The crystals already mentioned as occurring in tar are Pyrocatechin. They are easily sublimed at some degrees above their fusing point (104° C.), or removed by acetic acid, in which as well as in water they are readily soluble. Hence in some sorts of tar this substance does not occur, it having probably been removed by water.
Pyrocatechin, C₆H₄(OH)₂, can be obtained by the destructive distillation of many other substances, as catechu, kino, the extracts of rhatany and bearberry leaves, and other extracts rich in that form of tannin which produces greenish (not blue-black) precipitates in salts of iron. It is extracted from the granular sorts of wood-tar, by exposing them at a proper temperature to a current of heated dry air, or by exhausting them with water. Ether when shaken with the concentrated aqueous solution and left to evaporate, leaves colourless crystals of pyrocatechin which after purification are devoid of acid reaction. They have a peculiar burning persistent taste, and are very pungent and irritating when allowed to evaporate. A solution of pyrocatechin yields with perchloride of iron a dark green coloration changing to black after a few moments, and becoming red on the addition of potash. This mixture finally acquires a magnificent violet hue, like a solution of alkaline permanganate. No alteration is produced in a solution of pyrocatechin by protosalts of iron.
Among the few medicinal preparations of tar, is Tar Water, called Aqua vel Liquor Picis, made by agitating wood-tar with water. The presence in it of pyrocatechin is easily proved by the above-mentioned reactions, or by a few drops of red chromate of potassium, which produces a brownish black colouration. It may hence be inferred that pyrocatechin is perhaps the active ingredient in tar-water, and that for making this liquid the granular, crystalline sorts of tar should be preferred.[2314]
Commerce—Tar as well as pitch is manufactured in Finland, and shipped from various ports in the Gulf of Bothnia, as Uleaborg, Gamla Carleby, Jacobstad, Ny Carleby and Christinestad; also from Archangel and Onega on the White Sea. Some tar is also produced in Volhynia, and finds its way by the Dnieper to the Black Sea.
The North of Sweden likewise produces tar, chiefly about Umea and Lulea, the distillation being now performed in well-constructed apparatus of iron.
The pine forests of North America afford tar and pitch. Wilmington in North Carolina exported in 1871, 25,260 barrels of tar, and 3788 barrels of pitch.[2315]
The imports of tar into the United Kingdom in 1872, were 189,291 barrels, valued at £218,339. Of this quantity 145,483 barrels were shipped from the northern ports of Russia.
The barrels in which tar arrives hold about 30 gallons. Smaller sized vessels termed half-barrels are also used, though less frequently.
Uses—In medicine of no great importance: an ointment of tar is a common remedy in cutaneous diseases, and tar water is sometimes taken internally. The consumption of tar in shipbuilding and for the preservation of fences, sufficiently explains the large importations.
Other Varieties of Tar.