in natural history, an unctuous inflammable substance, drawn from several natural bodies, as animal and vegetable substances.

Animal oils are their fats, which are originally vegetable oils: all animal substances yield them, together with their volatile salts, in distillation.

Vegetable oils are obtained by expression, infusion, and distillation.

The oils by expression are obtained from the seed, leaves, fruit, and bark of plants; thus, the seed of mustard, and of the sunflower, almonds, nuts, beechmast, &c. afford a copious oil by expression; and the leaves of rosemary, mint, rue, wormwood, thyme, sage, &c. the berries of juniper, olives, Indian cloves, nutmeg, mace, &c. the barks of cinnamon, sassafras, and clove, yield a considerable proportion of essential oil by distillation. The method of procuring oils by expression is very simple: thus, if either sweet or bitter almonds, that are fresh, be pounded in a mortar, the oil may be forced out with a press, not heated: and in the same manner should the oil be pressed from linseed and mustard. The avoiding the use of heat in preparing these oils, intended for internal medicinal use, is of great importance, as heat gives them a very prejudicial rancidity.

This method holds of all those vegetable matters that contain a copious oil, in a loose manner, or in certain cavities or receptacles; the sides whereof being broke, or squeezed, makes them let go the oil they contain: and thus the zest or oil of lemon-peel, orange-peel, citron-peel, &c. may be readily obtained by pressure, without the use of fire. But how far this method of obtaining oils may be applied to advantage, seems not hitherto considered. It has been commonly applied to olives, almonds, linseed, rape-seed, beechnuts, ben-nuts, walnuts, bay-berries, mace, nutmeg, &c. but not, that we know of, to juniper-berries, cashew-nuts, Indian cloves, pine apples, and many other substances that might be enumerated, both of foreign and domestic growth. It has, however, been of late successfully applied to mustard-seed, so as to extract a curious gold-coloured oil, leaving a cake behind, fit for making the common table-mustard.

Certain dry matters, as well as moist ones, may be made to afford oils by expression, by grinding them into a meal, which being suspended to receive the vapour of boiling water, will thus be moistened so as to afford an oil in the same manner as almonds; and thus an oil may be procured from linseed, hemp-seed, lettuce-seed, white-poppy seed, &c.

As to the treatment of oils obtained by expression, they should be suffered to deurate themselves by standing in a moderately cool place, to separate from their water, and deposit their feces; from both which they ought to be carefully freed. And if they are not thus rendered sufficiently pure, they may be washed well with fresh water, then thoroughly separated from it again by the separating-glares, whereby they will be rendered bright and clear.

The next class of oils are those made by infusion, or decoction, wherein the virtues of some herb or flower is drawn out in the oil; as the oils of roses, chamomile, hypericum, alder, &c. However, these require to be differently treated: thus, for the scented flowers, particularly roses, inflorescence does best; because much boiling would exhale their more fragrant parts: but oils impregnated with green herbs, as those of chamomile and alder, require long boiling, before they receive the green colour desired. And, in general, no oils will bear to be boiled any longer than there remains some aqueous humidity, without turning black.

There are many compound oils prepared in the same manner, viz. by boiling and inflorescence, and then straining off the oil for use.

The same contrivance has likewise its use in making essences for the service of the perfumer; not only where essential oils cannot be well obtained in sufficient quantities, but also where they are too dear. The essential oil of jasmine-flowers, honey-suckles, sweet-briar, damask-roes, lilies of the valley, &c. are either extremely dear, or scarcely obtainable by distillation; and, in some of them, the odorous matter is so subtile, as almost to be lost in the operation. But if these flowers be barely infused in fine oil of nuts, or oil of ben, drawn without heat, and kept in a cool place, their subtile odorous matter will thus pass into the oil, and richly impregnate it with their flavour. And these essences may be rendered still more perfect by straining off the oil at first put on, and letting it stand again, without heat, upon fresh flowers; repeating the operation twice or thrice.

Oils or fats may likewise be obtained, by boiling and expression, from certain animal-substances; for the membranes which contain the fat, being chopped small, and set in a pan over the fire, become fit for the canvas bag, and, by pressure, afford a large quantity of fat; as we see in the art of chandlery, which thus extracting the oily matter, leaves a cake behind, commonly called grease.

As to the essential oils of vegetables, they are obtained by distillation with an alembic and a large refrigeratory. Water must be added to the materials, in sufficient quantity to prevent their burning; and they should be macerated or digested in that water, a little time before distillation. The oil comes over with the water; and either swims on the top, or sinks to the bottom, according as it is specifically heavier or lighter than water.

This process is applicable to the distilling of the essential oils from flowers, leaves, barks, roots, woods, gums, and balsams, with a slight alteration of circumstances, as by longer digestion, brisker distillation, &c., according to the tenacity and hardness of the subject, the ponderosity of the oil, &c.

Essential oils may be divided into two classes, according to their different specific gravities; some floating upon water, and others readily sinking to the bottom. Thus, the essential oils of cloves, cinnamon, and sassafras, readily sink; whereas those of lavender, marjoram, mint, &c. swim in water: the lightest of these essential oils is, perhaps, that of citron-peel, which even floats in spirit of wine; and the heaviest seems to be oil of sassafras.

For obtaining the full quantity of the more ponderous oils from cinnamon, cloves, sassafras, &c. it is proper to reduce the subjects to powder; to digest this powder for some days in a warm place, with thrice its quantity of soft river-water, made very saline by the addition of sea-salt, or sharp with oil of vitriol; to use the strained decoction, or liquor left behind in the still, instead of common water, for fresh digestion; to use for the same purpose the water of the second running, after being cleared of its oil; not to distil too large a quantity of these subjects at once; to leave a considerable part of the still, or about one fourth, empty; to use a brisk fire, or a strong boiling heat, at the first, but to slacken it afterwards; to have a low still-head, with a proper internal ledge and current leading to the nose of the worm; and, finally, to cohabitate the water, or pour back the liquor of the second running upon the matter in the still, repeating this once or twice.

The directions here laid down for obtaining the ponderous oils to advantage, are easily transferred to the obtaining of the lighter; so that we need not dwell particularly upon them. Many of the essential oils being dear, it is a very common practice to adulterate or debauch them several ways, so as to render them cheaper both to the seller and the buyer. These several ways seem reducible to three general kinds, each of which has its proper method of detection, viz. 1. With expressed oils. 2. With alcohol. And, 3. With cheaper essential oils.

If an essential oil be adulterated with an expressed oil, it is easy to discover the fraud; by adding a little spirit of wine to a few drops of the suspected essential oil, and shaking them together; for the spirit will dissolve all the oil that is essential, or procured by distillation, and leave all the expressed oil that was mixed with it, untouched.

If an essential oil be adulterated with alcohol, or rectified spirit of wine, it may be done in any proportion, up to that of an equal quantity, without being easily discoverable either by the smell or taste: the way to discover this fraud, is to drop a few drops of the oil into a glass of fair water; and if the oil be adulterated with spirit, the water will immediately turn milky, and, by continuing to shake the glass, the whole quantity of spirit will be absorbed by the water, and leave the oil pure at top.

Finally, if an essential oil be adulterated by a cheaper essential oil, this is commonly done very artfully: the method is to put fir-wood, turpentine, or oil of turpentine, into the still, along with the herbs to be distilled for their oil, such as rosemary, lavender, origanum, &c. and by this means, the oil of turpentine distilled from these ingredients, comes over in great quantity, and intimately blended with the oil of the genuine ingredient. The oils thus adulterated always discover themselves in time, by their own flavour being overpowered by the turpentine-smell: 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 the plant will fly off, and leave the naked turpentine-scent behind.

The virtues of oils being the same with those of the substances from whence they are obtained, may be learned under their several articles.

Method of Purifying Rancid Oils. See Chemistry, n° 497.