The principal elastic moduli of single crystal rods of NaCl were measured over the temperature range 20°C to 804°C, the melting point. The measurements were carried out by means of a tripartite piezoelectric oscillator consisting of a quartz crystal driver, an intermediate fused silica bar, and the NaCl specimen. The shear constants $C_{44}$ and ($C_{11}-C_{12}$) decrease nearly linearly with temperature, reaching non-zero values at the melting point. The compressibility increases with temperature, but exhibits a sudden drop within a few degrees of the melting point. The results are compared with certain predictions based on Born's theory of the stability of crystal lattices.

• Received 31 October 1941

DOI:https://doi.org/10.1103/PhysRev.61.84