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).