Abstract: In a lock for the transport of bulk material between chambers being under different pressure a lock chamber (4) is guided tightly and driven by a reciprocal movement in a preferably horizontal guide tube (2), which comprises on different sides of its wall a first opening (12) leading to the chamber with lower pressure and in an axial distance therefrom a second opening (15) leading to the chamber with higher pressure, wherein the chamber comprises openings (13, 14), which can be brought in coincidence with the first (12) and second (15) opening at the points or reversal of the reciprocal movement. Adjacent to the lock chamber (4) in axial direction is arranged a slide (3a) being tightly guided in the guide tube (2), which slide slides over the second opening (15).