Four versions of Fick's law for binary gas mixtures are given below. These assume: thermal diffusion is negligible; the body force per unit mass is the same on both species; and either pressure is constant or both species have the same molar mass. Under these conditions, Ref. shows in detail how the diffusion equation from the kinetic theory of gases reduces to this version of Fick's law: V i = − − D ∇ ∇ ln ⁡ ⁡ y i , where V i is the diffusion velocity of species i. In terms of species flux this is J i = − − ρ ρ D M i ∇ ∇ y i .