Measurements of helium density in aerogel near the liquid-vapor critical point

We investigate the behavior of He-4 confined in silica aerogels near the bu lk liquid-vapor critical point. Using a new mechanical technique to measure the density of the confined He-4 along isotherms, we find that the density continuously increases from a low, density phase up to a dense phase as th e pressure is increased up to slightly below the bulk saturated pressure. A n hysteretic behavior is observed between emptying and filling, which is no t uniquely due to thermal problems. We argue that our observations are more in favor of some kind of capillary condensation than of a genuine first or der phase transilion.