21:2 TAR. 



In France, the wood is generally distilled in large 

 brick ovens. Tar differs from common turpentine in 

 consequence of the mode in which it is obtained ; it 

 is darker colored, and contains less volatile oil, as a 

 good deal of the volatile oil is injured and decom- 

 posed, during the extraction of the tar, by the heat 

 of the fire. 



536. When tar is heated strongly and boiled, the 

 volatile oil which it contains is gradually driven off; 

 there then remains a brittle, black, shining, fusible 

 solid, called pitch. 



537. There is another variety of tar, which is ob- 

 tained during the manufacture of coal gas, and 

 formed by the destructive distillation of bituminous 

 coal. This substance resembles vegetable tar in na- 

 ture and properties. It contains a volatile oil called 

 naphtha, or coal-tar oil ; and when this substance is 

 driven off by distillation, there remains a kind of pitch. 

 Coal naphtha resembles the native rock oil, or mineral 

 naphtha ; it is very combustible, and forms a valuable 

 solvent for caoutchouc and some of the resins. It 

 consists of hydrogen and carbon — the same elements, 

 therefore, as oil of turpentine. 



538. Most of the simple resins, like common rosin 

 or colophony, the residue of the distillation of tur- 

 pentine, are brittle, fusible, and very combustible 

 solids, which easily dissolve in alcohol, or pyrox- 

 ylic spirit ; but are quite insoluble in water. They 

 are found to consist of mixtures of several distinct 

 acid substances. When resin is boiled in an alkaline 



