The absolute viscosity of the normal paraffins C₅H₁₂ to C₂₀H₄₂ was studied to determine a relationship more useful in predicting liquid viscosities than existing correlations such as the methods of Andrade, Souders, Thomas, and Doolittle.

It was found that a function of corresponding liquid states could be designed which related viscosity to the number of carbon atoms. This function, here called liquidity, is defined as the extent to which a substance exists as a liquid with respect to temperature. This function can be expressed mathematically as:

L_c = (t - t_m)/(t_c - t_m) or L_b = (t-t_m)/(t_b-t_m);

where:
L_c = liquidity based on the critical temperature
L_b = liquidit based on the normal boiling temperature
t = any temperature, °C
t_b = normal boiling temperature, °C
t_c = critical temperature, °C
t_m = normal melting temperature, °C

Once the basis for liquidity is determined, the denominator in the above expressions remain constant, and the % liquidity becomes a straight line function of the temperature.

It was further found that a plot of L_c vs. number of carbon atoms for the n - paraffins resulted in curves of iso - viscosity which, when fitted to straight lines, could be used for calculation, extrapolation, or interpolation of viscosity data. Thus, a method was developed for the viscosity prediction of the n - paraffins above C₄H₁₀ to apply at any temperature within the normal liquid range. Deviations from the experimental values of viscosity to those resulting from the designed liquidity function are within ± 10 per cent for the majority, and less than ± 20 per cent for the extreme cases. This approach to viscosity prediction is valid with or without the availability of experimental data.