The phenomena associated with the phase transition at the liquid-vapor critical point and at the consolute point in a binary liquid mixture have been investigated. Photo-electric measurements of turbidity were used to determine vertical density gradients within 0.2° C of the condensation point. The turbidity, determined either by measuring transverse light scattering, or transmission, was observed at fixed temperatures, as a function of the density, or composition in the case of a binary liquid system.