VEGETABLE SUBSTANCES. 129 



is of a gelatinous consistence, by distilling off the 

 alkohol. 



Fixed oils are mucli used for painting, as they 

 are of a drying nature : they are rendered still 

 more drying by boiling them with the oxides of 

 metals, as litharge. 



Volatile oil. — This is also called essential oil. 

 Many vegetables afford essential oil by expression, 

 or by distillation, When dissolved in water they 

 constitute perfumed essences and distilled waters. 

 They have much odour and taste. They are in- 

 flammable. They are volatilized by a gentle heat, 

 and evaporate entirely when pure so as to leave no 

 trace. The chief essential oils are, the oils of tur- 

 pentine, spike, cloves, oranges, lemons, lavender, 

 &c. Many of them bear a high price. 



Resin. — The resins are an important class of 

 vegetable substances from their application in the 

 arts. They are very numerous, and often exude 

 spontaneously from trees. Common resin is obtained 

 from the^r ; a juice exudes from this tree, which is 

 common turpentine: this consists of the oil of turpen- 

 tine and resin. When the essential oil is separated 

 by distillation, the resin remains. Mastich is a resin 

 obtained from a tree that grows in Turkey. San- 

 darach is the resin of a tree in Barbary. Copal is 

 a resin from a tree that grows in America, and is 

 a very valuable substance for varnishes. Lac is a 

 resin made by an insect in the East Indies. It is 

 very useful in varnishes, and in sealing-wax. Am- 

 ber is a substance resembling in its properties the 

 resins, but it is only found in the earth, or washed 

 out and driven on the shores. All the resins are 

 insoluble in water, but soluble in alkohol, especially 

 when assisted by heat. The greater number are 

 soluble in the essential oils, and some are so in the 



VOL, II. K 



