(but the same temperature), the following empirical equation has been suggested: where D is the diffusion coefficient, ρ is the gas mass density, P1 and P2 are the corresponding pressures. Population dynamics: dependence of the diffusion coefficient on fitness In population dynamics, kinesis is the change of the diffusion coefficient in response to the change of conditions. In models of purposeful kinesis, diffusion coefficient depends on fitness (or reproduction coefficient) r: where is constant and r depends on population densities and abiotic characteristics of the living conditions. This dependence is a formalisation of the simple rule: Animals stay longer in good conditions and leave quicker bad conditions (the "Let well enough alone" model). Effective diffusivity in porous media The effective diffusion coefficient describes diffusion through the pore space of porous media. It is macroscopic in nature, because it is not individual pores but the entire pore space that needs to be considered. The effective diffusion coefficient for transport through the pores, De, is estimated as follows: where D is the diffusion coefficient in gas or liquid filling the pores, εt is the porosity available for the transport (dimensionless), δ is the constrictivity (dimensionless), τ is the tortuosity (dimensionless). The transport-available porosity equals the total porosity less the pores which, due to their size, are not accessible to the diffusing particles, and less dead-end and blind pores (i.e., pores without being connected to the rest of the pore system). The constrictivity describes the slowing down of diffusion by increasing the viscosity in narrow pores as a result of greater proximity to the average pore wall. It is a function of pore diameter and the size of the diffusing particles. Example values Gases at 1 atm., solutes in liquid at infinite dilution. Legend: (s) – solid; (l) – liquid; (g) – gas; (dis) – dissolved. See also Atomic diffusion Effective diffusion coefficient Lattice diffusion coefficient Knudsen diffusion References Transport phenomena Diffusion