Einstein explains Boltzmann's relation between entropy and probability, S=k lg W, using the example of a particle suspended in a fluid which moves with a certain probability by the distance z from the bottom of a cylindrical container because of atomic thermal motion. He then deduces the law of Brownian Motion from an observation of the amount of the substance diffusing through the cross-sectional unit over a certain time [t]. He subsequently introduces the statistical law of motion of a Brownian particle.