What is Fick's Law of diffusion?

Fick's First Law states that the molar flux of a species is proportional to the concentration gradient: J = -D(dC/dx), where J is the molar flux, D is the diffusion coefficient, and dC/dx is the concentration gradient. The negative sign indicates diffusion occurs from high to low concentration. Fick's Second Law describes how concentration changes with time during unsteady-state diffusion.