Water Uptake by Pure Clays and Soil Crumbs*

Summary Wetting experiments on pure clays are described that lead to the conclusions that entrapped air is neither a necessary nor an important factor in slaking of dry soils, and that slaking is always associated with rapid intercrystalline swelling of the clay. If swelling is suppressed, or takes place slowly—as from a vapour phase— slaking does not occur, and it is suggested that slow swelling gives time for readjustment of the internal geometry of the clay, so producing some dislocation but no disruption. In N NaCl, a Na‐montmorillonite swells to about twice its dry thickness, and in an atmosphere of 98 per cent. relative humidity it gains about one‐third of its weight. Both these figures are in agreement with the usual estimate of surface area of 800 m. 2 /g. Under the same conditions a Na‐illite (from Willalooka) swells by about a third of its dry thickness, and gains about a quarter of its dry weight. Studies on rates of uptake of water vapour indicate that montmorillonite has taken up less than a quarter of its equilibrium water content at 98 per cent. relative humidity within 10 min., whereas illite has taken up one‐third of its water in the same time, the clays then being stable against subsequent flooding. In the field it is probable that water vapour diffuses ahead of a liquid wetting front, and the protective effect of the resultant slower pre‐swelling of the clay prevents the slaking action of rain from affecting more than a thin surface layer of soil.