78 



to the material employed for their preparation, but intensified to 

 the highest degree. Besides these, another class of oils occurs, 

 which like oil of bitter almonds, mustard, (kc, do not exist ready- 

 formed in the living organism, but owe their formation to a kind 

 of fermentation or breaking up of an inodorous suljstance into 

 different products, one of which is the res})ective volatile oil. 



Three processes are commonly em]i)loyed for obtaining the iso- 

 lated oils, viz.: — Pressing, Extractioa by means of solvents, and 

 Distillation. 



Hhe first of these methods is only practicable with materials 

 which, like orange-peels, are very rich in oils. As it is wasteful 

 and generally yields an oil contaminated with fat, wax, resin, 

 &c., this operation is restricted to a few materials only. 



The method of Extraction (commonly by means of ether) is 

 adopted either with small quantities of the raw material, or with 

 substances very poor in oil, or with oils, like those of turnsole, 

 jasmin, reseda, violets, suffering decomposition by heat. By this 

 process all the other constituents, soluble in ether, of the raw 

 material are of course likewise extracted, and accordingly the 

 remnant, after the spontaneous eA^aporation of the solvent, con- 

 tains generally also fixed oils, wax, chlorophyll, resins, Ac, 

 although the larger portion of these impurities, with the exception 

 of the fixed oils, separate during evaporation from the volatile oil. 

 It is therefore not possible to obtain a pure product by either 

 pressing oi- extracting in the usual way. 



Distillation by means of water is by far the most common 

 method of obtaining ethereal oils. Usually the properly commi- 

 nuted substance is mixed with six or eight times its weight of 

 water, and the mixture left to rest for a day. The whole is then 

 submitted to distillation (with proper means for refrigeration), until 

 the distilled water ceases to separate any oil, or until it is nearly 

 devoid of odour. To prevent any contamination of the oil, by 

 partial charring of the contents of the still ^vith empp-eumatic 

 products, the distillation is effected more properly by forcing steam 

 through the substance operated on. 



The essential oils obtained by distillation, and when freshly 

 prepared, are as a rule colourless or pale-yellowish, seldom, like 

 the oils of a few of the Compositse and I\Iyrtace?e, of a blue or 

 green colour. They are also commonly of thin fluidity (at mean 

 temperature) and of great light refracting and dispersing power ; 

 sometimes they are of thickish consistence, partly or completely 

 solid and in the latter case distinguished by the name of camphor 

 or stearopten, while the liquid part is called in contradistinction 

 elfsopten and which may often be obtained solid or in crystals by 

 refrigei-ating to 0° or below. The specific gi-avities of volatile 

 oils range from 0*800 to I'lOO; their boiling points are in all 

 instances higher than 100° seldom below 150°, but often as high 



