COAL-TAR

Coal-tar, one of the products of the destructive distillation of the coal employed in the manufacture of gas, is a very complex substance, consisting of various hydrocarbons, acids, and bases, together with certain resinoid and empyreumatic substances. The principal hydrocarbons yielded by coal-tar on distillation are: benzol, toluol, propyl, naphthalin, and anthracin; of these the first three are fluids, and the last two solids; the most important acids are: carbolic, cresylic, phlorylic, and nosolic; the chief bases are: aniline, chino line, and lepidine. The quantity as well as the quality of the tar obtained from the distillation of coals varies considerably with the kind of coal used, as well as with the temperature at which the distillation is carried on, the yield of tar being smaller at very high temperatures than when lower ones are employed. Coal-tar, from its antiseptic properties (due chiefly to the carbolic acid it contains), is painted on wood to preserve the latter from decay when exposed to wind and weather. Mixed with coal-dust, saw-dust, and peat-dust, it forms a useful artificial fuel, and when incorporated with pebbles makes an excellent artificial asphalt for pavements. The chief value of coal-tar, however, consists in its being the source of those brilliant dye-stuffs, the coal-tar colours. These, together with the naphtha obtained from its distillation, have converted coal-tar from a worthle and unwelcome waste product of gas manufacture—for the removal of which from their premises the gas makers were formerly only too glad to pay—into a very considerable and important branch of profit and revenue. The different constituents of the tar are separated from each other by distillation, the various products so obtained being further purified by various proce es. See Tar Colours , Naphtha , Benzol , Anthracene , .