135 We first describe the thermodynamic theory of surface tension and adsorption, by the method of the dividing surface of GIBBS. The use of a dividing surface or its equivalent is indispensable for the treatment of a curved interface, as otherwise the concepts of the area and curvature of the interface, cannot be pre- cisely defined. In the case of a plane interface, however, the concept of the dividing surface is not necessary and a valid alternative exposition has been proposed by GUGGEN- HEIM 3J, 4J in treating the interface zone as a separate entity of some definite thickness bounded...