OIL. 



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 

 exfracted 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 

 j>robably 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 

 bat 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 

 F 



