When coal-tar is heated in the still, there is a large amount of frothing due to the distillation of the ammoniacal liquor; it is therefore necessary to slake the fire to prevent this; if any tar boils over, pour water on the still head. After a time the mass " bumps" vigorously and then "rattles," owing to the escape of the water. As the water distils over, the first runnings pass along with it and float on the surface of the liquid. After all the water has distilled over, the "break" occurs, i.e. distillation slackens until the temperature rises and the distillation proceeds again. The different fractions are told by the smell, by gravity, and by the temperature in the still at the time, a thermometer being fixed in the still for this purpose. The first runnings pass over below 110° C., and their specific gravities are less than l.0. The light oil passes over from 110" to 120° C, and its gravity is about that of water = l.0. Carbolic oil or middle oil passes over between 120° and 140° C, and its gravity is over 1.0. Creosoteoilpasses over between 140' and 170° C.; it is heavier than carbolic oil. Anthracene oil passes over last. The residue is pitch, which is soft or hard, according to how far the distillation has been pushed.

The different fractions can be told by pouring a few drops of the oil into water; the first runnings float on the surface, the light oil will float anywhere in the water, whereas the carbolic and other oils sink; this test will tell when to change the receivers, but the temperature test is best. The first runnings and light oil are rectified by distillation with " close " or "open" steam yielding - (1) Up to 103° C , 65 to 70 per cent, benzol; (2) up to 1100C, 30 per cent, benzol; and (3) up to 130° C, a benzol none of which distils at 100' but 60 per cent, passes over at 120° C, this being usually put back with another charge; and (4) above 1360C. yields " solvent" naphtha. The 65 to 70 per cent, benzol is again rectified into two fractions called 90 per cent, benzol and 50 per cent, benzol respectively.