Diffusion is the process by which atomic and molecular size particles move from regions of high concentration to regions of lower concentration. This movement is quantified by Fick’s first law, which states that the magnitude of the particle flux is directly proportional to the concentration gradient; and that the direction of movement is toward lower concentrations. The proportionality constant in Fick’s first law is the diffusion coefficient. Tempera¬ture, pressure, particle size, and charge, in addition to properties of the surrounding media, affect the diffusion coefficient. Mass transport by diffusion is greatest initially when concentration gradients are large. The diffusion process subsequently reduces gradients in concentrations, which, in turn, slows transport of mass by diffusion.