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Streaming potentials of phosphor surfaces and of pyrex glass surfaces have been determined with the aid of non‐polarizable electrodes under conditions which promote varying degrees of adhesion of phosphor to glass. The materials and reagents were commercial products employed in the manufacture of television screens. Zeta potentials of the solid surfaces were calculated from the streaming potential data.

The mutual reactions of colloids attending the mixing of systems of opposite sign have been investigated by electrokinetic methods (1, 2, 3, 7). Of these, the method employing the ultramicroscope offers a distinct advantage, since mobilities of mixed systems can be observed throughout the concentration range including the flocculation zone. Accordingly, it is possible to study the mutual coagulation process as well as the mutual effects produced by varying concentrations of components in stable mixtures. In the latter case information can be obtained relating to the change in stability of the components and, in a restricted sense, to changes in the state of the particles, e.g., whether the components are completely independent, or combined (owing to capillary-active properties), or partially combined and partially free (7).Weiser and Chapman (9) are of the opinion that the mutual coagulation process may be determined by a number of factors: namely, electrical neutralization, mutual adsorption of the particles, interaction between stabilizing ions, and the presence of excess electrolyte in the sols. These same factors probably influence the state of the particles in stable mixed systems. On the other hand, exponents of a chemical mechanism explain mutual coagulation as a result of a reaction between the stabilizing electrolytes. Thus Thomas and Johnson (8) found for the undialyzed system ferric oxide-silicic acid that coagulation was at an optimum where SUMMARY1. An electrokinetic study of colloidal silicic acid has been made with the aid of an ultramicroscope.2. The effect of electrolytes, pH, and of colloids of opposite sign on the mobility has been investigated.