TERPENES AND THEIR DERIVATIVES. 595 



liquids obtained from plants, and having in common the properties 

 of being volatile, soluble in ether and alcohol, almost insoluble in 

 water, and having a distinct and in most cases even highly charac- 

 teristic odor. They stain paper as do fats or fat oils, from which 

 they differ, however, by the disappearance after some time of the 

 stain produced, while fats leave a permanent stain. 



The specific gravity of volatile oils ranges generally between 0.85 

 and 0.99. Being nearly insoluble in, and specifically lighter than, 

 water, they will float on it. The water, however, retains in most 

 cases enough of the oils to assume their odor (medicated waters). 

 Most volatile oils are optically active, turning the plane of polarized 

 light either to the right or left. While chemically pure oils are 

 colorless, many, even when freshly prepared, have a distinct color; 

 some are pale yellow, dark yellow, reddish or reddish brown, while 

 a few are green or blue. The oils generally darken with age, espe- 

 cially when exposed to light and air, the atmospheric oxygen acting 

 on them and converting the oils often into a sticky and resinous mass. 



Volatile oils are found in different parts of plants, and are the 

 principles imparting to the respective plants their characteristic odor. 

 The extraction of volatile oils from plants is accomplished generally 

 by distilling with water the vegetable matter containing the oil, the 

 oil passing over with the steam and floating on the surface of the 

 condensed water. In some instances mechanical pressure is used for 

 the separation, as in case of the oils of orange, lemon, bergamot, etc. 

 In other cases the oils are extracted by suitable solvents, or special 

 methods are used. 



In their chemical composition essential oils differ widely ; some are 

 compound ethers (oil of wintergreen is methyl salicylate), others are 

 aldehydes (oil of bitter almonds is benzaldehyde), but most of them 

 are hydrocarbons of the aromatic series, or mixtures of them, often 

 associated with oxygen derivatives, alcohols, phenols, ketones, alde- 

 hydes, esters, etc. 



Oil of doves. The principal constituent of this oil is eugenol, C 8 H S .C 3 H,. 

 OCH 3 .OH, a monatomic phenol ; a sesquiterpene, C 15 H 24 , culled caryophylleue 

 is also present. 



Oil of cinnamon consists chiefly of cinnamic aldehyde, C 6 H 5 CH.CHCOH, 

 a compound which has been prepared synthetically. Oil of cinnamon also 

 contains cinnamyl acetate, C 9 H 9 .C,H 3 O 2 , and a small amount of cinnamic acid 

 C 8 H 8 2 . 



Oil of peppermint. The oils found in the market differ widely from one 

 another, and there is no other volatile oil containing so many different con- 



