  A sponge subjected to an increase of the outside fluid pressure expands its
volume but nearly mantains its true density and thus gives way to an increase
of the interstitial volume. This behaviour, not yet properly described by
solid-fluid mixture theories, is studied here by using the Principle of Virtual
Power with the most simple dependence of the free energy as a function of the
partial apparent densities of the solid and the fluid. The model is capable of
accounting for the above mentioned dilatational behaviour, but in order to
isolate its essential features more clearly we compromise on the other aspects
of deformation.
