We set new regularities between the effective compressibility factor at the critical point (Z*) and unit compressibility line (Zeno-line) parameters. For classical liquids, Z* is the ordinary compressibility factor Z(c), but for quantum liquids Z* depends on the de Boer parameters. As a result, we show that a wide group of real substances with the classical thermodynamical properties has Z(c) < 0.32. Classical mercury and quantum H-2, He-4 and He-3 have Z(c) or Z* > 0.37. Using the low temperature part of the liquid-gas coexistence curve obtained from experiment, we can find the critical parameters for metals (Al, Cu, W, U, and Zr) for which these parameters are unknown in advance and lie in the domain of parameters still inaccessible for experiment.