• Fick's first law: J = -D grad c.  This is an empirical law and it is consistent with the theory of linear irreversible thermodynamics.

• Fick's second law: dc / dt = - grad J. This is a consequence of the conservation of matter. Note that if the diffusivity varies with c, the resulting differential equation is nonlinear.

The Law of Conservation of Matter states that matter cannot be created or destroyed, only redistributed. In chemistry, it is represented by the fact that the sum of the masses of the reactants are equal to the sum of the products formed in a chemical reaction.

In heat transfer analysis, thermal diffusivity (symbol: α) is the ratio of thermal conductivity to volumetric heat capacity.

In mathematics, a differential equation is an equation which involves the derivatives of an unknown function represented by a dependent variable.

• The equations of heat conduction are of identical form to Fick's laws: JQ = - k grad T and dT/dt = -grad JQ.

• Self-diffusion in a chemical pure material can be measured by using radioisotope that is easily tracked. The application of force-flux equations for the two isotopes yields a Fick's law-type expression for the radiotracer, KoM Eq. 3.4.

A radionuclide is an atom with an unstable nucleus. The radionuclide undergoes radioactive decay by emitting a gamma ray(s) and/or subatomic particles. Radionuclides may occur naturally, but can also be artificially produced.

Radionuclides are often referred to by chemists and biologists as radioactive isotopes or radioisotopes, and play an important part in the technologies that provide us with food, water and good health. However, they can also constitute real or perceived dangers.