In the absence of heat flows, Fick's law of diffusion is usually written: J ρ ρ = − − D ∇ ∇ ρ ρ , where D is the coefficient of diffusion. Since this is also a linear approximation and since the chemical potential is monotonically increasing with density at a fixed temperature, Fick's law may just as well be written: J ρ ρ = D ′ ∇ ∇ − − μ μ T where, again, D ′ is a function of thermodynamic state parameters, but not their gradients or time rate of change. For the general case in which there are both mass and energy fluxes, the phenomenological equations may be written as: J u = L u u ∇ ∇ 1 T + L u ρ ρ ∇ ∇ − − μ μ T J ρ ρ = L ρ ρ u ∇ ∇ 1 T + L ρ ρ ρ ρ ∇ ∇ − − μ μ T or, more concisely, J α α = ∑ ∑ β β L α α β β ∇ ∇ f β β