and 33 respectively each group of four ribs of the same length forms a four pointed star so that the passages opening into the bottom of the successive channels are distributed uniformly the passages 35 37 39 41 and 43 are not identical with each other their cross section is calculated so that the pressure drop introduced between the central dish 44 and the corresponding channel 29 to 33 is the same for all the channels uniform pressure and flow rate conditions are thus obtained for all the channels that is to say substantially for the whole surface of the separator very small differences are evened out by virtue of the permeability of the separator which acts as a diffuser the characteristic section of the channels 29 to 33 is sufficiently large not to perturb this uniformity it should be remembered in fact that the flow velocities in a chromatography apparatus are very low in the example described the passages 35 37 41 and 43 are simply drilled through the distribution plate 21 it would also be possible to provide calibrated constrictions fitted into pre drilled orifices this more complicated alternative form would make it possible to modify the pressure drops as a function of the working conditions in order always to obtain an optimum equalization of the.