  The experimental determination of freeze-out temperatures and densities from
the yields of light elements emitted in heavy ion collisions is discussed.
Results from different experimental approaches are compared with those of model
calculations carried out with and without the inclusion of medium effects.
Medium effects become of relevance for baryon densities above $\approx 5 \times
10^{-4}$ fm$^{-3}$. A quantum statistical (QS) model incorporating medium
effects is in good agreement with the experimentally derived results at higher
densities. A densitometer based on calculated chemical equilibrium constants is
proposed.
