VOLATILE OR ESSENTIAL OILS AND RESINS. 883 



water, and they dissolve only in the latter when pure and free 

 from acid or earthy salts. (Berzelius, Traite II. 367 and 467.) 



By boiling it with an excess of caustic alkali, pectic acid is 

 again modified, and converted into another acid, which dissolves 

 easily, is very fluid and has a sour taste. This second acid is 

 also isomeric in its salts with pectin. 



SECTION II. 



VOLATILE OR ESSENTIAL OILS AND RESINS. 



These oils or essences occur in all smelling plants, and are 

 the source of the fragrance of the vegetable kingdom. Some 

 plants, such as thyme, contain a volatile oil in all their parts ; 

 in others the oil is confined to particular parts, such as the 

 flower, the pollen, the leaves, the root or the bark. In most 

 plants the oil is contained in little sacs or vesicles, so well 

 closed that the plant may be dried without evaporation of the 

 oil, and the latter is preserved for years from the influence of 

 the air. In other species, and particularly in flowers, the oil is 

 constantly produced at the surface, and dissipated by eva- 

 poration the moment of its formation. Essential oils are gene- 

 rally obtained by distilling the plant with water. They are 

 themselves less volatile than water, but are carried over with the 

 steam, owing to the sensible tension of their vapour at 212, and 

 condensing in the refrigeratory are found on the surface of the 

 distilled water or at the bottom of the vessel. A few oils are 

 obtained by expression, such as those of the oranges and lemons, 

 where the oil resides in the epidermis of the fruit. Some other 

 oils which are not contained in vessels, such as those of 

 violet, jasmine, &c. are obtained by maceration of the flowers in 

 an inodorous fixed oil, and are used in this state in perfumery, 

 or are afterwards obtained apart by distilling the fixed oil with 

 water. 



The essences are generally liquid, but occasionally solid at the 

 usual temperature ; they have all a strong odour, more or less 

 agreeable, which is somewhat harsh immediately after distil- 

 lation but improves with keeping, although in general the odour 

 is never so agreeable as that of the fresh plant. Their taste is 

 acrid and burning, or only aromatic when it is greatly weakened 



2 M M 2 



