These are odoriferous oils, obtained from strongly fragrant plants; sometimes pervading all parts of the plant, but in many instances existing only in limited portions, as the petals of flowers, the bark of sassafras, the seeds of anise, and the rind of lemon. They are all more or less rapidly dissipated by heat; and those of petals disappear in most instances as the flowers dry, while the majority of plants lose only a moderate portion in drying. In most instances, these oils are obtained by distillation from water, as detailed in the department of Pharmacy. A few oils of this class are procured by pressure, as the oils of lemon, bergamot, and that from orange peel. The petals of flowers can not be treated by either of these processes to good advantage, when the following method is employed: Alternate layers are formed of the fresh flowers, and thin cotton fleece or woolen cloth-wadding previously saturated with some perfectly pure and inodorous fixed oil–as of olives or almonds, or, latterly, glycerin. They are allowed to stand in a pile thus formed till the petals have given out all their odor, which is absorbed by the fixed oil in the cotton. The old flowers are then replaced by other fresh ones, and thus, by repetitions, the fixed oil becomes thoroughly charged with the fragrance. The volatile oil may then be dissolved out by alcohol, or distilled over water in the usual way.
Most essential oils are yellow, some are reddish, a few have a distinct green tint, and a few–as of camomile, arnica, and yarrow–are blue. They have a powerful smell, resembling the plant from which they are obtained, but not so pleasant. The majority of them are quite fluid at even low temperatures; but lemon oil concretes at 4 deg. below zero, F.; fennel congeals at 14 deg.; anise forms into lamellar crystals at 50 deg.; while the oil of elder flowers is as solid as butter. They are not unctuous to the touch, but rather roughen the skin; and they are all more or less acridly stimulating to the taste, and quite diffusive when diluted and used inwardly. The greater number are lighter than water; but a few, as sassafras, cassia, cinnamon, and cloves, are heavier than water. When exposed to the air, they slowly change color, absorbing oxygen and becoming darker. They at the same time become thicker and more of a resinous character, and lose a portion of their intensity. Light hastens these changes; and hence it is advisable to keep these oils in dark rooms in thoroughly stoppered bottles.
Volatile oils are little soluble in water; yet enough so to impart to it, by agitation, a little of their smell and taste. A water which distills with any of these oils, is in general a saturated solution of it; and is used in medicine under the name of distilled water. By being first thoroughly rubbed with magnesia, carbonate of magnesia, or sugar, they may then be dissolved in water by careful trituration; and are used in medicine as medicated waters. They are very soluble in alcohol, the solubility increasing as the strength of the spirit increases. Such solutions are essences. They all dissolve the fixed oils, resins, and animal oils.
The essential oils are frequently adulterated with fat oils or resin. This fraud may be detected by putting a drop of the oil on paper, and exposing it to heat. A pure essential oil evaporates without leaving any residuum; whilst if any fat or fixed oil be mixed with it, a translucent stain will be left on the paper. Or if any terebinthinate fatty substance be present, it will remain undissolved on adding the specimen of essential oil to three times its own volume of eighty-three percent alcohol. Resin may be detected by distilling the specimen in an open test-tube, when the resin will remain after the oil has evaporated.
The facts in this article, as well as those on Fixed Oils, are condensed from Ure's Dictionary of Manufactures, published in New York by D. Appleton & Co.
The Physiomedical Dispensatory, 1869, was written by William Cook, M.D.
It was scanned by Paul Bergner at http://medherb.com