OILS, VOLATILE OR ESSENTIAL; Manufacture of. .The volatile oils occur in every part of oderiferous plants, whose aroma they diffuse by their exhalation ; but in different organs of different spa dies. Certain plants, such as thyme and the scented labiatte, in general contain volatile ell in all their parts; ; but others contain it only in the blossoms, the seeds, the leaves, the root, or the bark. It sometimes happens that different parts of the same plant contain different oils ; the orange, for example, furnishes three different oils, one of which resides in the flowers, another in the leaves, and a third in the skin or epidermis of the fruit. The qantity of oil varies not only with the species, but also in the same plant, with the soil, and especially the climate; thus in hot countries it isgene rated most profusely. In several plants, the volatile oil is contained in peculiar orders of vessels, which confine it so closely that it does not escape in the dry ing, nor is dissipated by keeping the plants for many years. In other species, and particularly in flowers, it is formed continually upon their surface, and flies off at the moment of its formation. Volatile oils are usually obtained by distillation. For this purpose the plant is introduced into a still, wafer is poured upon it, and heat being applied, the oil is volatilized by the aid of the watery vapor, at the temperature of 212°, though when alone it would probably not distil over un less the heat were 100 more. When the mingled vapors of the oil and water are condensed into the liquid state, by the refrigerator of the still, the oil separates, and either floats on the surface or sinks • to the bottom of the water. Some oils of a less volatile nature require a higher heat than 212° to raise them in vapor, and must he dislodged by adding com mon salt to the water, whereby the heat being augmented by 15°, they readily come over.
There are a few essential oils which may be obtained by expression, from the substances which contain them ; such as the oils of lemons and bergamot, found in the pellicle of the ripe fruits of the citrus aurantium and medico; or the orange and the citron. The oil comes out in this Case with the juice of the peel, and collects upon its, surface.
For collecting the oils of odoriferous flowers which have no peculiar organs for imprisoning them, and therefore speedily let them exhale, such as violets, jasmine, tuberose, and hyacinth, another process must be resorted to. Alternate layers are formed of the fresh flowers, and thin cotton fleece, or woollen cloth wadding, previously soaked in a pure and inodorous fat oil. Whenever the flowers have given out all their volatile oil to the fixed oil upon the fibrous mat ter, they are replaced by fresh flowers in succession, till the fat oil has become sa turated with the odorous particles. The
cotton or wool wadding being next sub mitted to distillation along with water, gives up the volatile oil. Perfumers alone use these oils ; they employ them either mixed as above, or dissolve them out by means of alcohol. In order to extract the oils of certain flowers t as for instance of white lilies, infusion in a fat oil is sufficient.
Essential oils differ much from each other in their physical properties. Most of them are yellow, others are colorless, red, or brown • some again are green, and a few are blue. They have a power ful smell, more or less agreeable, which immediately after their distillation is oc casionally a little rank, but becomes less so by keeping. The odor is seldom as pleasant as that of the recent plant. Their taste is acrid, irritating, and heat ing, or merely aromatic when they are largely diluted with water or other sub stances. They are not greasy to the touch, like the fat oils, but on the con trary make the skin feel rough. They are almost all lighter than water, only a very few falling to the bottom of this liquid ; their specific gravity lies between 0.847 and 1.096 ; the first number denot ing the density of oil of citron, and the second that of oil of sassafras. Although, when exposed to the air, the volatile oils change their color, become darker, and gradually absorb orygen. This absorp tion commences whenever they are ex tracted from the plant containing them ; it is at first considerable, and diminishes in rapidity as it goes on. Light contri butes powerfully to this action, during which the oil disengages a little carbonic acid, but much less than the oxygen ab sorbed ; no water is formed. The oil turns gradually thicker, loses its smell, and is transformed into a resin, which becomes eventually hard. De gauseure found that oil of lavender recently dis tilled had absorbed in four months, and at a temperature below 54° F., 52 times its volume of oxygen, and had disen gaged twice its volume of carbonic acid gases ; nor was it yet completely saturat ed with oxygen. The stearessence of anise-seed oil absorbed at its liquefying temperature, in the space of two years, 156 times its volume of oxygen gas, and disengages 26 times its volume of car bonic acid gas. An oil which has begun to experience such an oxydizement is composed of a resin dissolved in the un altered oil; and the oil may be separated by distilling the solution along with water. To preserve oils in an unchanged state, they must be put in vials, filled to the top, closed with ground glass stopples, and placed in the dark.