Effecting the separation of the various products of the distillation of coal, shale, etc., by taking advantage of the graduated temperature of successive portions of the length of mechanical retorts is described. The substances entering from the hopper are gradually heated in their progress and give off a series of products in their order of volatilization, which pass from openings in the retort through a series of ascension pipes into collecting-vessels. The vessels are designed to contain different portions of the distillate and are sealed against the escape of uncondensed vapor or gas by the condensed liquid. Each of the ascension pipes communicates above its vessel with a common pipe to convey away permanent gases. The flues for heating the retort may be so arranged as to give the greatest heat at the end farthest from the point of entrance, or the stages of heat may be self-regulated by the time necessary for the material to acquire heat as it travels. If necessary the pipes may be fitted with refrigerating-appliances.