Volatile Oils, as they are properly denominated by the Edinburgh College, are vegetable products, found in almost every part of many plants, except the cotyledons of the seeds, in which, almost always, the fixed oils are contained. In some plants, the volatile oil exists in distinct vesicles, and is obtained by simple expression, but in general it can only be obtained by distillation; whence the name distilled oils, given to this class of substances by the London College: and as the odour of plants generally depends on their volatile oils, the Dublin College, following the example of the elder chymists, who denominated them essences, have adopted the term essential oils. The expressed volatile oils are now rejected from all the Pharmacopoeias; and the whole of those used in pharmacy, retained, are procured by distillation.

Volatile oil is obtained from both recent and dried plants. When fresh plants are to be employed, they require no previous treatment; but when the plants are dry, or woods or barks are to be employed, the plants must be macerated in salt and water for some time, and the woods and barks be previously rasped. The distillation is performed in the following manner: - The plants, or the parts of them containing the oil, are to be put into a tinned copper still, and closely pressed down; after which, as much water is to be poured in as will be sufficient to cover the materials. The head of the still, which should be low, is then to be luted on; the fire lighted, and so regulated as to keep the contents of the still scarcely up to the boiling point; and the distillation should be continued, until the condensed vapour comes over nearly insipid and inodorous. During this process the volatile oil rises with the watery vapour, from which, however, the greater part of it again separates, after it has remained at rest for some hours in a cool place, and either floats on the surface of the water, or sinks to the bottom, according to its specific Gravity. The complete separation of the oil is effected by an instrument called a Separator (see Part I.) : and the water is to be again used for a second distillation of fresh materials, by which, as it is already impregnated with as much of the oil as it can dissolve, the product of oil of the second and every subsequent distillation will be consequently greater than that of the first; but it is not until "the tenth distillation, in some cases, that the product attains its maximum."1 By the same process, volatile oils are obtained from balsams, resins, gum resins, and turpentines.

They have not their characteristic qualities in perfection immediately after their distillation, but have a disagreeable, empyreumatic odour; to dissipate which they must be allowed to stand for some days in vessels loosely covered with paper, before they be put into the bottles in which they are to be preserved; which should he opaque.

Although all volatile oils agree in their chymical properties sufficiently to constitute them members of the same class of substances, yet they differ greatly in their qualities from each other, and in the proportions in which they are obtained.

Volatile oils have a penetrating odour, and hot taste. They are completely evaporated when heated in the open air; a property which is taken advantage of as a test of their purity : for if they be adulterated with fixed oil, which is not unfre-quently the case, by heating a small portion of the oil on a piece of clean paper a greasy spot will remain, whereas, if the volatile oil be pure, the paper will be left perfectly clean. In a higher temperature, volatile oils are readily ignited, and burn with a bright white flame, emitting a large quantity of black dense smoke; and with the production of a large proportion of carbonic acid and water.

Volatile oils, exposed to the light, are deepened in colour, or become colourless: when exposed to the air they become more viscid, less odorous, redden the tincture of turnsole, and gradually assume the form of resins; which changes, Dr. Priestley ascertained2 to depend upon the absorption of oxygen; and hence the necessity of preserving volatile oils in small phials, full, and well corked. An oil, which has become thick and scentless may be rectified by redistilling it with some of the same kind of plant from which it was originally extracted, or with alcohol or sulphuric aether3 : a limpid, odorous oil comes over, and resin remains in the retort.

These oils are very sparingly soluble in water, rendering it milky when agitated with it: they communicate to it their odour: they are all soluble in alcohol, aether, and the fixed oils in various proportions. From their solubility in alcohol they are sometimes adulterated with that fluid; but the fraud may be detected by agitating some of the suspected oil with water; when, if the oil contain alcohol, an increase of temperature will be indicated by the thermometer, but not if the oil be pure.1 The more expensive oils are also occasionally adulterated with the cheaper, particularly with oil of turpentine, which, however, is readily discovered by its peculiar odour, if a piece of paper be dipped in the suspected oil and dried with a gentle heat. They are also, sometimes, adulterated with castor oil; and as the mixture, when the ingredients are in equal proportions, is soluble in alcohol, the fraud cannot be detected by that test; but it is rendered obvious by the adulterated oil leavings greasy stain upon paper, which has been touched with it, and held before the fire, whereas no stain is left by the genuine oil.

1 Aikin's Dictionary of Chemistry, art. Oil. 2 Priestley on Air, ii. 232

3 Nicholson's Journal, 8vo. vii. 6.

Volatile oils unite with sulphur, in a temperature sufficient to melt the sulphur, and form brown-coloured, fetid mixtures, which have been denominated balsams of sulphur. The alkalies and earths combine imperfectly with them, and constitute a class of bodies, which the French chymists have denominated savonules. The action of the acids is much more violent on them than on the fixed oils: and several of them detonate when rubbed with chlorate of potassa.

As medical agents, volatile oils are stimulant and aromatic. They are chiefly employed to remove nausea and flatulence, to correct the griping qualities of some purgatives, and the disagreeable taste of other remedies. They may be given, triturated with water and mucilage; or dropped first on a lump of sugar, and through its medium diffused in water, forming a solution denominated oleum saccharum. The quantity of sugar must be more than ten times the weight of the oil; and when they are well triturated together, the oil becomes thus completely soluble in water, and may be diluted to any extent.

Some of the more stimulant of these oils are added to embrocations to be used as rubefacients, where counter-irritation is necessary.

The three British Colleges give the following general rules for the preparation of volatile oils:-

1 Marqueron, Annales de Chimie, xlviii. 267.