Of a single pure liquid or solid phase α, w C α is the work done on the system when a column α of unit area is split, reversibly, normal to the axis of the column to form two new surfaces each of unit area in contact with the equilibrium gas phase. \[w_{\text{C}}^{\unicode[Times]{x3B1} }=2\ \gamma ^{\unicode[Times]{x3B1} }\] where γ α is the surface tension between phase and its equilibrium vapour or a dilute gas phase.