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kaolin containing free silica. Two properties of clay plasticity and cleavage are of prime importance to the potter. On the CLEAVAGE depends the ease or difficulty with which the clay can be mined. Cleavage is due to the unlike cohesion in various directions of individual crystals. Only with the crystals of the regular system is there perfect equilibrium of the mutual attractive forces. In other crystals, such as felspar and mica, there is a tendency to cleavage in one or two definite directions. In the process of weathering, there- fore, the break naturally takes place in those directions in which the least mechanical resistance is to be overcome that is, along the planes of cleavage, and the minerals are accordingly broken up into laminae which become thinner and thinner. When at length these laminae are washed away and deposited in some new position, it is obvious that the most stable position of equilibrium is parallel to the surface of the sediment. J. S. Owen has shown (Geog. Journ., 1911, 76) that a body falls in water with the greatest possible surface horizontally disposed. The whole bed of sedimentary clay thereby assumes a 14 CERAMIC CHEMISTRY. plane of cleavage which was at first peculiar to the in- dividual crystal. PLASTICITY is the chief characteristic of ball clays, and the phenomenon has not yet been satisfactorily explained. As it is destroyed at 415, the temperature necessary to