The principles underlying the design of a satisfactory cell for the direct study of the Soret effect are discussed, and an improved cell, suitable for use with most non-aqueous mixtures, is described. It has been used, in conjunction with a phase-contrast schlieren technique of measuring refractive index gradients, to study thermal diffusion in some solutions of carbon tetrachloride in benzene at a mean temperature of 25. The results confirm that carbon tetrachloride migrates to the cold wall, the average Soret coefficient (σ) for an equimolar solution being 6.40 × deg.-1, a figure based on 49 separate values obtained in four independent experiments (standard deviation = 0.08 × 10-3 deg.-1). The coefficient varies only slightly with concentration down to N(CCl4) = 0.2, though some preliminary measurements at lower mole fractions suggest that σ may rise rapidly as the system becomes richer in benzene.
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