  The method employed by Einstein to derive his famous relation between the
diffusion coefficient and the friction coefficient of a Brownian particle is
used to derive a generalized Einstein relation for the mutual diffusion
coefficient of a binary fluid mixture. The expression is compared with the one
derived by de Groot and Mazur from irreversible thermodynamics, and later by
Batchelor for a Brownian suspension. A different result was derived by several
other workers in irreversible thermodynamics. For a nearly incompressible
solution the generalized Einstein relation agrees with the expression derived
by de Groot and Mazur. The two expressions also agree to first order in solute
density. For a Brownian suspension the result derived from the generalized
Smoluchowski equation agrees with both expressions.
