Simultaneous measurements of the longitudinal sound velocity and relative volume were made on freezing nitrogen at pressures up to 20 kbar in a pistonndash;cylinder apparatus. Results in both the fluid and solid phases were combined with data at lower pressures from the literature to derive equations giving the melting properties of nitrogen at high pressure. The solid in equilibrium with the fluid is apparently bgr;hyphen;N2(hcp) throughout the experimental range. An estimate was made of the transverse sound velocity in bgr;hyphen;N2at high pressure. Analysis of the total entropy change on melting shows that the entropy component due to disordering of the molecules from solid to fluid nitrogen has a fairly constant value of about ln1.7 along the entire melting curve. There is no indication that the melting curve will terminate in a critical point.
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