X. THE CHEMICAL CONSTITUENTS OF PLANTS 201 



The essential oils are, as a rule, very slightly soluble in water, but 

 sufficiently so to impart to the water a strong odour. They are 

 generally readily soluble in or miscible with ether, chloroform and 

 most organic solvents. 



The density is usually less than that of water though some are 

 heavier than that liquid. Most of them are optically active, and have 

 a high refractive power. Chemically they show great variation. As 

 a rule, the essential oil derived from any one source is a mixture of 

 several compounds and'the relative proportions of the constituents are 

 liable to considerable variation. 



In the essential oils, many chemical compounds of very different 

 constitution have been detected. 



They may be divided into the following groups : 



1. Terpenes. 



2. Camphors. 



3. Benzene derivatives. 



4. Aliphatic or open chain compounds. 



The chemistry of the essential oils is, however, too complicated to 

 allow of anything but the merest sketch being given here. 



1. Terpenes. These are the most abundant constituents in many 

 essential oils. They are, with the exception of camphene, volatile liquids 

 of the empiric formula C 10 H 1f) . Many isomeric bodies of this com- 

 position exist, differing greatly in odour, density, optical activity and 

 other properties. Some of the best known are 



Pinene, which occurs as a dextro-rotatory form in American turpen- 

 tine and as a laevo-rotatory body in French turpentine. Its specific 

 gravity is about 0'85 and it boils about 155. Its probable constitution 

 is thought to be 



C- CH, 



Limonene is a large constituent of many essential oils. It occurs 

 as dextro-limonene in oils of orange, lemon, bergamot, dill, etc., and 

 ;as laevo-limonene in Russian peppermint oil and oils of certain pines. 

 Its specific gravity is about 0-844, it boils at 177 to 188 and has a 

 rotation of about + 105. An inactive form is known as dipentene. 

 Its probable constitution is 



C-CH 3 



HC CH 2 



I I 

 H.,C CH.} CHg 



' \/ / 

 CH C=CH., 



