ointment the beneficent properties of which were known to
Dioscorides in the beginning of the present era. Its chemical
composition is exceedingly complex, and specially remarkable
on account of the considerable proportions of cholesterol and
isocholesterol it contains.
Commerce.—The sperm oils are generally sold in the same markets as the fish and blubber oils (see above). For beeswax London is one of the chief marts of the world. In Yorkshire, the centre of the woollen industry, the largest amounts of wool-fat are produced, all attempts to recover the hitherto wasted material in Argentine and Australia having so far not been attended with any marked success. Spermaceti is a comparatively unimportant article of commerce; and of Chinese wax small quantities only are imported, as the home consumption takes up the bulk of the wax for the manufacture of candles, polishes and sizes.
2. Essential or Ethereal Oils.
The essential, ethereal, or “volatile” oils constitute a very extensive class of bodies, which possess, in a concentrated form, the odour characteristic of the plants or vegetable substances from which they are obtained. The oils are usually contained in special cells, glands, cavities, or canals within the plants either as such or intermixed with resinous substances; in the latter case the mixtures form oleo-resins, balsams or resins according as the product is viscid, or solid and hard. A few do not exist ready formed in the plants, but result from chemical change of inodorous substances; as for instance, bitter almonds and essential oil of mustard.
The essential oils are for the most part insoluble or only very sparingly soluble in water, but in alcohol, ether, fatty oils and mineral oils they dissolve freely. They ignite with great ease, emitting a smoke freely, owing to the large proportion of carbon they contain. Their chief physical distinction from the fatty oils is that they are as a rule not oleaginous to the touch and leave no permanent grease spot. They have an aromatic smell and a hot burning taste, and can be distilled unchanged. The crude oils are at the ordinary temperature mostly liquid, some are solid substances, others, again, deposit on standing a crystalline portion (“stearoptene” in contradistinction to the liquid portion (“elaeoptene”). The essential oils possess a high refractive power, and most of them rotate the plane of the polarized light. Even so nearly related oils as the oils of turpentine, if obtained from different sources, rotate the plane of the polarized light in opposite directions. In specific gravity the essential oils range from 0·850 to 1·142; the majority are, however, specifically lighter than water. In their chemical constitution the essential oils present no relationship to the fats and oils. They represent a large number of classes of substances of which the most important are: (1) Hydrocarbons, such as pinene in oil of turpentine, camphene in citronella oil, limonene in lemon and orange-peel oils, caryophyllene in clove oil and cumene in oil of thyme; (2) ketones, such as camphor from the camphor tree, and irone which occurs in orris root; (3) phenols, such as eugenol in clove oil, thymol in thyme oil, saffrol in sassafras oil, anethol in anise oil; (4) aldehydes, such as citral and citronellal, the most important constituents of lemon oil and lemon-grass oil, benzaldehyde in the oil of bitter almonds, cinnamic aldehyde in cassia oil, vanillin in gum benzoin and heliotropin in the spiraea oil, &c.; (5) alcohols and their esters, such as geraniol (rhodinol) in rose oil and geranium oil, linalool, occurring in bergamot and lavender oils, and as the acetic ester in rose oil, terpineol in cardamom oil, menthol in peppermint oil, eucalyptol in eucalyptus oil and borneol in rosemary oil and Borneo camphor; (6) acids and their anhydrides, such as cinnamic acid in Peru balsam and coumarin in woodruff; and (7) nitrogenous compounds, such as mustard oil, indol in jasmine oil and anthranilic methyl-ester in neroli and jasmine oils.
Preparation from Plants.—Before essential oils could be prepared synthetically they were obtained from plants by one of the following methods: (1) distillation, (2) expression, (3) extraction, (4) enfleurage, (5) maceration.
The most important of these processes is the first, as it is applicable to a large number of substances of the widest range, such as oil of peppermint and camphor. The process is based on the principle that whilst the odoriferous substances are insoluble in water, their vapour tension is reduced on being treated with steam so that they are carried over by a current of steam. The distillation is generally performed in a still with an inlet for steam and an outlet to carry the vapours laden with essential oils into a condenser, where the water and oil vapours are condensed. On standing, the distillate separates into two layers, an aqueous and an oily layer, the oil floating on or sinking through the water according to its specific gravity. The process of expression is applicable to the obtaining of essential oils which are contained in the rind or skin of the fruits belonging to the citron family, such as orange and lemon oils. The oranges, lemons, &c., are peeled, and the peel is pressed against a large number of fine needles, the exuding oil being absorbed by sponges. It is intended to introduce machinery to replace manual labour. The process of extraction with volatile solvents is similar to that used in the extraction of oils and fats, but as only the most highly purified solvents can be used, this process has not yet gained commercial importance. The process of enfleurage is used in those cases where the odoriferous substance is present to a very small extent, and is so tender and liable to deterioration that it cannot be separated by way of distillation. Thus in the case of neroli oil the petals of orange blossom are loosely spread on trays covered with purified lard or with fine olive oil. The fatty materials then take up and fix the essential oil. This process is principally employed for preparing pomades and perfumed oils. Less tender plants can be treated by the analogous method of maceration, which consists in extracting the odoriferous substances by macerating the flowers in hot oil or molten fat. The essential oil is then dissolved by the fatty substances. The essential oil itself can be recovered from the perfumed oils, prepared either by enfleurage or maceration, by agitating the perfumed fat in a shaking machine with pure concentrated alcohol. The essential oil passes into the alcoholic solution, which is used as such in perfumery.
Synthetic Preparation.—Since the chemistry of the essential oils has been investigated in a systematic fashion a large number of the chemical individuals mentioned above have been isolated from the oils and identified.
This first step has led to the synthetical production of the most characteristic substances of essential oils in the laboratory, and the synthetical manufacture of essential oils bade fair to rival in importance the production of tar colours from the hydrocarbons obtained on distilling coal. One of the earliest triumphs of synthetical chemistry in this direction was the production of terpineol, the artificial lilac scent, from oil of turpentine. At present it is almost a by-product in the manufacture of artificial camphor. This was followed by the production of heliotropin, coumarin and vanillin, and later on by the artificial preparation of ionone, the most characteristic constituent of the violet scent. At present the manufacture of artificial camphor may be considered a solved problem, although it is doubtful whether such camphor will be able to compete in price with the natural product in the future. The aim of the chemist to produce essential oils on a manufacturing scale is naturally confined at present to the more expensive oils. For so long as the great bulk of oils is so cheaply produced in nature’s laboratory, the natural products will hold their field for a long time to come.
Applications.—Essential oils have an extensive range of uses, of which the principal are their various applications in perfumery (q.v.). Next to that they play an important part in connexion with food. The value of flavouring herbs, condiments and spices is due in a large measure to the essential oils contained in them. The commercial value of tea, coffee, wine and other beverages may be said to depend largely on the delicate aroma which they owe to the presence of minute quantities of ethereal oils. Hence, essential oils are extensively used for the flavouring of liqueurs, aerated beverages and other drinks. Nor is their employment less considerable in the manufacture of confectionery and in the preparation of many dietetic articles. Most fruit essences now employed in confectionery are artificially prepared oils, especially is this the case with cheap confectionery (jams, marmalades, &c.) in which the artificial fruit esters to a large extent replace the natural fruity flavour. Thus amyl acetate is used as an imitation of the jargonelle-pear flavour; amyl valerate replaces apple flavour, and a mixture of ethyl and propyl butyrates yields the so-called pine-apple flavour. Formic ether gives a peach-like odour, and is used for flavouring fictitious rum. Many of the essential oils find extensive use in medicine. In the arts, oil of turpentine is used on the largest scale in the manufacture of varnishes, and in smaller quantities for the production of terpineol and of artificial camphor. Oil of cloves is used in the silvering of mirror glasses. Oils of lavender and of spike are used as vehicles for painting, more especially for the painting of pottery and glass.
The examination of essential oils is by no means an easy task. Each oil requires almost a special method, but with the progress of chemistry the extensive adulteration that used to be practised with fatty oils has almost disappeared, as the presence of fatty oils is readily detected. Adulteration of expensive oil with cheaper oils is now more extensively practised, and such tests as the determination
