Volatile oils are distinguished from the fixed oils by their volatility, fragrance, and acrid taste. They are also known under the name of aromatic oils, from their odour ; or essential oils, or simply essences, from being supposed to consti tute the essence or the existence of the vegetable matters which furnish them. Volatile oils are not limited to particular parts of plants, but are found to exist in every part of the plant, excepting in the seed, which furnishes the fixed oils. A great number of roots, which are general ly distinguished by an aromatic odour, and have more or less of an acrid taste, af ford volatile oils. They are furnished also by many woods, such as those of the pine and fir tribe, and by many of those which are natives of warm climates. The leaves of a great number of plants be longing to the Didynamia class also afford volatile oil, as well as many of the umbel liferous plants. It is obtained also from many flowers of vegetables, and also from the covering of many fruits, as the skin of oranges and lemons. It is likewise ob tained from a great number of seeds ; but it is never found in the cotyledons or lobes themselves, but only in the external covering. The quantity of volatile oil which is obtained from vegetables, varies according to the age, the soil in which they grow, and the state of the plant. Some plants while green furnish it in greatest abundance, while others yield most when they are dry. There are two processes by which volatile oil may be obtained. When it exists in plants in great abundance, and in vesicles in a fluid state, it may be separated by mechanical means. Thus, by simple expression, the volatile oils are extracted many plants, as, for instance, from the fruit of the orange and the iemon. From the outer rind of these fruits, when they are fresh, the volatile oil is obtained in the liquid form ; but in general the volatile oils of plants are neither so abundant, nor do they exist in that state of fluidity, It.y which they can be procured by so simple a process. In most cases they are sub jected to the process of distillation ; and for this purpose they are macerated for some hours in water. They are then in troduced into a still with the water ; a moderate heat is applied, and continued till the fluid boil, when a great quantity of vapour of water, mixed with the vola tile oil, passes over, and is received in proper vessels. The oil collects on the surface of the water, from which it may be easily separated. The water itself is of a milky colour, on account of a small quantity of oil suspended in it ; and even after the water becomes transparent by the particles of the oil separating from it, and rising to the top, it is still loaded with the peculiar odour of the plant. The vo latile oils are particularly distinguished by their fragrance, which varies in the oils extracted from different plants. The consistence of the volatile oils also varies considerably. Sometimes they are as fluid as water, which is the case with those oils obtained by expression. Some are thick and viscid, as those generally are which are extracted from woods, roots, barks, and fruits of the warmer regions. Some congeal, or assume a gra nulated solid consistence at different tem peratures. Of these last some are al ways found to be in the concrete state. Several of the volatile oils are suscepti ble of crystallization, depositing in the re. maining portion of the oil, which con tinues liquid, transparent polyhedrons, more or less of a yellow colour, which are found to be pure oil. This last change is probably owing to an incipient oxyda tion ; for it never takes place unless oils have been kept for some time. There is great variety of colour among volatile oils. Some indeed are nearly colourless, as the oil of turpentine ; but in general they are of different shades of colour. Some are yellow, as the oil of lavender ; some are of a reddish'yellow or brown, as the oil of cinnamon or of rhodium ; some are blue, as the oil of chamomile ; and some are green, as that of parsley. But the
most prevailing colour among volatile oils is yellow or reddish.
Volatile oils have almost always an acrid, hot, and even burning taste. It is observed that the most acrid vegetable matters do not yield an oil possessed of this quality. The specific gravity of vo latile oils is generally less than that of water. Some volatile oils, however, as those of sassafras and canella, have a greater specific gravity. The specific gravity of oils varies from 0.87 to 0.99, in those which are lighter than water ; but those which are heavier are from 1.03 to 1.40. When volatile oils are exposed to the light, the colour becomes considera bly deeper ; they become thicker, and increase in specific gravity. When vola tile oils are exposed to heat, they evapo rate very readily. They are much more combustible than the fixed oils ; and in burning give out a great quantity of smoke, a very bright white flame, and a good deal of heat. They require a greater proportion of oxygen than the fixed oils, and yield a greater quantity of water. This arises from a greater proportion of hydrogen, and a smaller quantity of carbon, which they contain. The volatile oils are in some degree so luble in water. When they are agitated with this liquid they combine with it, and communicate a very strong odour, and a slightly acrid taste. Phosphorus and sul phur are soluble in volatile oils. With phosphorus the solution is luminous in the dark, is extremely fetid, and gives out by the force of heat phosphorated hydrogen gas. Some of these oils are employed in medicine. They are used also for the solution of those substances which are to be employed as varnishes ; and many of them are used in perfumery. As many of the volatile oils are produced but in small quantity, they are conse quently high priced. There is therefore some temptation to adulterate them with fixed oils, with cheaper volatile oils, or with other substances, to increase the quantity. It is therefore of some im portance to be able to detect such frauds. When a volatile oil is adulterated with a fixed oil, there is a very easy test to dis cover it. Let a single drop of the oil that is suspected fall on clean paper, and expose it to a gentle heat. If the oil is pure, the whole will be evaporated, and no trace remain on the paper ; but if it has been mixed with a fixed oil, a greasy spot remains behind. Volatile oils are frequently adulterated with oil of turpen tine ; but this can only be detected by its peculiar odour, which continues for a long er time than most ofthe other volatile oils. When they are adulterated with alcohol, it is easily detected by mixing a little of the oil with water, which immediately produces a milkiness, by the abstraction of the alcohol from the oil, and its com bination with the water. There is ano ther class of oils, known under the name of empyreumatic oils, which have dif ferent properties from those which have been described. These oils are acrid and stimulating, with a strongly fetid and disagreeable odour. It would appear that these properties are owing to a par tial decomposition of other oils. These oils are produced, as the name imports, by the action of fire. They are obtained when oils are forced to rise in vapour, and pass over in common distillation, with a greater degree of heat than that of boiling water, or by the application of a strong heat to substances from which no oil was previously extracted. These em pyreumatic oils agree in some of their properties with the volatile oils. They combine in small proportion with water, and they are soluble in alcohol ; and pro bably any difference that exists between them is owing to a partial decomposition; for when they are distilled, the oil is re stored to a state of purity, and the car bonaceous matter which had been sepa rated remains behind. See Thomson's Chemistry_