Brownian motion - it is heat motion of smallest particles, weighted in liquid or gas. It was discovered by English botanist Brown (1827) and appeared as a proof of chaotic molecular motion. Brownian particles move under the influence of collisions of molecules. Because of chaotic heat motion of molecules these collisions never equalize each other. As a result the velocity of a Brownian particle constantly changes in size and direction, and its trajectory represents a complicated zigzag. Molecular-kinetic theory of Brownian motion was developed by A. Einstein (1905).

The main point of the theory is that square of displacement r² of Brownian particle from initial position, averaged by many Brownian particles, changes proportionally to time (diffusion law): r² = D * T. Coefficient of diffusion D is proportional to the absolute temperature T. Einstein's theory was experimentally proven in experiments of French physicist G. Perrene (1908).