Figure 4 is another plot of the same equation showing T r as a function of ω ω for various values of p r s . It includes data from 51 substances including the vdW fluid over the range − − 0.4 < ω ω < 0.9 . This plot shows clearly that the vdW fluid ( ω ω = − − 0.302 ) is a member of the class of real fluids; indeed it quantitatively describes the behavior of the liquid metals cesium ( ω ω = − − 0.267 ) and mercury ( ω ω = − − 0.21 ) whose values of ω ω are close to the vdW value. However, it describes the behavior of other fluids only qualitatively, because specific numerical values are modified by differing values of their Pitzer factor, ω ω .