Cluster phase transitions and critical phenomena in fluid mercury

New detailed measurements of sound velocity in mercury have been carried out at a frequency of 10 MHz for temperatures up to 1900 K and pressures up to 1750 bar, i.e., in the liquid and gas phases together with the coexistence curve up to the critical point and in the supercritical region. The estimated error of the sound velocity data is 0.4%. Great attention was paid to the vicinity of the critical point. Measured critical parameters of mercury are 1764 K and 1670 bar. We conclude that the behavior of the sound velocity of mercury in the gas phase near the coexistence curve and in the vicinity of the critical point is abnormal. The critical behavior of sound absorption is also discussed.