According to the particle theory, there is a random movement between particles in a fluid or in a gas. This random motion of the particles is called Brownian motion. or. The experimental evidence of thermal agitation was given by famous Scottish botanist Robert Brown in 1827. He put tiny pollen grains of flower in water and illuminated them by intense light and observed under a microscope. He found that pollen grains we are under a continuous vigorous zig-zag motion in water though the water was perfectly still. This zig-zag motion of small particles is called Brownian Motion .