As is well known, in 1909, Perrin investigated the sedimentation equilibrium of colloidal suspensions and obtained a logarithmic decrease in concentration with the height, the result being confirmed by other workers. This was in agreement with the distribution law for "ideal" behaviour of the sol, log n = log n0 + vDg d (ρ1 - ρ2/kT, (1) where n and n0 are the number of particles per cubic centrimetre at depths d of particles and dispersing medium respectively, g, k, T having their usual meaning. Costantin, however, obtained departures from this condition for n greater than 8 x 1010 and interpreted his results in terms of repulsive forces due to the charges on the particles. Perrin modified equation (1) by assuming a law of the van der Waals' type and obtained agreement with observations up to the highest concentrations investigated (n = 6 x 1011). In all these results normal behaviour was found up to values of n greater than 1010. Burton, however , working with vessels about 100 cm long, has shown that the concentration is permanently uniform below a very thin layer near the upper surface.