A series of experiments was designed to test the phenomenological theory of thermogravitational thermal diffusion columns by obtaining doto on the steady stote separation and approach to equilibrium in a batch column as a function of column length. Data were obtoined on the separation of an equimolar n-heptane-benzene solution a t five different values of column length varying from 14.5 in. to 6 ft. All measurements were made in one concentric cylinder column in which the effective length was varied by controlling the liquid level in the annulus.Helium gas was used to displace the air in the annulus above the liquid in order to provide even temperature distribution throughout the column. The data were found to substantiate the effect of length predicted by application of the phenomenological theory and to be in very good agreement with other aspects of the theory.