that water may affect it. Memoires Acad. Par. 1 721, In order to difcover what are the parts of a plant in which the ejj'ential oil moft abounds, it will be proper to examine what part yields the moft fmell ; and this will be found diffe- rent in different plants. In fome the feat of odour occupies the whole body of the flower; in others only fome particular part of the flower has it, and in fome ; it is principally lodged in the covering ; and in fome it is difperfed evtry way, thro* the body of the fruit. In fome inftances it is found lodged in other parts of the plant, and fometimes it is equally ftron£ in every part of it.

This principle of the effential cils varies alfo not only in re- gard to the place where it refides, but in the different fcent? it yields in different places ; or in the fame parts of the plant in different feafons. Thus the leaves, ftalks, and roots of the violet have no fmell, while the flowers have a great deal. All parts of the jafmine tree are without fcent in like manner, except the flowers, which arc very fweet. And the cafe is the fame in the tuberofe, the jonquillc, &c. in all thefe plants the efhi ial oil is very volatile, and in very little quan- tity ; their refervoirs are fo obfeure, that we cannot find the leaft tr.ice of them; and, inftead of an effential oil, all that can be obtained from them by diiHILition, is a fweet-fcented water, and even this lofes its fmell in a very little time. Of this foit alfo are the flowers of the lime tree, the lilly, and the clove julyflower; in thefe there is no part fcented but the extremities of the flowers, and then not till they are tho- roughly opened ; and, for this rcafou, if we would obtain a fweet fcented water from thefe, we are to uCc no part bur the flower, and that not til! perfectly expanded on the ex,

tremities