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 by two mathematical planes. We make, however, little mention of this method, since it seems to be of only minor importance in connec?? tion with the statistical treatment of an interface. To avoid any ambiguity, the treatment of a spherical interface given in this article is based not on the original method of GIBBS but on the method modified by HILL [8J and KONDO [9]. This method, however, is not applicable to non?? spherical interfaces, which will not be dealt with in this article. Although all the relations for a plane interface can be deduced from the cor?? responding ones for a spherical interface by putting the curvature equal to zero, the planar and the spherical cases are considered separately because of the prac?? tical importance and easy physical visualization of a plane interface.