Essential Oils (called also volatile oils, and distilled oils), oily products derived from plants, generally by distilling portions of them with water. The aqueous vapor which passes over carries with it the vapor of these oils, though their boiling point is often higher than that of water. They condense together in the receiver of the still, the oil commonly floating upon the water, sometimes sinking beneath it. A portion appears to be taken up by the water, giving to it the peculiar odor and properties of the oil in a less degree. This is called medicated and perfumed water. The oils contain in a concentrated form the fragrance and essential properties of the plants, or of the portion of them employed, and when kept dissolved in alcohol constitute the essences. They may sometimes be obtained by expressing the parts containing them, as the rind of the orange and lemon; and sometimes they are so evanescent as to escape in the ordinary mode of securing them by distillation with water. The method then adopted, as in securing the oil in which lies the delicate fragrance of the tuberose, narcissus, jasmine, etc, is to arrange the flowers in layers with cotton imbued with some fixed and inodorous vegetable oil.
This gradually absorbs the volatile oil of the flowers, and when the cotton is digested in alcohol, the volatile oil is taken up by this fluid, and an essence is obtained. In some cases it may be separated also by distilling the, cotton with water or alcohol. The odor of the oil is often less agreeable than that of the plant, which is probably owing to its greater concentration, as by dilution it is made more pleasant. The oils are often colored some shade of red, brown, yellow, green, or blue, but this is not always fixed. Their taste is hot and pungent, but made pleasantly aromatic by dilution. Some are poisonous. They burn with a bright and often smoky flame. The feeling of them upon the hand is not greasy like that of the fixed oils, but rough, and a cork moistened with them grates harshly when turned in the phial. Their specific gravity varies from 0.847 to 1.17. They boil at various degrees, some at 320° F., and a few others require a higher temperature. Exposed to the air and light at ordinary temperatures, they absorb oxygen, become darker and thicker, and are finally changed into resin, sometimes into acid compounds.
Most of them consist, like the fixed oils, of a thin fluid and a solid product, which may be separated at a low temperature by compressing the substance between folds of paper. The camphor-like product called by Berzelius stearoptene is retained within the folds, while the oily fluid called elaioptene passes through. The ultimate analysis of the essential oils affords in most instances carbon, hydrogen, and oxygen. Some prove to be hydrocarbons, containing no oxygen; and in these the proportion of carbon is between 88 and 89 per cent., and of hydrogen between 11 and 12 percent., which would be expressed by the formula C5H4. Nitrogen is found as a constituent of some of them, and sulphur is met with in the oils of mustard and of horse radish. - The agreeable odors retained by many of the oils cause them to be largely used in perfumery. Their medicinal properties also render many of them valuable agents. These properties depend upon the plants from which they are derived. Most of them are aromatics and stimulants. They are chiefly used in medicine to render other articles grateful to the taste or acceptable to the stomach, though some of them are employed for other purposes, as when the oil of cubeb is given to obtain the physiological actions of the berry from which it is derived.
For a more detailed account of the remedial use of these oils the reader is referred to the articles upon the plants that yield them. Their dose is commonly only a few drops, which may be administered on a lump of sugar, or triturated with a dozen times their weight of sugar, and then mixed with water. Some are largely employed in the manufacture of paints and varnishes, some in printing calico, and some have been used for illuminating purposes. - Essential oils are frequently adulterated. The presence of fixed oils added to them for this purpose may be detected by the greasy stain left upon paper moistened with the liquid and exposed to heat sufficient to drive off the volatile oil. Alcohol is detected by various tests, as by adding water and agitating the mixture, which becomes milky if alcohol is present; and the bulk of the oil is reduced as the fluids separate on standing, by the alcohol leaving it and going with the water. A piece of potassium as large as the head of a pin will remain nearly 15 minutes in contact with a dozen drops of pure oil without change; but if it disappears in five minutes, the oil contains at least 4 per cent. of alcohol; if it disappears in one minute, it contains at least 25 per cent.
Fused chloride of calcium is also used to abstract alcohol from the oils. When the high-priced oils are adulterated with the cheaper kinds, a thorough practical acquaintance with the physical properties of the oils can alone serve to detect the imposition. The odor of oil of turpentine when used for this purpose is concealed until the oil is dissolved in alcohol, and water is added, when both the odor and flavor are easily recognized. The oils require to be kept in small bottles entirely filled, well stopped, and excluded from the light. - By recently devised chemical processes artificial essences imitating the flavor of various choice fruits are prepared from substances which would seem entirely unfitted for producing such results. Thus butyric acid, a product of butter or putrid cheese, being converted into an ether, cannot be distinguished from that prepared from the pineapple, and may be used equally well with the latter to produce the celebrated pineapple rum. The fetid fusil oil, separated from brandy and whiskey in rectifying these liquors, produces, when distilled with sulphuric acid and acetate of potash, an essence of pears; and if for the acetate of potash bichromate of potash be substituted, the product is an essence of apples.
By similar methods a variety of other flavors are obtained; and though when concentrated they are acrid, they become very agreeable when used as flavoring in proportions of a drop to an ounce or two ounces. Some of the choicest perfumes are by similar chemical processes prepared from substances which seem strangely foreign to their nature.