Based on the contention that the driving force for the motion of a species relative to the mass flow in a particular direction is the negative gradient of total potential of that species in that direction, equations for both the sedimentation velocity and the diffusion of a solvated substance are derived. They are reduced to the usual expressions for a nonsolvated substance. The partial specific volume term in the equations of sedimentation velocity is shown to have theoretical justification. The advantages of expressing the sedimentation velocity in terms of solvated species are discussed. It is also shown that for the diffusion of a solvated species the ``thermodynamic factor'' calculated in the usual way is always greater than the gradient of chemical potential of the solvated species, except at zero concentration, by a previously neglected term arising from solvation. The effect of solvation on diffusion coefficient and on the relation between the frictional coefficients in the sedimentation and diffusion are also discussed in the light of this theory.
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