so PROXIMATE CONSTITUENTS OF PLANTS 



of a citrate, a white precipitate of silver citrate, soluble 

 (like the tartrate) in ammonia. On being warmed, 

 however, the ammoniacal solution does not deposit 

 metallic silver (distinction from tartrate). 



{h) The Essential Oils. 



Classed together under the generic term of essential oils, are 

 a number of plant products that resemble each other in that they are 

 all bodies possessed of a penetrating, generally pleasant aromatic 

 smell, but which differ very largely in chemical constitution. Strictly 

 speaking, the essential oils are not pure chemical substances, but 

 are more often mixtures of different oils. Some are true hydro- 

 carbons (terpenes), while many contain oxygen in their com- 

 position, and are ethereal salts, aldehydes, or phenols. The 

 essential oils are all readily volatile, and can be distilled with 

 steam at temperatures much below their boiling-points. They 

 are generally present in plants in only small quantities (less than 

 I per cent.) but are of great importance in certain animal food- 

 stuffs (hay) in imparting palatability. 



Some of the more important essential oils are the following : — 

 Terpenes (all of formula CjoHje). 



Pinene, in oil of turpentine, oil of eucalyptus. 



Limonene, in oil of orange, oil of lemon. 



Cymene, in oil of caraway, oil of eucalyptus. 

 Ethereal Salts. 



Linolyl acetate, CjoHj^. OOC. CHg, in oil of lavender. 



Allyl isothiocyanate, C3H5. CNS, in oil of mustard. 



Allyl sulphide, (C3H5)2S, in oil of garlic. 



Propyl acetate, CH3. COOC3H5r, in essence of pears. 



Ethyl butyrate, C3H7. COOCgH^, in essence of pineapple. 

 Aldehydes. 



Cinnamic aldehyde, CgHg. CH : CH . CHO, in oil of cinnamon. 



Benzaldehyde, CgHg. CHO, in oil of bitter almonds (see p. 43). 

 Phenols. 



Thymol, CioHjgOH, in oil of thyme. 



Carvacrol, C10H13OH, in oil of hops and oil of cloves. 



The essential oils are generally prepared by distillation of the 

 plants with steam. In the case of the more delicate perfumes 

 (rose, violet, orange-flower), the oil is extracted by macerating 

 with lard. 



