Coal Tar

Coal Tar. The tar obtained during the manufacture of coal gas (q.v.) is a thick viscid black liquid, which was formerly used only for tarring wood, etc., and was of no commercial value. By distillation, however, it was found a number of products were obtained which were classified as light, intermediate, and heavy oils. The light oil was used as an illuminant, and the heavy or creosote oil was employed for "creosoting" timber to prevent decay. At the present day, however, the coal tar is chiefly valuable as being the primary source of the large class of compounds derived from benzene, and especially of the important and ever-increasing group known as the "coal-tar colours." The first step towards this was the discovery of benzene itself in coal tar by Hoffman in 1845, and the preparation in 1856, by Perkin, of mauveine, the first aniline dye. Since that time colouring matters of every variety of colour and shade have been added to the list, the most important addition, perhaps, being that of alizarin (q.v.), the colour principle of the madder. Naphthalene (q.v.) and anthracene (q.v.) were also discovered in the previous waste product, and with their derivatives form an important class of commercial substances. The colour industry is not the only one affected by the study of coal tar. To it we owe, among many others, the well-known pharmaceutical preparations kairine and saccharin, and the photographer is indebted for his isochromatic plates and the developers hydroquinone and eikonogen. After the distillation of the volatile products from the tar, a black substance, pitch, is left behind, used largely for preparation of asphalte, varnishes, patent fuel, etc.