The melting and freezing phase transitions of mercury in a porous glass were studied by NMR and acoustic techniques. The NMR measurements provided direct information on the total amount of liquid mercury versus temperature. A depression of the phase transition temperatures and pronounced hysteresis between melting and freezing were found. Acoustic measurements showed that the freezing process was irreversible while the melting process consisted of reversible and irreversible temperature ranges. The use of longitudinal and transverse acoustic waves made it possible to obtain information about the origin of reversible and irreversible behavior upon melting. In particular, we found that the complete melting of confined mercury can be acoustically detected using only longitudinal and not transverse waves. The broadening of melting is explained by the formation of a liquid layer on the mercury solid surface, and freezing was driven by the pore geometry with no visible precursor effects. ͓S0163-1829͑98͒06033-0͔

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