The relative mobility of ions in cellulose-acetate membranes used for desalination of water by reverse osmosis depends strongly on the size of the hydrated ions. The preferential exchange of ions by compacted clays shows a similar dependence. It is concluded that the same dependence must exist for the mobility of ions in compacted clays and shales acting as semi-permeable membranes. Qualitative results of the processes of osmosis and reverse osmosis through these materials thus should be similar to those obtained with the desalination membranes: modification of both chemical composition (ion ratios) and concentration of the solutions involved, and the occurrence of some flow-phenomena that are not in accordance with Darcy's law.

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