c and v>vv , where vv is the mass-specific volume of saturated vapor. In the adjacent regions with negative internal energy values U<0 the mean distances between particles are short enough to yield negative energy contributions to U due to interparticle attraction that exceeds the thermal, positive energy contributions due to particle motion. The critical isochor vc is the line of equal positive and negative energy contributions and thus represents a line of vanishing internal energy U=0. At this level along the critical isochor the ever present microscopic fluctuations in energy and density become macroscopic fluctuations as the pressure decreases on approaching the critical point; these are to be observed in experiments on the critical opalescence. Crossing the isochor vc from U>0 to U<0, the change in energy ΔU>0 happens without any discontinuity. The saturation line vv also separates the regions between U>0 and U<0 , but does not represent a line U=0. The internal-energy values of saturated vapor Uv and condensate Ui can be determined absolutely as functions of vapor pressure p and densities (M/V)v and (M/V)i , repectively, yielding the results Ui<0v, U=Ui+Uv<0, for T
 Elsner, A. (2012) Applied Thermodynamics of the Real Gas with Respect to the Thermodynamic Zeros of the Entropy and Internal Energy. Physica B: Physics of Condensed Matter, 407, 1055-1067.
 Elsner, A. (2015) Thermodynamic Equilibrium of the Saturated Fluid with a Free Surface Area and the Internal Energy as a Function of the Phase-Specific Volumes and Vapor Pressure. Engineering, 7, 577-596.