The tar obtained from Newcastle coal 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 coal 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 by the name of
Coal Oil.
To obtain this oil, a common still is charged with 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 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 naptha, which is very useful in painting ships, and for making common varnishes. It has lately been employed as a substitute for whale oil, to be burnt in out door lamps.
The contrivance by means of which this oil is burnt in lamps[61] consists of a fountain reservoir to supply and preserve a constant level. The burner with its wick is placed in the axis of the lamp, and supplied with the oil from the fountain reservoir, placed on the outside of the lamp. The air is admitted by an aperture at the bottom of the lamp. The current of air in passing through the lamp envelopes the burner and urges the flame, which is extremely bright; but it is essential that the flame should be small. The draught tube proceeding from the centre of the reflector above the flame carries away the smoke.
[61] All the lamps on Waterloo Bridge, and the streets adjoining the bridge are lighted by means of tar oil.
1430 pounds of coal tar, produce 360 pounds of essential oil. The residue left after the distillation is
Pitch.
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 spout or rim is added 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.