this phenomenon as the retentive power of the unburnt clay. A comparison of procedures (2) and (3) shows that the percolator method is superior to the Soxhlet method, since not only is the apparatus simpler, but the time required is less. It is not yet possible to state any definite threshold value of the soluble salt content, beyond which these salts can be reckoned to be deleterious, since efflorescences are also dependent on pore space, pore size and the type of distribution of the pores. In

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... fired ware with coarse pores, greater quantities of soluble salts can crystallize out in the body itself without causing damage, than in ware with fine pores. For laying down a standard of quality the only thing that can be considered is an efflorescence test on the fired ware carried out according to some definite procedure. During such a test the soluble salts which are present in the body migrate to the surface. In carrying out this experiment, a three centimeter wide fragment is broken off from the top of a brick and is placed in a vessel with distilled water so that 1/3 of the material is immersed in the water. The water is heated to 80~ and is held at this temperature with a constant water level for 15 hours. Following on this the material is dried at ll0~ This method permits efflorescences to be detected in a short time. By carrying out this experiment with material burnt at more than 980 ~ it has been shown that at this temperature most sulphates are already dissociated. In a letter to "Nature" in 1937, Mitchell and Muir directed attention to the existence, in certain Scottish soils derived from basic igneous parent material, of a very high total cation exchange capacity relative to the clay content--as high as 9 m. eq./gm, clay in some cases. These values were so high as effectively to preclude the possibility of the clay itself being solely responsible for the cation exchange capacity, and in fact in those cases where it was very high relative to clay content, the clay content itself was generally low, so that the cation exchange capacity of the whole soil was quite moderate, of the order of 0.3 m. eq./gm. The high cation exchange capacity could not be due to organic material, for it often occurred in lower layers where the organic matter content was likely to be low.