Animation showing the distribution of velocities in particles of a mixed gas. As the particles have the same average kinetic energy, the larger particles have lower velocities. Kinetic energy is equal to half a particle's mass multiplied by the square of its velocity. The larger, slower particles are seen to have a path featuring sudden changes of direction after a short distance, due to the collisions with smaller particles. This can be seen as Brownian motion, first noticed by the botanist Robert Brown when looking at pollen grains under a microscope in 1827.