Prof. Miller makes a second group of fatty acids, of which oleic acid is the type, and which have the general formula 021I,„-,0,; but as oleic acid is the only member of this grqup which is of any practical importance, it is sufficient to refer the reader to the special article on that acid.
A complete list of even the chief fats and fixed oils would take up far more space than we can command. In the article "Fixed Oils, ' in The English C*lopaalia. ihe reader will find 04 Of the most imPottant of these•substances mentioned, with, in most cases, a brief notice of the origin and properties of each. The British pharmacopoeia contains hog's lard, mutton suet, cod-liver oil, concrete oil (or butter) of nutmeg, and almond, castor, croton, linseed, and olive oils, besides the closely allied substances spermaceti and wax.
The Volatile or'Essential Oils exist, in most instances, ready formed in plants, and are believed to constitute their odorous principles. They form an extremely numerous class, of which most of the members are fluid; a few (oil of aniseed, for example) being solid at ordinary temperatures, but all of them are capable of being distilled without undergoing change. They 'resemble the fixed oils in their inflammability, in their solubility in the same fluids, and in their communicating a greasy stain to paper or any other fabric; but the stain in this case soon disappears, and they further differ in coin munivating a rough and harsh rather than an unctuous boil ingto the skin. Their boil in points are in almost all eases far higher than that of water, but when heated with NVaie: they pass off with the steam—a property on which one of the chief modes of ob :lining them depends. See PERFUMERY. The oils have characteristic penetrating odor s, which are seldom so pleasant as those of the plants from which they are obtained, aura their taste is hot and irritating. They vary in their specific gravity, but most of them are lighter than water, and refract light strongly. Most of them are nearly color less when fresh, but darken on exposure to light and air; but a few are green, and two or three of a blue color. By prolonged exposure they absorb oxygen, and become con verted into resins.
By far the greater number of them are products of the vital activity of plants, in which most of them exist ready formed, being inclosed in minute cavities, which are often visible to the naked eye. Although diffused through almost every part of a plant, the nil is especially abundant in particular organs of certain families of plants. In the umbelliferw, it is most abundant in the seeds; in the rosaceoe, in the petals of the flowers; in the myrtacece and (abiatce, in the leaves; in the aurantiacece, in the rind of the fruit. As in the case of the animal and vegetable fats and fixed oils, so most of the essential oils occurring in plants are mixtures of two or more distinct chemical compounds, one of which usually contains no oxygen, while the others are oxidized. Of t112.se, the former, which is a pure hydrocarbon, is the more volatile, and acts as a solvent for the others. Most of these oils, when cooled, separate into a solid and a fluid portion, to which the terms stearopten and elceopten have been applied. .
In the comparatively few cases iu which the oils arc not formed naturally, they are produced by a species of fermentation, as in the case of oil of bitter almonds and oil of mustard (q.v.), while others are the product of the dry distillation or of the putrefaction of many vegetable bodies. Some of the natural oils, as those of cinnamon, spirma, and winter-green, have also been artificially produced.
The essential oils are much employed in the fabrication of perfumery (q.v.), for the purpose of flavoring liquors. confectionary, etc., for various purposes in the arts (as in silvering mirrors), and in medicine. The special uses of the most important of these oils in medicine will be noticed subsequently.
The members of this group, which is an extremely numerous one (more than 140 essential oils being noticed in the article on that subject in the The English Cyclopedia), admit of arrangement under four heads. 1. Pure hydrocarbons; 2. oxygenoue essential oils; 3. sulphurous essential oils; 4. essential oils obtained by fermentation, dry distil lation, etc.