Moderately dense gas
Estimating the viscosity of pressurized gases

The absence of a rigorous theory for estimating the transport properties of moderately dense real fluids means that it has been necessary to use evaluation methods based on an approximate theory: the principle of corresponding states, or on empiricism. The only approximate theory to have been employed to any extent is Enskog's theory, in a form modified to predict the initial density dependence of transport properties. As was made clear in 1