Any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations.
It was named for Robert Brown , who was investigating the fertilization process of flowers in 1827 when he noticed a "rapid oscillatory motion" of pollen grains suspended in water. He later discovered that similar motions could be seen in smoke or dust particles suspended in air and other fluids . The idea that molecules of a fluid are constantly in motion is a key part of the kinetic theory of gases , developed by James Clerk Maxwell , Ludwig Boltzmann , and Rudolf Clausius (18221888) to explain heat phenomena.