CONSTITUENTS OF PLANTS, ETC. 321 



when subjected to this process. They are these: 1. Coke, 

 which is nearly pure carbon ; 2. Illuminating gas ; 3. Tar- 

 water, a watery empyreumatic liquid containing some am- 

 monia ; 4. Coal-tar, a dark, viscid liquid. This process of 

 dry distillation of bituminous coal is employed for the pro- 

 duction of the gas so much used for illumination. The 

 coke which is left in the retorts of the gas-works is a valu- 

 able article of fuel. The ammonia in the tar-water is the 

 chief source of the commercial article ; it is derived from 

 the nitrogen, which is always present in coal in small quan- 

 tities, uniting with the hydrogen during the distillation. 



Coal-tar is used for covering roofs, to protect them from 

 moisture, and, mixed with chalk and other substances, it is 

 employed as a cement. A great variety of products can 

 be obtained from it. By distilling it we can obtain two 

 oils, one of them a light oil, called benzol, which can be- 

 used for many purposes in place of spirits of turpentine ; 

 the other a heavy oil, used in the arts for lubrication and 

 for dissolving India rubber, and also sometimes for illumi- 

 nation. 



Benzol is one of the large class of bodies called hydro- 

 carbons, so many of which are found in petroleum. Its for- 

 mula is C 6 H 6 . It is a colorless, volatile liquid, having a 

 low boiling-point. By treating it with nitric acid it is con- 

 verted into nitro-benzol C 6 H 5 (NO 2 ) commonly called ar- 

 tificial oil of almonds, from its odor, which resembles bitter 

 almonds. It is much used in perfumery. Aniline is made 

 from benzol, and is the basis of many of the beautiful dye- 

 stuffs which have been introduced of late years. These 

 dyes are called aniline colors, or simply coal-tar colors; 

 their manufacture is interesting, but too complicated to 

 give here. Aniline is also a constituent of the heavy oil 

 mentioned above. This heavy oil also contains naphtha- 

 line, a solid hydrocarbon also yielding dye-stuffs, and car- 



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