The departures from Joule's Law that the internal energyUis independent of volumeVare first expressed in dense liquids in terms of specific heats and the isothermal compressibility. For condensed rare gases, described by density independent pair potentials,(∂U/∂V)Tis shown to be closely related to the vacancy formation energyEv, a correction term dependent on the density dependence of the pair function being displayed explicitly. For metals with density-dependent pair potentials,(∂U/∂V)Tis shown to involve the vacancy formation volume.