Source, Etc

Tar or, as it is better termed, wood-tar (Pix liquida), to distinguish it from coal-tar (Pix carbonis), is a bituminous liquid obtained from the wood of Pinus sylvestris, Linne (N.O. Coniferoe), and other species of Pinus by destructive distillation.

Various methods have been adopted for the destructive distillation of wood. The simplest consists in constructing a mound of the material and covering it with earth, leaving spaces by which air can be admitted and through which the tar produced can drain off. The wood is fired, and, the heat being carefully controlled, charcoal is left as a residue in the mound, whilst a tarry and an aqueous liquid are collected.

More modern apparatus consists of either upright or horizontal iron retorts arranged singly or in a battery of about twenty. These are heated from without, and the products of distillation may be ranged under three heads:

(i) Gaseous products analogous to coal-gas; these are conducted back to the hearth and burnt, serving to economise fuel, or they are stored in a gasometer until required for some such purpose.

(ii) Watery products; water containing acetic acid, methyl alcohol, acetone, etc.; these pass over and are condensed. They serve as a source of acetic acid, methyl alcohol, etc.

(iii) Tarry products; these separate from the watery liquid on standing.

Coniferous wood is chiefly employed, as it yields more tar than the wood of foliage trees (beech, birch, &c). From the former 15 to 20 per cent. of tar may be obtained, from the latter 6 to 8 per cent. For further details see Thorpe, 'Dictionary of Applied Chemistry': Wood, Destructive Distillation of.

Description

Tar is a dark-brown or blackish, semi-liquid substance, with a peculiar aromatic odour and a bitter, pungent taste; it is heavier than water, its specific gravity varying from 1.02 to 1.15. By keeping, it becomes thicker, and acquires a granular appearance, due to the formation of minute crystals (probably pyrocatechin, resin acids, &c). Water shaken with it acquires a yellow colour and acid reaction (distinction from coal-tar, which imparts to water an alkaline reaction). The filtered aqueous liquid is coloured red by very dilute (0.1 per cent.) solution of ferric chloride, (distinction from juniper-tar oil). Tar is completely soluble in ten volumes of alcohol.

Constituents

The following are the chief constituents of tar: benzene, toluene, xylene, and styrolene (styrol); phenol, cresol, guaiacol and its homologues; pyrocatechin and paraffin. The most important of these, as far as the medicinal activity is concerned, are probably pyrocatechin, phenol, cresol, guaiacol, and their homologues.

Tar from Coniferous woods, which alone is official, is especially rich in guaiacol and its homologues, beech-tar in guaiacol and pyrogallol derivatives, birch-tar in guaiacol and benzophenol derivatives.

Uses

Tar is used as an external stimulant and antiseptic in certain skin diseases; given internally in the form of pills or syrup, it acts as a disinfectant and deodorant of offensive discharges from the bronchial tubes.