  The acoustic sound dispersion of nitrogen in its liquid and supercritical
phases has been studied by Inelastic X-Ray Scattering. Approaching
supercritical conditions, the gradual disappearance of the positive sound
dispersion, characteristic of the low temperature liquid, is observed. In the
supercritical state, evidence for a crossover between adiabatic and isothermal
sound propagation regimes is inferred by an analysis of the dynamic structure
factor based on generalized hydrodynamics.
