On the theory of fluids

The theory of fluids based on the radial distribution function, g(r), is related to the direct correlation function, f(r), of scattering theory. The simplest form of the Born-Green theory is shown to be intimately related to the first approximation to f(r). The second approximation to f(r) is considered in detail, and shown to effect a considerable improvement on earlier theories. It leads to a critical point at which p/$\rho $kT = $\frac{1}{3}$. Fournet's application of the Born-Green theory to X-ray scattering by fluids is shown to be correct only to the first power in the density $\rho $. The terms in $\rho $ and $\rho ^{2}$ are considered in detail for the case of hard spheres.