Tar

Tar, a product of the destructive distillation of organic substances intimately related to the natural asphalts and bitumens (q.v.). There are two chief varieties, coal-tar (q.v.) and wood-tar. Wood-tar is a dark-brown or black, semifluid, pungent, acid substance, consisting of a mixture of heavy, non-volatile hydrocarbons. It is mainly obtained from the roots and stools of Pinus sylvestris in Northern Europe, where it is known as Stockholm and Archangel tar, and from those of P. palustris in the Southern United States. It is extracted by a slow combustion of the wood when covered with turf; or more economically in closed retorts, for which the gas evolved serves as fuel. In the latter method roots yield 16 to 20 per cent. of tar, and on further distillation tar yields wood vinegar, acetic acid, creosote, and oil of tar leaving a residue of the black, brittle, glossy solid known as pitch. Tar and pitch are largely used for the protection of shipping and other timber from the weather, and the latter also in Berlin or Brunswick black. Tar contains also some pyrocatechin, which is soluble and gives to tar-water the slight medicinal birtue which was so enthusiastically overstated by Bishop Berkeley (q.v.).