272 POPULAK ECONOMIC BOTANY. 



The Essential Oils differ materially from the fixed 

 oils already described, both in their characters, and also in 

 the methods by which they are obtained. As a group, they 

 are again divisible into three distinct classes, distinguish- 

 able by their chemical characters. Thus, the first class 

 are simple compounds of carbon and hydrogen (C H) ; by 

 chemists they are termed non-oxygenated oils. They are 

 very inflammable, burning like coal-gas, of which they 

 appear to be a mere concentration. Although they con- 

 tain no oxygen when pure, if exposed to the atmosphere 

 they gradually absorb it, and are converted into resins. 

 They are very volatile, and rapidly decrease by evaporation 

 at the ordinary temperature ; if the temperature be raised, 

 they evaporate entirely. They are nearly all obtained by 

 distillation. 



The Oil or Spirit of Turpentine, distilled from common 

 turpentine, is the type of this class. They are all odoriferous, 

 usually having the odour of the material from which they 

 are procured. Besides turpentine, the other non-oxygenated 

 oils met with in commerce are — 



Oil or Essence op Lemons. — This oil exists so 

 abundantly in the cysts which are found in great numbers 

 in the fiavedo, or yellow portion of the rind of the fruit of 



