The patent is concerned with the distillation of dried materials, the distillation being facilitated by injection of hot air into the retorts. Figures show apparatus for heating the air, consisting of a series of pipes, connected together and placed horizontally in the interior of the furnace on bricks arranged in such a way that the flames and smoke circulate, as shown, around each pipe, touching first all the surface of the large one placed in the center. The air enters this tube, and from it passes into the others which it runs through successively, coming finally into the last pipe, being heated in this journey to a very high temperature. The last tube ends in a bell from which different branches start, each supplied with stop-cocks, to lead this hot air into the different sections of the retort. With the stop-cocks the quantity of air can be regulated at will, in the compartment of the retort, for accelerating the operation more or less.