When vitreous silica is submitted
to high pressures under a helium
atmosphere, the change in volume observed is much smaller than expected
from its elastic properties. It results from helium penetration into
the interstitial free volume of the glass network. We present here
the results of concurrent spectroscopic experiments using either helium
or neon and molecular simulations relating the amount of gas adsorbed
to the strain of the network. We show that a generalized poromechanical
approach, describing the elastic properties of microporous materials
upon adsorption, can be applied successfully to silica glass in which
the free volume exists only at the subnanometer scale. In that picture,
the adsorption-induced deformation accounts for the small apparent
compressibility of silica observed in experiments.