Brownian motion
Atoms and molecules had long been theorised as the main parts of matter. Albert Einstein published a paper in 1905 that explained in precise detail how the motion that Brown had observed was a result of the pollen being moved by individual water molecules. This was one of his first big contributions to science, and convinced many scientists that atoms and molecules exist.  It was further verified experimentally by Jean Perrin in 1908.  The direction of the force of atomic bombardment is constantly changing, and at different times the particle is hit more on one side than another, leading to the seemingly random nature of the motion.