Einstein’s second 1905 paper, “On the Movement of Small Particles Suspended in Stationary Liquids Required by the Molecular-Kinetic Theory of Heat,” used statistical mechanics to analyze other scientists’ observations of a phenomenon known as Brownian motion.
About 80 years earlier, Scottish botanist Robert Brown, after whom the effect is named, observed pollen grains suspended in a fluid through a microscope. Brown noticed that the grains moved jerkily along irregular paths. In the years after that, other scientists made precise measurements of Brownian motion and reported their results.
Einstein realized that those irregular jogs were the result of collisions with the molecules of the fluid. He calculated how far and how fast the particles would be expected to move between collisions, and how the details of the zigs and zags would be affected by changes in temperature. His compared his calculations to experimental measurements and found that they matched. Though single atoms and molecules had still not been observed, Einstein’s calculations provided direct evidence that they existed
~ Physics – Decade by Decade, Alfred B. Fortz, Ph.D