  We have studied 4 He confined in a 95% porosity silica aerogel in the
vicinity of the bulk liquid gas critical point. Both thermodynamic measurements
and light scattering experiments were performed to probe the effect of a
quenched disorder on the liquid gas transition, in relation with the Random
Field Ising Model (RFIM). We find that the hysteresis between condensation and
evaporation present at lower temperatures disappears at a temperature T ch
between 25 and 30 mK below the critical point. Slow relaxations are observed
for temperatures slightly below T ch , indicating that some energy barriers,
but not all, can be overcome. Above T ch , no density step is observed along
the (reversible) isotherms, showing that the critical behavior of the
equilibrium phase transition in presence of disorder, if it exists, is shifted
to smaller temperatures, where it cannot be observed due to the impossibility
to reach equilibrium. Above T ch , light scattering exhibits a weak maximum
close to the pressure where the isotherm slope is maximal. This behavior can be
accounted for by a simple model incorporating the compression of 4 He close to
the silica strands.
