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, beechnuts, &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, saffron, 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 broken, 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 va-
pour 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 faces; 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-glass, 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 are 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 exhaust 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 infusion, 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-buckles, sweet-briar, damask-roses, lilies of the valley, &c. are either extremely dear, or scarcely obtainable by distillation; and, in some of them, the odorous matter is so subtle, 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.