THE outstanding work on hydrolytic adsorption has been carried out with purified charcoal (see, for example, Bartell and Miller, J. Am. Chem. Soc., 44, 1866; 1922: 45, 1106; 1923), and definite positive results have been obtained. The measure of the hydrolytic effect in the case of the numerous negative hydrophobic (acidoid) sols, such as colloidal mastic, platinum, gold, arsenious suiphide, etc.—all of which are remarkably alike in structure and in reactions—is complicated by the well-established phenomenon of catalytic interchange. For example, in the presence of salts, such as barium chloride, the hydrogen ion at the colloid surface suffers an interchange with the added barium ion, and the corresponding increase in acidity is superimposed upon any change in acidity due to the hydrolytic action at the surface. It has become the general practice to ascribe the whole of the increase in acidity of acidoid sols (on the addition of neutral salts) solely to the cationic replacement, and to neglect any probable hydrolytic effect. That the latter effect is real is evident from the following consideration.