Comment on a Theory of Melting Proposed by Y. Ida

A theory of melting of Ida based on a lattice instability is analyzed. The theory is reformulated, showing that the critical condition at which melting occurs may be related to an infinity in the derivative with temperature of the velocity of ultrasonic waves. It is also shown that Lindemann's hypothesis is equivalent to the constancy of the vibrational elongation $Q_m$ at the critical point, but that experimental data give values of $Q_m$ that vary appreciably for different substances. Subsequently derived values for the Lindemann constant are consistent with a higher temperature for the critical point than the thermodynamic melting temperature.