If the dependence of the chemical potential of the gas on the extent of surface coverage is known, the Gibbs adsorption isotherm, eqn 19.50, can be integrated to give a relation between Va and p, as in a normal adsorption isotherm. For instance, suppose that the change in the chemical potential of a gas when it adsorbs is of the form du =-c,(RT/CY)dVa, where c2 is a constant of proportionality: show that the Gibbs isotherm leads to the Freundlich isotherm in this case.