Brownian Motion, constant erratic movement of tiny particles suspended in a fluid or gas. The phenomenon was discovered in 1827 by the British botanist Robert Brown. The inherent motion of the molecules of the fluid causes the molecules to strike the suspended particles at random. The impact makes the particles move. Albert Einstein in 1905 arrived at a mathematical explanation of the phenomenon and integrated it into kinetic theory. One of the earliest estimates of the value of Avogadro's number was made by the French scientist Jean-Baptiste Perrin by a quantitative study of the Brownian motion. See Colloid.