ESSENTIAL OILS 51 



30. Preparation of Essential Oils by Distillation 

 with Steam, 



Some heads of lavender, pieces of orange-peel, or 

 peppermint leaves are placed in a distilling flask, half- 

 filled with water, and fitted with a condenser and 

 receiver. About one-third of the water is distilled 

 off, whereby a large proportion of the essential oil, 

 being volatile in steam, will be carried over into the 

 distillate. 



The distillate is extracted with small quantities of 

 ether in a separating funnel or burette, the ethereal 

 solution separated from the aqueous portion, and the 

 ether distilled off on the water-bath. The residue of 

 essential oil will be found to possess the characteristic 

 odour of the substance taken. 



(/) Miscellaneous Substances — Tannins, Resins, 

 Chlorophyll. 



Tannins. — The tannins are mostly derivatives of gallic acid or 

 trihydroxybenzoic acid, C6H2(OH)3. COOH, and occur in plants 

 mainly as glucosides, although the best known and most important 

 member, ordinary tannin or gallotannic acid, exists in the free 

 state. It occurs in comparatively large quantities in oak galls, of 

 which it makes up half the weight, in oak-bark, and in smaller 

 proportions in other plant products, such as tea. 



The function of the tannins in the economy of plants is at present 

 unknown, although they are thought to be elaborated from the 

 carbon dioxide of the air, in a somewhat similar way to that in 

 which the sugars and starches are produced. Tannin is of import- 

 ance owing to its extensive use for the conversion of hides into 

 leather, an action depending on the fact that certain substances 

 (gelatine, etc.) present in the hides are converted into complex 

 insoluble substances impervious to water. 



The presence of tannin in food-stuffs is undesirable, as the 

 astringency of the substance leads to a decreased flow of the 

 gastric juices, with the result that digestion is to a certain extent 

 impaired. 



