456 OILS, VOLATILE 



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. See PERFPMEBY. 



Essential oils differ much from each other in their physical properties. Most of 

 them are yellow, others are colourless, red or brown ; some again are green, and a 

 few are blue. They have a powerful smell, more or less agreeable, which immediately 

 after their distillation is occasionally a little rank, but becomes less so by keeping. 

 The odour is seldom as pleasant as that of the recent plant. Their taste is acrid, 

 irritating, and heating, or merely aromatic when they are largely diluted with water 

 or other substances. They are not greasy to the touch, like the fat oils, but, on the 

 contrary, 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 0'84:7 

 and T096 ; tho first number denoting the density of oil of citron, and the second 

 that of oil of sassafras. Although styled volatile oils, the tension of their vapour, as 

 well as its specific heat, is much less than that of water. The boiling-point differs in 

 different kinds, but it is usually about 316 or 320 Fahr. Their vapours sometimes 

 render reddened litmus-paper blue, although they contain no ammonia. When dis- 

 tilled by themselves, the volatile oils are partially decomposed ; and the gaseous 

 product of tho portion decomposed always carry off a little of the oil. When they 

 are mixed with clay or sand, and exposed to a distilling heat, they are in a great 

 measure decomposed ; or when they are passed in vapour through a red-hot tube, 

 combustible gases are obtained, and a brilliant porous charcoal is deposited in the 

 tube. On the other hand, they distil readily with water, because the aqueous vapour 

 formed at the surface of the boiling fluid carries along with it the vapour of the oil 

 produced in virtue of the tension which it possesses at the 212th degree Fahr. In the 

 open air tho volatile oils burn with a shining flame, which deposits a great deal of 

 soot. The congealing-point of the essential oils varies greatly ; some do not solidify 

 till cooled below 32, others at this point, and some are concrete at the ordinary 

 temperature of the atmosphere. 



When exposed to the air the volatile oils change their colour, become darker, and 

 gradually absorb oxygen. This absorption commences whenever they are extracted 

 from the plant containing them ; it is at first considerable, and diminishes in rapidity 

 as it goes on. Light contributes powerfully to this action, during which the oil disen- 

 gages a little carbonic acid, but much less than the oxygen absorbed ; no water is formed. 

 The oil turns gradually thicker, loses its smell, and is transformed into a resin, which 

 becomes eventually hard. De Saussure found that oil of lavender, recently distilled, 

 had absorbed, in four winter months, and at a temperature below 54 Fahr., 52 times its 

 volume of oxygen, and had disengaged twice its volume of carbonic acid gas ; nor was 

 it yet completely saturated with oxygen. The stearescence of anise-seed oil absorbed at 

 its liquefying temperature, in the space of 2 years, 156 times its volume of oxygen gas, 

 and disengaged 26 times its volume of carbonic acid gas. An oil which has begun to 

 experience such an oxidisement is composed of a resin dissolved in the unaltered 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 phials, filled to the top, closed with 

 ground-glass stopples, and placed in the dark. 



Volatile oils are little soluble in water, yet enough so as to impart to it by agitation 

 their characteristic smell and taste. 



They are soluble in alcohol, and the more so the stronger the spirit is. Some 

 volatile oils, devoid of oxygen, such as the oils of turpentine and citron, arc very 

 sparingly soluble in dilute alcohol ; while the oils of lavender, pepper, &c. are con- 

 siderably so. Such combinations form the odoriferous spirits which the perfumers , 

 incorrectly call waters, as lavender water, eau de Cologne, eau de jasmin, &c. They 

 become turbid by admixture of water, which seizes the alcohol, and separates the 

 volatile oils. Ether also dissolves all the essential oils. 



These oils combine with several vegetable acids, such as the acetic, the oxalic, the 

 succinic, the fat aciis (stearic, margaric, oleic), the camphoric, and suberic. 



With the exception of the oil of cloves, the volatile oils do not combine with the 

 salifiable bases. They have been partially combined with caustic alkali, as in the case 

 of Starkey's soap. This is prepared by triturating recently-fused caustic soda in a 

 mortar, with a little oil of turpentine, added drop by drop, till the mixture has 

 acquired the consistence of soap. The compound is to be dissolved in spirits of wine, 

 filtered and distilled. What remains after tho spirit is drawn off, consists of soda 

 combined with a resin formed in the oil during the act of trituration. 



The essential oils dissolve all the fat oils, the resins, and the animal fats. 



In commerce, these oils are often adulterated with fat oils, resins, or balsam of capivi 

 dissolved in volatile oil. This fraud may bo detected by putting a drop of the oil on 

 paper and exposing it to heat. A pure essential oil evaporates without leaving any 



