Other products obtainable from Coal, namely:—Coal Tar—Pitch—Coal Oil—Ammoniacal Liquor, and conversion of the latter into Carbonate, and Muriate of Ammonia.
Coal Tar.
The coal tar is so called from its resembling common tar in its appearance, and most of its qualities.
This substance is deposited in the purification of the coal gas, in a separate vessel destined to receive it. See fig. 3, [plate I.]
In the year 1665, Becher, a German chemist, brought to England his discovery for extracting tar from coal, this distillation he performed in close vessels. It is not mentioned in the records of the time, whether Becher obtained, or rather collected, any other articles than the tar.
Several works have been, at different times, erected both in England and on the continent, to procure from coal a substitute for tar; but they have turned out unprofitable speculations.
In 1781, the Earl of Dundonald invented a mode of distilling coal in the large way, which enabled him not only to form the coke, but, at the same time, to save and collect the tar. Even this process, however, for which a patent was taken out, gained very little ground. Its object was too limited; for though some of the proximate constituent parts of coal were procured, they were obtained at an expence that nearly balanced the profits; and no attention whatever was paid to the coal gas, which constitutes by far the most valuable part obtainable from pit coal.
Coal tar is now used with advantage largely in the Royal Navy, and also for painting and securing wood that is exposed to the action of air. The wood being warmed, the tar is applied cold, and penetrating into the pores, gives the timber an uncommon degree of hardness and durability.
The quantity of tar obtainable from a given quantity of coal, varies according to the manner in which the decomposition of the coal is affected. See [page 122].