Special Relativity was a spectacular achievement, but Einstein was not finished yet for the year. His third paper of 1905 concerned statistical mechanics, an area of physics concerned with the probabilities of events. One thing that had puzzled scientists for years was the incessant movement of very fine dust particles in water, and effect called Brownian Motion. Einstein proved that this movement was caused by the dust particle's continuous bombardment by water molecules, and he calculated the average path of such a particle. Einstein's research provided persuasive evidence for the existence of atom-sized molecules, which, though commonly accepted today, was still being debated among physicists at that time.
"In long intervals I have expressed an opinion on public issues whenever they appeared to be so bad and unfortunate that silence would have made me feel guilty of complicity."
University of Oslo --Norwegian Einstein page