Abstract
For the purpose of investigating elastic properties of rocks at the melting point, the velocity of ultrasonic dilatational wave in polycrystalline sodium was measured from room temperatures up to its melting point. Very marked drops in elastic moduli with the corresponding decrease in the velocity were observed as the melting point is approached. The marked drop in the velocity near the melting point was compared with the order-disorder transition phenomena of a metal alloy. The nature of the core-mantle boundary (Bullen's region D") was discussed from such experimental evidences.
