illustrate the fact that the bottom can also be contoured to a certain extent thus the bottom 50 illustrated in fig3 for example comprises a number of sloping sections 52 which slope relative to the plane 51 of the bottom and are connected to one another by vertical sections 54 this bottom contour can be formed by embossing the bottom 50 with an embossing tool from an originally flat plate for example depending on requirements the slots can then be provided in the sloping sections 52 as illustrated by slot 56 for example or can be provided in the vertical sections 54 as represented by slot 58 here too the aperture cross section is such that the latter slopes relative to the plane 51 of the bottom with the result that a motion component in the direction of the plane 51 of the bottom is imposed on a treatment medium passing through the slots 56 and 58 in this case too as indicated by the flow arrows in the case of the bottom 60 illustrated in fig4 the latter is of waved construction i e wave tips and troughs project above and below the plane 61 of the bottom here too there are once.