ESSENTIAL OILS. 577 during the act of distillation, the fragrance of the plant is obtained and concentrated by dissolving it in a fixed oil, which is itself destitute of odour, such as oil of poppy-seeds. The leaves of the flower are in such instances spread in thin layers upon woollen cloths saturated with the fixed oil, the cloths thus charged are then piled one upon another, and submitted to the action of a press. The essential oils vary in specific gravity, but in general they are lighter than water. These oils are usually liquid at ordinary temperatures, but oil of aniseed is solid at all temperatures below 59° (15° C.). If cooled slowly, many of them separate into a solid and a fluid portion, respectively termed stearopten and elæopten. This fact is readily explained by the circumstance that most of the essential oils are mixtures of two or more distinct chemical compounds which differ in volatility and fusibility; one of these compounds generally contains no oxygen, whilst the others are often substances formed from it by oxidation. Of these the pure hydrocarbon is generally the more volatile; it acts as a solvent to the oxidized compounds, which, by a depression of temperature, may frequently be separated from it in a crystallized form. Most of the essential oils have a yellowish colour, which deepens when exposed to the air; during such exposure they absorb oxygen more or less rapidly, and are thus gradually converted into a solid, resinous, or camphreous mass. Those oils which absorb oxygen the most rapidly have in general the most powerful odour. This absorption of oxygen is sometimes attended with a simultaneous extrication of carbonic anhydride and water: this occurs in the case of the oil of anise, and of lavender; and in such instances the resulting resinous mass is not a simple oxide of the hydrocarbon. The essential oils are insoluble in potash, and are not capable of saponification by treatment with alkalies: but if the oils be transmitted in the form of vapour over heated caustic potash, hydrogen is frequently disengaged, and the oil becomes oxidized, forming an organic salt with the potassium. Oil of cinnamon may thus be converted into potassic cinnamate: Chlorine, iodine, and bromine act upon most of the essential oils, and form compounds in which a certain number of atoms of one of these elements displaces a corresponding number of atoms of 578 ESSENTIAL OILS-PURE HYDROCARBONS. hydrogen. Nitric acid oxidizes most of the essential oils with great violence. The essences may be arranged under three principal divisions, viz.-A. Pure hydrocarbons; B. Oxidized essences; C. Sulphuretted essences. The pure hydrocarbons are generally lighter than water; the other two varieties often have a specific gravity greater than that of water. The essences which belong to the first class always occur in a state of admixture with a larger or smaller proportion of an oxidized compound. (A.) Pure Hydrocarbons. (1411) The group of chemical compounds formed by the essential oils which contain no oxygen presents many interesting features; sixteen or twenty of these substances are isomeric. These isomeric bodies may be subdivided into two metameric classes; in one of these, the terebenes, the molecule is represented by 10H16, and to this class oil of turpentine belongs; in the other, the molecule of the oil, like that of essence of cubebs, is represented by €15H24. The members of each of these groups, notwithstanding the diversity of their odour and taste, are strictly metameric; equal weights of all the members of the same group yielding exactly equal volumes of vapour. The group of terebenes, or camphogens, is the more numerous and important. Many characters, both chemical and physical, are common to all the members of the group: for example, they have a specific gravity in the liquid form closely approaching to 0.860; and the boiling point, though subject to greater variations, in most instances is very near 320° (160° C.). Although the general formula of the group contains 4 atoms of hydrogen less than that of the olefine with an equal number of atoms of carbon, no transformation of the olefines into terebenes, or of terebene into olefine derivatives, has yet been accomplished. Many of the terébenes cannot be distinguished from each other except by their action upon polarized light: some varieties of oil of turpentine, for example, cause left-handed rotation of a ray of polarized light; oil of lemons produces rotation to the right hand; whilst other oils are destitute of rotatory action upon the plane of polarization. A slight change in the molecular arrangement of the constituent particles of these bodies modifies their action upon polarized light, for it has been found that oil of turpentine by peculiar treatment at a high temperature may be deprived of ESSENTIAL OILS-ARTIFICIAL CAMPHORS. 579 its rotatory power, although it retains its usual chemical properties (1414). The terebenes rapidly absorb dry hydrochloric acid gas, and yield compounds termed artificial camphors; some of these bodies crystallize, and in appearance and properties much resemble natural camphor; while others of the oils form combinations with the acid equally definite, but which preserve the liquid form, and often retain their optical rotatory power. Oil of turpentine and oil of lemons both form solid compounds with hydrochloric acid as well as liquid ones; but the oils of black pepper and of bergamotte form only liquid compounds : Analogous compounds with the other hydracids, such as the hydrobromic and the hydriodic, may also be obtained. Another remarkable feature of the terebenes is the power which they possess of combining with water, and thus forming solid volatile crystalline substances which have a still closer analogy to camphor, both in composition and properties, than the hydrochloric compounds. Mere contact of the oil with water, or with alcohol slightly acidulated with nitric acid, in most instances gives rise to these compounds; oil of turpentine furnishing not fewer than three such hydrates, viz.: terpine hydrate (C10H16 3 H2O); terpine (Є10H16, 2 H2O); and terpinol [(E10H16)2H2O)]: the first two are solid, the last is liquid. Oil of lemons gives two such compounds, viz.: (Є10H16, 3 H2O) and (Є10H16, 2 H2O); and oil of juniper one such hydrate (C10H16, 2 H): the camphor of the Dryobalanops camphora, or Borneo camphor, may be represented as €10H16,H2O. H2 2 All these varieties of camphor, when distilled with phosphoric anhydride, lose their water, and yield hydrocarbons, having the composition Є0H16; ordinary camphor €10H16, when similarly treated, also gives a hydrocarbon termed cymol (E10H14). Indeed, the simple act of sublimation deprives the camphor or solid hydrate (C10H16 3 H2O) of the oils of turpentine, and lemon, of an atom of water; the sublimed camphor in each case being represented by the formula, €10H169 2 H2Ð. The terebenes when exposed to the air absorb oxygen; they become brown and viscid; and are ultimately converted into resins. In many instances this oxidation may be regarded as a simple substitution of oxygen for hydrogen, the number of atoms of oxygen absorbed corresponding exactly to the amount of hydrogen removed in the form of water; but in other cases this constitutes only the first stage of the process, and an additional quantity of oxygen combines with the newly formed oxide: a good example of the latter mode of oxidation is seen in the conversion of the liquid oil of lemons into the solid : : €10H10H6+ 42=¤10H1003,02,+ 3 H2Ð. (1412) OIL OF TURPENTINE (H16, or C20H16); Sp. gr. of liquid o864; of vapour 4'76; Rel. wt. 68; Boiling pt. 320° (160° C.). -Various species of pine, when wounded, pour out a semi-solid resin, of which there are different varieties: that obtained from the Pinus abies constitutes common turpentine; that from the larch, Larix Europea, is known as Venice turpentine; and that from the Pistacia lentiscus forms Chian turpentine. If the turpentine of the Pinus abies, or of the Pinus sylvestris, be distilled with water, it yields nearly one-fourth of its weight of an essential oil: this oil passes over with the vapour of water as a volatile, limpid, very inflammable liquid, of a penetrating wellknown balsamic odour. The residue in the retort constitutes common rosin, or colophony. Oil of turpentine may be distilled unchanged. It mixes freely with alcohol and ether, but not with water; it dissolves the fixed and essential oils, and is largely used in the preparation of many kinds of varnish, since it readily dissolves the resins, and on volatilizing leaves them behind in the form of a transparent coating upon the surface of the objects to which it had been applied. Oil of turpentine dissolves sulphur and phosphorus with facility; it is also one of the best solvents for caoutchouc. Most of the oil of turpentine sold as camphine in England produces a right-handed rotation (=18°.6), and is said to be furnished by the Pinus australis of the Southern States of North America. Commercial oil of turpentine frequently consists of a mixture of several isomeric hydrocarbons, which act differently on polarized light. The rotation occasioned by Bordeaux turpentine, which is produced chiefly from the Pinus maritima, is left-handed, but the amount of the rotation in different samples varies with the proportions in which the oils are mingled. According to Berthelot, if the ordinary Bordeaux turpentine be distilled in vacuo, after saturating the acids which it contains, a homogeneous hydrocarbon, terebenthene (Sp. gr. 0·864, at 59°), is obtained. It MODIFICATIONS OF OIL OF TURPENTINE. 10 16 581 boils at 32108 (161° C.). It is endowed with left-handed rotatory action upon a polarized ray=-42°3. With hydrochloric acid it gives a solid and a liquid hydrochlorate of similar composition, (10H16,HCl). When the solid compound is heated with dry soap at a temperature of about 400° (204° C.), a white solid, resembling camphor, is obtained; it fuses at 103° (45° C.), and boils at about 320° (160° C.): it has a left-handed rotation=-63°. To this substance H1 the name of terecamphen has been given. If English camphine be distilled in vacuo from potassic carbonate, it gives a corresponding liquid, australene, of sp. gr. c.864, boiling at 321°8, with a right-handed rotation = +21°.5. It yields with hydrochloric acid compounds corresponding to those of terebenthene, and with soap furnishes a similar solid, austracamphen (rotation + 22°). If either of the solid hydrochlorates of the terebenthenes be heated with barytic stearate, or sodic benzoate, a solid inactive camphen, with no rotatory power on polarized light, is obtained. (1413) Modifications of Oil of Turpentine.-Deville (Ann. de Chimie, II. lxxv. 37, and III. xxvii. 80) and Berthelot (Ib., III. xxxix. 5) have carefully studied the modifications of which oil of turpentine is susceptible without undergoing any change in the proportion of its components. Some of these modifications retain their rotatory power upon polarized light, whilst others are inactive in this respect : a. Active Modifications.-When English essence of turpentine is simply heated in a closed vessel to 460° or 480°, it becomes converted into a mixture of several compounds, which boil at different temperatures; two of these, isoterebenthene and metaterebenthene, may be separated from each other by fractionated distillation. These modifications may be produced at a lower temperature if the essence be heated with water, or with the chloride of calcium, of strontium, of zinc, or of ammonium. Fluoride of boron, as well as many organic acids, such as the acetic, oxalic, and tartaric, also produces similar effects. 1. Isoterebenthene (Є10H16); Sp. gr. 0·843, at 71°; Boiling pt. about 350° (177° C.).—This is a colourless liquid, having an odour of stale lemons. It exerts a left-handed rotation upon polarized light, but the intensity of this power appears to vary with the duration and intensity of the heat to which it has been exposed. It may be made to yield a crystalline hydrate and a solid hydrochlorate. 20 32 2. Metaterebenthene (C2H2; Sp. gr. 0913, at 68°).—This body forms at least one-third of the entire quantity of the essence |