Robert Brown in 1927 observed that pollen grains when placed in a liquid and seen through a microscope show a zig-zag motion. He also found that colloidal particles present in a sot. show the same type of motion. This random zig-zag motion of the colloidal particles is known as Brownian movement. This movement is caused when unequal number of medium molecules strikes the colloidal particles from different directions. The Brownian movement of colloidal particles counteract the force of gravity acting on them and is thus responsible to a certain extent for the stability of sols.