We try to follow the ideas of the original works of 1905 in order to present the main ingredients of Einstein's theory of the Brownian movement. Using the competition between viscous friction and osmotic pressure, we obtain the diffusion coefficient of the Brownian particles in a fluid. We then present Einstein's statistical deduction of the diffusion equation, leading to the well-known expression of the mean-square deviation of the positions of the Brownian particles, which was duly confirmed by Perrin's experiments. According to the original work of 1908, we also present the much simpler theory of Langevin, which is usually discussed in modern textbooks of statistical physics.
Brownian movement; diffusion; Langevin equation
