The tar obtained from Newcastle coal-tar is specifically heavier than that produced from cannel-coal; hence it sinks in water, whereas the latter swims on the surface of that fluid.
To render the tar fit for use, it requires to be evaporated to give it a sufficient consistence. If this process be performed in close vessels, a portion of an essential oil is obtained, which is known to colourmen by the name of oil of tar. To obtain this oil, a common still is filled with the coal-tar, and, being properly luted, the fire is kindled and kept up very moderate, for the tar is very apt to boil up in the early part of the process. The first product that distils over is principally a brown ammoniacal fluid, mixed however with a good deal of oil. As the process advances, and the heat is increased, the quantity of ammoniacal liquor lessens, and that of oil increases, and towards the end of the distillation the product is chiefly oil.
The oil and ammoniacal water which distil over do not mix, so that they may be easily separated by decantation. The oil is a yellowish inferior kind of oil of turpentine, which is very useful in painting ships, for making varnishes, and other coarse out-door work.
Two hundred pounds of tar produce, upon an average, fifty-three pounds of essential oil.
If the coal-tar is wanted to be converted into pitch, without obtaining the oil which it is capable of furnishing, the evaporation of it may be performed in a common boiler; but as it is extremely liable to boil over, the greatest precaution is necessary in conducting the evaporation. A boiler constructed on the following plan is very convenient for the conversion of coal-tar into pitch. The contrivance consists in adding a spout, or rim, to the common boiler, into which the tar spreads itself as it rises, and by this means becomes cooled, and the boiling over is checked.
Kettle for boiling Tar.
1000lb. of coal-tar produce, upon an average, from 460 to 480lb. of pitch. A subsequent fusion, with a gentle heat, converts the coal-pitch into a substance possessing all the characters of asphaltum.
Ammoniacal Fluid.—The properties of the ammoniacal liquor, which accompanies the tar, and which is deposited in the tar-cistern, has not yet been fully investigated. It is employed already in the manufacture of muriate of ammonia (sal ammoniac). One chaldron of coal affords from 220 to 240lb. of this ammoniacal fluid, which is composed chiefly of sulphate, and carbonate of ammonia.—Such are the products obtainable from coal.
However certain the practicability of extending the new lights to the dwelling houses of every town and village is, it cannot be expected that such an event should take place speedily and generally. To eradicate prejudice, and to alter established habits, is a work which nothing but time can effect; because prejudice is the effect of habit, and can seldom be eradicated from the minds of such individuals as consider the ready occurrence of a proposition as a test of its truth. To establish a new philosophical theory has, in every instance, required time sufficient to educate an entire generation of men. The rejection of the Aristotelian philosophy—the adoption of experimental research—the substitution of the doctrine of gravitation instead of that of vortices, and the rejection of phlogiston by modern chemists, are sufficiently illustrative of this assertion. New arts, and new practices, are still more difficult to be introduced. The new art of bleaching need merely be mentioned to prove this assertion. The new grammar—the new rudiments of science—the new stile—or the new instrument, however superior to the old in simplicity, facility, and truth, must be less valuable to the ordinary teacher or artisan, whose memory is familiarized with the precepts of the latter, and whose only ambition is to earn his subsistence with the least possible exertion.