Einstein, Perrin, and the reality of atoms: 1905 revisited

We have repeated Perrin’s 1908 experiment for the determination of Avogadro’s number by determining the mean square displacement of small particles undergoing Brownian motion. Our apparatus differs from Perrin’s by the use of a CCD camera and is much less tedious to perform. We review Einstein’s 1905 analysis of Brownian motion and Langevin’s alternative derivation of the Einstein equation for the mean square displacement. We also show how Einstein’s thinking was a reflection of his belief in the validity of molecular-kinetic theory, a validity not universally recognized 100 years ago.