Stills of waggon-shape with side flues are employed; the legs are sheltered from the direct furnace heat, and apparatus for injecting steam into them is provided. The feed pipe to the primary still is vertical; as the more volatile portions of the oil evaporate the heavier portions sink into the legs from which they are drawn off by pipes entering secondary stills horizontally. In the same manner the heavier portions are removed from these stills to coking ovens. A feed-heating apparatus of the kind described in Specification No. 540, A.D. 1883, is employed.