Superfluid density of thin /sup 4/He films adsorbed in porous Vycor glass

We have studied the superfluid density of thin superfluid /sup 4/He films absorbed in porous Vycor glass. At low temperatures we have used a third-sound technique to examine the Landau elementary excitations. We observed phonons with a T/sup 2/ temperature dependence and rotons with a 5-K energy gap. At higher temperatures we have examined the superfluid density in the critical region. The superfluid density follows a power law similar to that of bulk /sup 4/He with an exponent at the bulk value. We conclude that these effects are due to the three-dimensional geometry of the substrate.