It is shown that the change in thermodynamic potential of a solid when elastically strained by the application of the principal stresses px, py, and pz is given by the equation[Formula: see text]where v is the volume per unit mass and [Formula: see text] is the shear strain work done on unit mass of the solid, at T and (px + py + pz) constant, as the relative values of px, py, and pz vary, i.e., as the values of the principal stresses vary from their mean.