ESSENTIAL OILS. 141 



extent of this traffic may be imagined when it is known that in 1817 

 the importation of palm-oil into England did not much exceed 

 140,000 lbs., and that in 1836 it exceeded 70,000,000 lbs ! In tak- 

 ing an acre of surface for unity, I find that on an average the 



Spring oleaginous plants yield 3201bs. of oil. 



Winter oleaginous plants 534 " 



The olive (south of Europe) 534 " 



The Palm (America) 801 " 



OF ESSENTIAL OILS. 



Aromatic plants owe the odors which characterize them to 

 certain volatile principles, which by reason of certain properties 

 which they have in common with fat oils, such as insolubility in 

 water, solubility in ether and alcohol, inflammability, &c., are gen- 

 erally designated as essential oils. They are met with in all parts 

 of plants ; but in one plant the oil is principally found in the flower, 

 in another in the leaves, in another in the bark,, <fec. It sometimes 

 happens that diflferent parts of the same plant contain oils of different 

 kinds. From the orange-tree, for instance, three distinct oils are 

 obtained, as the flower, the leaf, or the rind of the fruit is treated. 

 In some cases the volatile principle is so thoroughly imprisoned in 

 the vegetable cells, that drying does 'not dissipate it; in others, as 

 in the greater number of flowers, the oil is formed on the surface, 

 and is volatilized immediately after its formation. 



Essential oils are less volatile than water ; nevertheless they rise 

 with the vapor of water, and it is by distillation that they are gen- 

 erally extracted. The plant is put into a still or alembic containing 

 water, and heat is applied : the vapor formed is condensed in the 

 receiver, and the essence, by reason of its less density, is found 

 swimming on the surface of the water which has been distilled. 

 Some volatile oils are obtained by pressure, those of the citron and 

 bergamotte, for example. 



The volatile principles of plants present somewhat varied physical 

 properties. They are generally limpid and lighter than water ; yet 

 there are some which are more dense, and some, such as camphor, 

 which are solid. With reference to their composition, volatile oils 

 may be divided into three classes ; 1st. Oils composed entirely of 

 carbon and hydrogen. 2d. Oils composed of carbon, hydrogen 

 and oxygen. 3d. Essential oils containing sulphur ; in addition to 

 which, the essential oil of mustard seed contains azote. 



The essential oils undergo a change by long contact with the air : 

 they absorb oxygen, an i many of them become acidified ; under the 

 influence of this gas, the oil of bitter almonds is changed into ben- 

 zoic acid, the oil of cinnamon into cinn-amic acid ; in a general 

 way, acetic acid is produced. The volatile oil obtained from any 

 plant almost always contains two distinct principles, which may be 

 separated by careful distillation ; one of these principles is a car- 

 buret of hydrogen, the other an oxygenated oil. Camphor is com- 

 bined vyith essential oils in many plants of the labiate family. It 



