Brownian motion is the random motion of particles in a liquid or a gas. The motion is caused by fast-moving atoms or molecules that hit the particles. Brownian Motion was discovered in 1827 by the botanist Robert Brown. In 1827, while looking through a microscope at particles trapped in cavities inside pollen grains in water, he noted that the particles moved through the water; but he was not able to find out what was causing this motion.