Nonsteady condensation and evaporation waves

Recent scaling results for the ac conductivity of ionic glasses by Roling et al. Phys. Rev. Lett. 78, 2160 (1997)] and Sidebottom Phys. Rev. Lett. 82, 3653 (1999)] are discussed. We prove that Sidebottom's version of scaling is completely general. A new approximation to the universal ac conductivity arising in the extreme disorder limit of the symmetric hopping model, the "diffusion cluster approximation," is presented and compared to computer simulations and experiments.