Brownian motion is caused by the impact of fluid molecules or atoms in rapid and random motion from heat on small particles suspended in the fluid. Brownian motion, which tends to disperse particles as widely as possible, is the major force in diffusion. Brownian motion occurs in small suspended particles, regardless of any overall movement or current in the suspending fluid.
Brownian motion was first noticed and described by botanist Robert Brown in 1827 while observing the movement of pollen grains in still air. His initial thoughts were that the movements were caused by some self-powered movement of the plant sperm, but later experiments showed that the motion occurred with dead and even inorganic particles. Robert Brown never came up with a satisfactory explanation for the motion.
The current explanation was first offered 50 years later with the kinetic theory of heat, which stated that heat was actually the random vibration of the atoms and molecules that composed a substance. These random movements within a fluid hit suspended particles rapidly and randomly, producing motion. This theory was initially resisted, as it seemed to violate the tendency of energy to degrade, with friction always converting motion into heat. Nonetheless, experiments confirmed that this is, in fact, the mechanism of Brownian motion.