80 PLANT COMPOUNDS 



in all parts of plants, except in the seeds where the fixed 

 oils predominate. The glucosides hydrolyze under certain 

 conditions with the help of enzymes to glucose and the 

 volatile oil. The volatile oils apparently serve no true 

 physiological function in plants, being for the most part 

 by-products or end products in metabolism. They frequently 

 serve as valuable helps to the plant, however, in attracting 

 insects whereby fertilization is effected; or as a repellent 

 to keep off harmful insects. 



73. Properties. — Volatile oils are usually colorless or 

 yellow when fresh, but darken on standing. They have very 

 characteristic odors and tastes and are valuable commerciallv 

 for these properties. Most of them are lighter than water, 

 varying in specific gravity from 0.850 to 0.990, although a 

 few are less than 0.850 and many of the more common oils 

 are heavier than water. They will for the most part distill 

 unchanged, especially under reduced pressure. They are 

 slowly volatile at ordinary temperatures, hence the dis- 

 tinction in names between the fixed oils, which are not 

 volatile, and the volatile oils, which are like their name. 

 Their boiling points are rather high. Volatile oils are 

 insoluble in water, although many of them dissolve to a 

 slight extent, imparting the characteristic odor and taste 

 to the water. They are readily soluble in cold alcohol as 

 well as in hot alcohol, and in ether, chloroform, carbon 

 tetrachloride, and carbon disulphide. They are also mis- 

 cible in all proportions with fixed oils. Only those which are 

 esters saponify. 



74. Methods of Extraction. — There are several ways of 

 obtaining the volatile oils for commercial use : 



(a) By Expression, very much as in the case of the fixed 

 oils. 



(6) By Distillation with Water or Steam whereby 

 the crushed material to be extracted is either boiled with 

 water or has a current of steam passed through it. The 

 volatile oil is carried over with the steam and condensed. 

 The oil and water can easily be separated because of their 

 insolubility in each other and because of the difference in 

 their specific gravities. 



