Essential Oils, (called also volatile and ethereal,) are distinguished from fixed or fat oils, from the circumstance of their rising in distillation at temperatures below that of 320° Fahr, by themselves. They are mostly obtained from odoriferous vegetable substances, although some of the principles are found in animal matter. In different vegetables these oils are found variously lodged, - sometimes in the bark, as in cinnamon; sometimes in the root, as in the plant that yields the true camphor; sometimes in the wood, as in the cedar; sometimes in the leaves, as in mint, balm, etc.; sometimes in the flowers, as in the carnation and rose; sometimes in the rind, as in the orange and lemon; and, in a great variety of instances, in the seeds or fruit. The plants should be collected at the time their scents are the most powerful, and in most instances it is preferable to dry them previous to distillation, to get rid of some of the acid juices of the sap of the plant, which, by enabling the water with which they are distilled to dissolve more of the oil than it otherwise would, would diminish the produce. The drying of the plants should, if possible, be in the sunshine, but where this is not practicable, it should be effected as quickly as possible on a kiln.
Woods must be reduced to shavings; barks, and similar substances, to a gross powder; and these in general require to be soaked for some days before they are distilled, in water sharpened with salt, or a little muriatic acid. The still, if of copper, should be well tinned inside, and have a low head, that the oil may not have to rise high. The old alembic answers well, with the simple mode therein adopted of condensing at the capital or head, either by a small reservoir of water fixed thereon, or by enveloping the head with flannel, and allowing water to drip upon it. No more water should be added than is necessary to bring over the oil, and prevent the matter from burning in the still; hence, the goods should be first floated with water, and then more added by weight or measure, to determine the necessary quantity. In goods that yield their oil easily, about six times their weight is sufficient; but in others which yield their oils with difficulty, as the woods, about ten times their weight must be added.
The distillation is conducted with a quick fire, until the quantity of water that wa3 added is come over; and if the last portions bring over any oil with them, the fire is slackened, and the distilled water returned into the still, and brought over a second time; sometimes it is found necessary to distil it a third time. As some oils congeal at a low temperature, it becomes necessary that the condenser be not kept so cold as to produce that effect, but at that temperature by which the oils would preserve their liquid form, in order that they may flow out into a receiver. And as it is difficult to clean out the convoluted worms of ordinary condensors, those with straight or zigzag tubes are preferable where more than one kind of oil is made, otherwise the odour of one would be mixed with another. The quantity of oil which comes over being extremely small in comparison with that of the water, it is proper to have a receiver that will allow the water to run off into another vessel, while it retains the oil. The water used in a previous distillation may be advantageously used in a second, and sometimes a third distillation, to save that portion of the oil which always combines with fresh water.
It is, however, to be observed, that by frequent cohobation, the water acquires acid properties, and then takes up a larger quantity of oil, and diminishes the produce. Roses should be distilled with their green flower cups, and be torn open with the nails, as the liquid or scented oil is lodged in a cell at the claw of each petal. By adding a little muriatic acid to the water, and digesting for a few days, the produce is doubled. The essential oil of bitter almonds requires particular treatment, and the distillation should be conducted in the open air, to prevent the deleterious effects' of its vapours, which cause severe head-ache and fainting to all persons within its influence. The usual process consists first n pressing the almonds, to separate the fixed oil, and then grinding the resulting oil cake to a coarse powder. Thirty pounds of this powder is then to be distilled with eight gallons of water, until the whole is come over, when there will be found floating about three-quarters of an ounce of essential oil; this is to be taken off, and then as much salt as the water will dissolve is to be added to it, and about a gallon of this being distilled, a further produce of four or five ounces of the essential oil will result therefrom.
The essential oil of turpentine, called also spirit of turpentine, is prepared by distilling turpentine in iron stills with a condensing apparatus, until the drops of oil begin to grow coloured. One hundred-weight of turpentine yields from twelve to twenty pounds of oil; the product is found to be greater in proportion to the slowness of the operation. For medical purposes the turpentine is either distilled with water, or rectified with it, but as a portion of water thus combines with it, it is not adapted to painters' use. The essential oils of aniseed, camomile, caraway, cassia, cinnamon, cloves, dill, juniper berries, mint, nutmegs, penny-royal, peppermint, rue, sassafras, savine, and wormwood, are used in medicine as carminatives and stimulants. Those of aniseed, caraway, cassia, cinnamon, cloves, juniper, and pepper, are used in compounding the cordial waters of the spirit dealers. A third class, as those of balm, citron flowers, lavender, orange flowers, roses, rosemary, sandal, thyme, are used to scent and flavour spirits of wine, to make what are called toilet waters, as eau de Cologne, Hungary water, etc. which are employed as cordials.
The essential oils of balm, calamus aromaticus. camomile flowers, caraway seeds, hyssop, lavender flowers, marjorum, milfoil, parsley, rosemary, sage, sassafras, thyme, are used to scent soaps.