COAL-TAR, or GAs-TAR. The thick, black, opaque liquid that comes over and condenses in the pipes when gas is distilled from coal. It is slightly heavier than water, and has a strong, disagreeable odor. Coal - tar is a mixture of many distinct liquid and solid substances, and the separation of the more useful of these con stitutes an important branch of manufacturing chemistry. By distilling from wrought - iron stills, the tar is first broken up into five frac tions, which are then further subjected to frac tional distillation separately: (I) Crude Naphtha or Light Oil is the frac tion distilling over before the temperature of the tar has risen to 170° C. This portion contains a number of valuable hydrocarbons, including benzene, toluene. xylene, etc. Another impor tant product obtained from this fraction is the so-called solvent or burning naphtha of com merce, which is largely used for burning in lamps. as a solvent for india-rubber and gutta percha, and for a variety of other purposes. The benzene obtained from this fraction is also used as a solvent, though most of it is converted into aniline, all of the vast amount of aniline manu factured at present being derived from nitro benzene, which is, in its turn, made from ben zene. To separate its constituents, the crude naphtha is first divided into three fractions by distillation; each of these fractions is washed successively with sulphuric acid and caustic soda, as well as with water, and subjected to further fractional distillation.
(2) Middle Oil or Carbolic Oil is the crude fraction distilling over from tar between the temperatures of 170° and 230° C. This fraction contains large quantities of naphthalene and carbolic acid, the former separating out in the form of a crystalline mass, while the latter re mains liquid. The naphthalene thus obtained is purified by washing with caustic soda and sulphuric acid, and distilling. On the other hand, the crude liquid is treated with caustic-soda so lution, which takes up all of the carbolic acid and from which the latter is separated by adding sulphuric acid; the impure carbolic acid thus obtained is further purified by distillation. Naphthalene is extensively used in the manufac ture of colors. Carbolic acid is extensively used as a disinfectant and for the manufacture of picric acid.
(3) Creosote Oil is the crude fraction distilling over from coal-tar between the temperatures of 230° and 270° C'. This somewhat heavy oil is largely used for the preservation of timber. See