In the following only media which are @I03353@ with respect to mass transport (i.e. the transport coefficients are independent of direction) are being considered. In the @L03558@ (not too far from equilibrium), for uniform temperature and neglecting external fields such as the earth's gravitational field, the flux density of species B is related to the gradients of the electrochemical potentials of all species by the phenomenological equation: \[N_{\text{B}}- c_{\text{B}}\ v_{\text{A}}=- \sum _{\begin{array}{c} i\\ i\neq A \end{array}}L_{\text{B}i}^{\text{A}}\ \nabla \mu _{i}\] with \[i=\text{B},\text{C},...\] where \(\nabla \mu _{i}\) is the @G02669@ of the @E01945@ of species i. The proportionality factors \(L_{\text{B}i}^{\text{A}}\) are called phenomenological coefficients. Their values depend on the frame of reference. The latter is taken here to move with the velocity \(v_{\text{A}}\) of species A, and hence: \[L_{\text{A}i}^{\text{A}}=0\]