Referred to unit of mass of the solid, the difference (at constant temperature) between the enthalpy of a solid completely immersed in a wetting liquid, and that of the solid and the liquid taken separately. It must be specified whether the solid in the initial state is in contact with vacuum or with the vapour of the liquid at a given partial pressure. Measurements of the enthalpy of wetting of a solid equilibrated with varying relative pressures of the vapour of a pure wetting liquid may be used to derive the differential enthalpy of adsorption of the vapour.