Third Sound inHe4Adsorbed on Nuclepore

Measurements of the velocity of third sound $C_3$ in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on Nuclepore filter material with and without pores have been made and reveal striking structure as a function of film thickness for the porous case. In the absence of capillary condensation the data are understood in terms of a model which predicts the porous index of refraction as a function of coverage. It is likely that the techniques employed will be a useful alternative to the Brunauer-Emmett-Teller method for the determination of surface areas. For helium film coverages above those necessary for capillary condensation a fast propagating mode with speed $v\sim C_3^{\frac{1}{2}}$ is observed.