So called Brownian Motion was first observed and described by Jan IngenHousz in 1785, but was named after Robert Brown who investigated the characteristic motions of grains of pollen suspended in water in 1827.  These microscopic random motions of very small particles (≤ 50 microns) are caused by atoms or molecules of the suspending fluid (air or water) colliding with the particles from different directions in unequal quantities within the same time interval. This produces unequal forces to be applied to the particles resulting in particle motion in the direction of the net force.
The videos seen here were produced by observing 1 micron polystyrene spheres suspended in water through a microscope augmented with a digital camera. Note the apparent randomness of the motion of the individual spheres within the field of view. The umbra effects on many of the particles derives from the fact that they are slightly out of the focus of the microscope. See if you can spot the effects of gravity on the system.
Dr. Lejeune, Certified Member
1. Lemons, D. S. An Introduction to Stochastic Processes in Physics, Baltimore: The John Hopkins Press, 2002.