VOLATILE OILS 725 



seed oils are also used in veterinary practice. The fixed oils 

 are obtained by expression. They consist of two or more 

 fatty acids oleic, margaric, palmitic, and stearic in com- 

 bination with the sweet basic principle glycerin. They 

 contain 76-79 parts of carbon, with 11-13 of hydrogen, and 

 10-12 of oxygen. Their consistence varies according to the 

 proportion of the fluid olein. When fresh, they are gener- 

 ally almost colourless, and are inodorous and tasteless. 

 When exposed to the air, the traces of albuminoids which 

 they contain oxidise, a species of fermentation ensues, as in 

 saponification, resulting in the breaking up of the neutral 

 fatty matters, with the production of disagreeable rancidity, 

 which may, however, be removed by boiling the faulty oil 

 with water, and subsequently washing it with a weak soda 

 solution. Oils and water are mutually but very slightly 

 soluble, but oils and fats are readily dissolved by carbon 

 disulphide, benzine, oil of turpentine, ether, and chloroform. 

 They are miscible, and hence sometimes conveniently ad- 

 ministered, in milk. Castor and croton oils are soluble in 

 absolute alcohol. They vary in their combustibility and 

 their melting and freezing points. Their specific gravity 

 ranges from 900-970. The bland oils such as olive, 

 linseed, palm, and cod-liver in small quantities are nutri- 

 ents, but administered in large doses they are purgatives. 

 Croton oil is irritant, whether applied to a mucous surface 

 or to the skin. 



The volatile or essential oils are mostly of vegetable 

 origin, being found generally in the flowers, leaves, fruit, 

 or seeds of plants, but they occur in all parts of the coni- 

 ferae. The several volatile animal odorous principles am- 

 bergris from the sperm whale, civet, musk, and castoreum 

 are occasionally used. The chemical constitution of the 

 volatile oils differs from that of the fixed oils ; most are 

 pure hydrocarbons, with the molecular formula of oil of 

 turpentine (C 10 H 16 ), and are termed terpenes. With this 

 terpene is generally associated an oxidised product, analo- 

 gous to the colophony or resin (C 2 oH 30 2 ) of turpentine. 

 Some, such as oil of garlic, contain sulphur ((C 3 H 5 ) 2 S). 

 They produce no permanent grease stain. Unlike the fat 

 oils, they are not unctuous, but make the skin rough or 



