  Fluid phase transitions in porous media are a powerful probe of the effect of
confinement and disorder on phase transitions. Aerogel may provide a model
system in which to study the effect of dilute impurities on a variety of phase
transitions. In this paper we present a series of low frequency acoustic
experiments on the effect of aerogel on the liquid-vapor phase transition.
Acoustic resonators were used to study the liquid-vapor transition in two
fluids (helium and neon) and in two different porosity aerogels (95% and 98%).
While effective coexistence curves could be mapped out, the transition was
sometimes difficult to pinpoint, leading to doubt as to whether this transition
can be treated as an equilibrium macroscopic phase transition at all.
