Two laws, formulated in 1855 by the German physiologist and physician Adolf Eugen Fick (1829–1901) that describe how one substance moves into another by diffusion. The first law states that molecules move from a region of higher concentration to a region of lower concentration at a rate proportional to the concentration gradient. The second law describes how diffusion causes the concentration gradient to weaken over time. Fick studied diffusion in fluids, but his laws are now known to apply equally to diffusion in solids.