The Ficks law of diffusion reference article from the English Wikipedia on 24-Apr-2004
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Ficks law of diffusion

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Fick's Law of Diffusion states that the rate of diffusion of a gas across a fluid membrane is Fick's law is also important in radiation transfer equations. However, in this context it becomes inaccurate when the diffusion constant is low and the radiation becomes limited by the speed of light rather than by the resistance of the material the radiation is flowing through. In this situation, one can use a flux limiter.

The exchange rate of a gas across a fluid membrane can be determined by using this law together with Graham's law.

See also : lung -- alveoli