CARBAZOLE, a constituent of coal tar, occurring with crude anthracene, crystallizing in plates or tables, melting point 238° C, and readily subliming. It may be separated from crude anthracene by fusing it with caustic potash when it is converted in to carbazole-potassium, which can be easily separated by distilling off the anthracene. The separation is, however, preferably effected on a large scale by extracting the crude anthracene with heavy coal tar pyridine (b.p. 14o-18o° C) in which solvent the carbazole dis solves more readily than anthracene. Carbazole, may be prepared synthetically by passing the vapours of di phenylamine or aniline through a red-hot tube ; by heating di orthodiaminodiphenyl with 25% suphuric acid to 200° C for 15 hours ; or by heating thiodiphenylamine with copper powder. It is also obtained as a decomposition product of brucine or strych nine, when these are distilled with zinc dust. Carbazole dissolves readily in the common organic solvents. Melted in combination with oxalic acid it gives carbazole blue. The potassium salt reacts with the alkyl iodides to give N-substituted alkyl derivatives. It gives the pine-shaving reaction, in this respect resembling pyrrole (q.v.) . The chief importance of carbazole lies in the production of hydrone blue (Alizone blue), a vat dye which in many respects rivals indigo. This dye is prepared by condensing carbazole with nitrosophenol in concentrated sulphuric acid. The resulting indophenol is then heated with alcoholic sodium tetrasulphide. Greener shades of hydrone blue are obtained from the N-alkyl carbazoles.