VOLATILE OILS AND RESINS 81 



(c) By Hydrolysis and Distillation. — Some oils occur 

 as glucosides not only of the oil but also of other compounds. 

 Occurring with them in the plant is an enzyme which under 

 favorable conditions of moisture and temperature acts on the 

 glucosides, forming in addition to glucose the volatile oil 

 and the other compound if it is present. Commercially the 

 oil is obtained by crushing the material in water, allowing 

 the enzyme to act, and distilling off the oil. 



(d) By Solution in a Fixed Oil, such as olive oil or lard. 

 The product is used as a perfumed oil or an unguent. 



(e) By Extraction with a Solvent much more volatile 

 than the oil. The solvent is removed by distillation. This 

 method is applicable only for such oils as have high boiling 

 points. 



75. Classification. — The volatile oils as obtained from 

 plants are not single chemical compounds, but are very 

 frequently mixtures of several chemical individuals, and the 

 difficulty often experienced in obtaining an extracted oil 

 which resembles the original is due either to a failure to 

 obtain every compound, or to obtain them in the proper 

 proportions. Each oil, however, generally has one principal 

 compound to which are due most of the odor and taste. For 

 convenience, these compounds may be classified into three 

 groups : 



(a) Carbon-Hydrogen Compounds.— These oils belong to 

 that class of hydrocarbons called terpenes — ring compounds 

 which have a general formula, C10H16, or a multiple thereof. 

 They may be terpenes proper, O0H16, or sesquiterpenes, 



C15H24. 



(6) Carbon-Hydrogen-Oxygen Compounds. — These 

 compounds may be alcohols, aldehydes, or ketones of the 

 aliphatic or carbocyclic series, or esters of any of the 

 alcohols and acids. 



(0) Carbon-Hydrogen-Sulphur Compounds. — In addi- 

 tion these compounds sometimes contain nitrogen. They are 

 usually organic sulphides, or thiocyanates. The oils con- 

 taining oxygen or sulphur have much stronger odors and 

 flavors than the terpenes, and are more useful for these 



properties than are the commoner terpenes. 

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