Abstract: A substance to be treated for the production therefrom of useful products (usually decomposition products) is formed into a bed on a perforate support, the bed having a thickness of up to about one inch. An inert atmosphere is maintained above and in contact with the bed in a substantially closed space of constant volume. The bed is heated to progressively increasing temperatures by means of a radiant heat source located in the closed space at a distance of up to one and one-half inches above the upper surface of the bed. As the bed is heated, liquid products are educted from the substance and are removed by the force of gravity from the bottom of the bed through the perforate support.