True vegetable tar has always a decidedly acid reaction. It is readily miscible with alcohol, glacial acetic acid, ether, fixed and volatile oils, chloroform, benzol, amylic alcohol or acetone. It is soluble in caustic alkaline solutions, but not in pure water or watery liquids. The sp. gr. of tar from the roots of conifers is about 1·06 (Hessel) yet at a somewhat elevated temperature, it becomes lighter than warm water.
Water agitated with tar acquires a light yellowish tint, and the taste and odour of tar, as well as an acid reaction. On evaporation the solution becomes brown, and at last microscopic crystals are obtained with a brown residue like tar itself, which is no longer soluble in water. A microscopical examination of tar which has been exhausted with water, shows that all crystals have disappeared.
Chemical Composition—Dry wood may be heated to about 150° C. without decomposition; but at a more elevated temperature, it commences to undergo a change, yielding a large number of products, the nature and comparative quantity of which depend upon circumstances. If the process is carried on in a closed vessel, a residue will be got which has more or less resemblance to coal. By heating fir-wood enclosed with some water to 400° C., Daubrée (1857) obtained a coal-like substance, which yielded by a subsequent increase of temperature scarcely any volatile products.
The results are widely different if a process is followed which permits the formation of volatile bodies; and these substances are formed in largest proportion, if the heat acts quickly and intensely. At lower degrees of heat, more charcoal results and more water is evolved.
Among the volatile products of destructive distillation, those alone which are condensed at the ordinary temperature of the air are of pharmaceutical interest; and of these, chiefly the portion not soluble in water, or that which is called Tar or Liquid Pitch. The aqueous portion of the products consist principally of empyreumatic acetic acid, to which tar owes its acid reaction.
The tissue of wood is chiefly formed of cellulose, intimately combined with a saccharine substance, which may be separated if the wood is boiled with dilute acids. The remaining cellulose is however not yet pure, but is still united to a substance which, as shown by Erdmann,[2313] is capable of yielding pyrocatechin.
It is well known that sugar subjected to an elevated temperature, yields a series of pyrogenous products; and the same fact is observed if purified cellulose is heated in similar manner. But for tar-making, wood is preferred which is impregnated with resins and essential oils, and these latter furnish another series of empyreumatic products. From these circumstances, the components of wood-tar are of an extremely complicated character, which is still more the case when other woods than those of conifers form part of the material submitted to distillation. In the case of beech-wood, Creasote is formed, which is obtained only in very small quantity from the Coniferæ. Volatile alkaloids and carbolic acid, which are largely produced in the destructive distillation of coal, appear not to be present in wood-tar.
The components of the latter may be considered under two heads:—first, the lighter aqueous portion, which separates from the other products of distillation, forming what is called Impure Pyroligneous Acid. This contains chiefly acetic acid and Methyl Alcohol or Wood Naphtha, CH₄O; Acetone, C₃H₆O; besides other liquid products abundantly soluble in water and acetic acid. In this portion, some pyrocatechin also occurs.
The second class of pyrogenous products of wood consists of a homologous series of liquid hydrocarbons, sparingly soluble in water, and which therefore are chiefly retained in the heavy layer below the pyroligneous acid, forming the proper wood-tar. The liquid in question furnishes Toluol or Toluene, C₇H₈ (boiling point 114° C.), Xylole C₈H₁₀, and several other analogous substances.
If tar is redistilled, an elevated temperature being used towards the end of the process, some crystallizable solid bodies are obtained, the most important of which is that called Paraffin, having the formula CₙH₍₂ₙ₊₂₎, n varying from 20 to 24.