(1) The action of salts in liberating hydrogen ions, by ionic interchange, from the micelles in (Raffo) sulphur and stearic acid sols has been investigated with regard to the coagulating action of the salts.
(2) The data support the view that in the case of sulphur sols, the amount of interchange which accompanies the coagulation of a given fraction of the colloid is the same for all salts with monovalent, divalent, and (in the absence of hydrolysis) trivalent metallic kations.
(3) The data for the stearic acid sols indicate that in the case of these systems two types of interchange are to be distinguished, one of which is involved in the coagulation of the colloid by salts.
(4) It has been observed that stearic acid ,sole "age" in respect to stability and degree of ionic interchange.
(5) The importance of the study of ionic inter- :change for a general theory of coagulation is discussed. It is concluded that the results of the present investigation are consistent with the view that in sols stabilised by adsorbed ions or ions originating from the dissociation of surface molecules, the formation of electrically neutral complexes with the coagulating kations is an essential condition for coagulation.