Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/808

Rh 748 OILS instances there is no obvious relation betAveen the bodies. A third class of essential oils, limited in number, consists of those into the composition of which sulphur enters. Of the hydrocarbons which constitute the principal pro portion of essential oils terpene, C 10 H ]6, is the most im portant. Terpene is the chief constituent of the various kinds of oil of turpentine ; and a hydrocarbon of precisely the same composition is contained in the oils of bergamot, orange, and other species of Citrus, and in a great number of other essential oils. But although agreeing in chemical formula these terpenes differ considerably in physical pro perties, such as specific gravity, boiling point, and rotatory influence on the plane of polarization. Under the name of terpenes are also included two series of hydrocarbons, polymeric with the C 10 H 16 series, having respectively the formulae C 15 H 24 and C 20 H 32. The former of these, C 15 H 24 , is of comparatively frequent occurrence, being found in the essential oils of cloves, cubebs, patchouli, and several others. The terpenes of formula C 10 H 16, by their oxida tion in presence of water, give off indirectly peroxide of hydrogen, H 2 O 2, and yield cymene, C 10 H 14 , a hydrocarbon found naturally in the essential oils of cumin, thyme, and some others. The property of evolving peroxide of hydro gen, possessed by cymene in common with terpene, explains the well-known oxidizing and antiseptic influence of com mon turpentine oil, and has been turned to account in the preparation called &quot; sanitas,&quot; a disinfectant and antisep tic agent, consisting of an aqueous solution of oxidized turpentine oil prepared by blowing air into a mixture of turpentine oil and water. The oxygenated bodies in essential oils when formed by direct oxidation of the hydro carbon generally have feebly acid properties developing through viscid oleo- resins and balsams into solid resins. The oxygenated compounds are very varied in their consti tution, some being of the nature of acids or aldehydes, while others exhibit the characters of alcohols or of ethers. The most characteristic oxygenated compounds in essential oils are camphors, the type of which is common or Japan camphor, C 10 H 16 O. Compounds isomeric with common camphor occur in the volatile oil of Pyrethrum parthenium, wormwood, mint, and other labiate plants, and in chamomile and galbanum. Camphors isomeric with Borneo camphor, C 10 H 18 O, are found in coriander oil, cajeput oil, and Indian geranium oil. Isomeric camphors having the formula C 10 H 20 O form the menthol of peppermint and the euca- lyptol of Eucalyptus Globuhis, and patchouli camphor has the composition C 15 H., S 0. The other oxygenated bodies in crude essential oils mostly ally themselves with compounds of the aromatic series, so called on account of the large proportion of its members that are represented by natural products obtained in essential oils, balsams, and resins. The essential oils which contain sulphur, of which the allyl sulphide, C 6 H 10 S, or oil of garlic is a type, have generally a pungent, penetrating, and disagreeable odour. The essential oils are obtained from their sources in four principal ways, by distillation, by expression, by enfleurage or absorption, and by maceration. The process of distillation is the most important, and is applicable to a large number of substances owing to the ease with which essential oils distil unchanged. Their general insolu bility in water is turned to account in the process, the odoriferous materials being placed in a simple still with a small quantity of water, the steam from which carries over with it the vapour of the essential oil. In distilling from certain bodies it is necessary to cohobate or return into the still the first distillate, and that opera tion may require to be repeated more than once before the raw material is quite exhausted. Again, in dealing with some substances, solutions of common salt or of chloride of calcium must be used in place of pure water, and these, by raising the boiling point, send over the vapour more richly laden with essential oil. After con densation and resting a sufficient time, the distillate separates into two portions, the oil floating or sinking according to its specific gravity. The process of expression is applicable to the obtaining of the essential oils which reside in the rind of the orange, lemon, and other citric fruits. Enfleuraye is a method by which the odours of several substances are dealt with. The aroma in such cases is S resent to a small extent, and is too tender and liable to loss and eterioration to permit of being separated by way of distillation. The process consists of exposing the flowers in contact with purified lard or with fine olive oil in suitable frames, whereby the fatty substances take up and become impregnated with the essential oil. The pro cess is principally employed in preparing pomades and perfumed oils (see PERFUMERY), as is also the analogous method of maceration, which consists in extracting the aromatic principles by macerating the raw materials in heated oil or molten fat, whereby the essential oils dissolve out into the fat. By subsequently digesting the im pregnated fats and oils prepared either by enfleurage or maceration with spirit 60 over proof the essential oils are obtained as alcoholic- essences, in which form they are much used for perfumery and flavour ing purposes, &c. Alcoholic solutions of essential oils prepared by macerating the raw materials in alcohol also form a part of the tinctures of pharmacy. Essential oils have a wide range of uses, of which the principal are their various applications in perfumery. Next to that in many ways they play an important part in connexion with food. The value of flavouring herbs, condiments, and spices is due in largest measure to the essential oils they contain ; and, further, the com mercial value of tea, coffee, wine, and other beverages is largely dependent on the delicate aroma which they owe to minute quan tities of such oils. For the flavouring of liqueurs, of aerated beverages, and of other drinks essential oils are extensively used, and their employment is not less considerable in the manufacture of confectionery and in the preparation of many dietetic articles. In the cheaper kinds of confectionery a large quantity of artificial oils called fruit essences are now employed, although the flavours as such are distinctly crude, and the wholesomeness of the prepara tions is more than doubtful. The acetate of amyl gives an imitation jargonelle-pear flavour ; valerate of amyl yields apple flavour ; a mix ture of butyrate of ethyl and butyrate of propyl gives the so-called pine-apple flavour. Formic ether has a peach-like odour, and is used for flavouring factitious rum, and there are numerous other artificial compounds used in flavouring and in perfumery. Many of the essential oils form most important medicinal agents. In the arts the cheaper oils, such as oil of turpentine, are used in the manufacture of varnishes ; the reducing influence of the oil of cloves is utilized in the silvering of mirror glass ; and oils of tur pentine, lavender, and spike are used as vehicles for painting, more particularly for the painting of pottery and glass. The essentials oils are subject to extensive adulteration. The presence of fatty oil is easily detected by the formation of a per manent grease spot on paper on which the suspected mixture is dropped. The admixture of fixed oil -may also be demonstrated by distillation, when the volatile portion goes over, leaving the fatty oil, or by treatment of the suspected sample with strong spirit, which dissolves out the essential oil, and leaves the fatty oil as a separate layer. The presence of spirit of wine in an essential oil may be readily proved by shaking iip a specimen of the oil with a measured quantity of water in a graduated glass. If the oil is pure there will be little or no change in the volume of the water layer after the complete separation of the oil and water by repose ; but if, on the other hand, the oil has been mixed with spirit, the water will have extracted the spirit and decreased the apparent amount of essential oil, and increased the watery layer in propor tion to the extent of the falsification. In dealing with very small samples a test by sight is obviously inapplicable, and in such cases the existence of alcohol in the watery solution may be demonstrated by the molybdenum test. This consists of treating a drop or two of water separated from the oil under examination with a solution of molybdic acid in ten parts of sulphuric acid. If the water contains alcohol, a characteristic intense blue reaction at once exhibits itself. The behaviour of essential oils towards alcohol is also made the basis of a test of their purity. The test is effected by mixing the essential oil with two volumes of absolute alcohol, and there after observing the volume of diluted alcohol sp. gr. 839 required to render the clear solution opalescent. The falsification of a costly essential oil with one of inferior value is a fraud which can only be detected with much difficulty. In practice a fair estimate of the purity of an essential oil and of its individual character may be made by some experience of its smell as nibbed on the hands ; colour, boiling point, specific gravity, and other physical properties may also be usefully observed in aiding to form an estimate of the genuineness and value of a sample. Colour tests with various reagents are also used for the identification of the various essential oils. Among these are Frohde s test (a solution of molybdate of sodium in sulphuric acid), sulphuric acid itself, fuming nitric acid, alcoholic solution of hydrochloric acid and picric acid, &c. In the following list the essential oils ordinarily met with in commerce, with their soiirces, general applications whether in medicine, in perfumery, in the arts, or as flavouring materials, and some of their physical properties, are tabulated. The specific gravities and power of turning the plane of polarization, given for a column 10 inches long, are extracted from Dr Gladstone s paper