The principle of corresponding states has been adopted to the correlation of densities and found to be applicable for nonopolar substances when the critical compressibility factor is introduced as an independent parameter. For the inert gases, densities available in the literature for the gaseous and liquid states have been referred to their critical densities to produce reduced-state correlations which are identical only when their corresponding critical compressibility factors are the same. As a result, a consolidated reduced density correlation for zc = 0.291 has been developed from experimental data for argon, krypton, and xenon. Although the zc values for helium and neon are essentially alike, different reduced-state correlations for these substances were found to exist. This behavior is expected in view of the radical nature of helium, which can be accounted for by its excessive quantum deviation. Therefore the reduced density correlation for neon (zc = 0.307) is selected as representative of that class of substances having similar zc values.
The validity of both correlations has been extended beyond the monatomic gases to include substances having comparable critical compressibility factors. With the generalized chart for zc = 0.291 gaseous densities at elevated temperatures and pressures have been calculated for nitrogen, oxygen, carbon monoxide, and methane to produce an average deviation of 1.4% from reported values in the literature. For these substances the average deviation becomes 1.5% for densities in the liquid state. With the reduced density chart for neon (zc = 0.307) densities calculated for hydrogen (zc = 0.305) in the gaseous region produced an average deviation of 2.5% and covered pressures in excess of 500 atm. and temperatures as high as 500°C.