Tar
a product of the destructive distillation of various organic substances; but the tars of commerce are obtained 1st from the distillation of coal, etc., for gas (gas tar or coal tar), and 2d from the distillation of wood (wood tar). Gas or coal tar, which was formerly regarded as a troublesome and almost useless byproduct product of the gas manufacture, is now a substance of so much value that it is second only in importance to the gas itself. Its value has arisen almost entirely from the fact that it is the source of the wide range of important dyeing substances, which, derived from aniline, phenol (carbolic acid), and anthracene respectively, may all be classed as tar colors. Coal naphtha obtained by distillation from coal tar is a mixture of several hydrocarbons of different degrees of volatility, the lightest being benzol and toluol, and the earlier part of the distillate is composed of a mixture of these two, and that part is utilized for the preparation of aniline as the basis of the aniline colors. The material remaining in the retort after the light oils are dis- TAR tilled over constitutes artificial asphalt, but this on further distillation at a higher heat gives off "heavy oils," leaving in the retort pitch, a substance which when cold is hard, black, and shining, and breaks with a glassy fracture. The principal constituents of the heavy oil are carbolic acid (phenol), cresylic acid and in the later stages of the distillatory process anthracene is obtained. All these substances are industrially important.
Wood tar is obtained as a by-product in the destructive distillation of wood for the manufacture of pyroligneous acid (wood vinegar), and methyl alcohol (wood spirit). Wood tar has several constituents in common with coal tar; its most characteristic fluid ingredients being hydrocarbons with methylic acetate, acetone, eupione, and creosote, with solid resinous matters, paraffin, anthracene, chrysene, etc. It possesses valuable antiseptic properties, chiefly owing to the creosote it contains, whence its principal value for most purposes is derived. In addition to its various uses in the arts of coating and preserving timber and iron in exposed situations, and for impregnating ships' ropes and cordage, it has various applications for external use in medicine owing to its antiseptic properties.