"In the Brownian motion paper, Einstein... calculations involved the relationship between osmotic pressure, viscosity, and the way individual particles suspended in the liquid diffuse... He realized that the kick produced by a single molecule hitting a particle as large as a pollen grain could not produce a measurable shift... But the large particle is constantly being bombarded... if you take a very small time interval, then just by chance at that instant the particle will be receiving more kicks on one side. The combined effect will shift the particle by a minute amount... Einstein discovered that it gradually moved farther from its starting point... as a random walk. He showed the distance ... depends on the square root of the time... This is called "root mean square" displacement and the equation Einstein worked out for displacement involves the temperature of the liquid, its viscosity, the radius of the particle and Avogadro's number. ...He also realized that if the predicted displacement could be measured... the same equation... could be used to give a value of Avogadro's number. ...It was extremely difficult to make the observations... but in 1908... Jean-Baptiste Perrin finally succeeded. ...Perrin's results exactly matched the predictions from Einstein's theory. ...The whole package finally established the reality of atoms and molecules, and the validity of the kinetic theory..."