344 CHEMICAL DISCOVERY AND INVENTION 



has been achieved, for although the preparation of perfumes 

 from plants is an industry which dates back many centuries, 

 any knowledge of the composition of the " essential oils " has 

 been derived almost wholly from chemical researches conducted 

 within living memory. Before proceeding to the most recent 

 advances it will be in the interest of those who are quite un- 

 acquainted with the technology of the subject to explain briefly 

 what is understood by an essential oil. An oil is usually under- 

 stood to be a liquid fat which is practically insoluble in water 

 and which floats on that liquid. When boiled with an alkaline 

 liquid, such as solution of caustic soda, it slowly dissolves 

 forming a solution of soap. And if a drop of oil is placed on 

 paper it forms a translucent spot which is permanent, for common 

 oil does not evaporate away when exposed to the air. 



An essential oil is distinguished from the fatty oils first by a 

 strong and characteristic odour ; it usually floats on water, but it 

 is slightly soluble, for the odour is commonly communicated to 

 the water, as in such instances as the familiar rose-water or 

 orange-flower water. An essential oil is usually changed by 

 contact with caustic alkali, but it does not produce a soap. And 

 lastly if a drop of essential oil is placed on paper the translucent 

 stain disappears gradually as the oil evaporates away. 



Most commonly, though not invariably, an essential oil is a 

 mixture of two chief ingredients. One of these is a terpene a 

 compound of carbon and hydrogen only the other is usually a 

 compound of carbon and hydrogen with oxygen, and consists of 

 an aldehyde, a ketone, a compound ether or " ester " or some- 

 thing else. To the latter ingredient the characteristic odour of 

 the oil is mainly due. 



Some essential oils consist of one constituent only with only 

 slight impurities. Such, for instance, are the essential oils 

 following : 



Name of oil. Composed almost entirely of 



Turpentine, American . . Dextro-pinene C 10 H 16 . 



,, French . . Lsevo-pinene C 10 H 16 . 



Bitter almond 



Apricot and peach kerne] 



Benzaldehyde C 6 H 5 .CHO. 



Cherry laurel leaf . 



Cinnamon bark . . . | Cinnamic aldehyde 



Cassia ... j CeH 5 .CH : CH.CHO. 



Mustard seed .... Allyl isothiocyanate 



SC : NC 3 H 5 . 



