A method wherein the particles are homogeneously distributed like rain to constitute a horizontal bed of particles on the entire section of the chamber (1). The invention is characterised in that it consists in temporarily modifying at least once the loading conditions, so that the profile of the surface (8) of the bed of particles forms, at least at the periphery thereof, an angle &agr; with the horizontal ranging between 6° and 25°, preferably, between 10° and 20°, and in subsequently restoring the loading conditions to obtain a horizontal bed.