  We report on thermodynamic and optical measurements of the condensation
process of $^4$He in two silica aerogels of same porosity 95%, but different
microstructures resulting from different pH during synthesis. For a
base-catalyzed aerogel, the temperature dependence of the shape of adsorption
isotherms and of the morphology of the condensation process show evidence of a
disorder induced transition,in agreement with recent theoretical predictions.
This transition is not observed for a neutral-catalyzed aerogel, which we
interpret as due to a larger disorder in this case.
