This chapter begins the study of the physical study of Brownian motion. The theory of Smoluchowski is based on a kinetic model of colliding spheres, at first for an ambient dilute gaseous medium. This result is then generalized, approximately to a dense ambient medium. It is indicated how fluctuations may give an appreciable effect when the effect of an individual collision is negligible. Essentially the same result is obtained by Einstein's theory, using probabilistic, instead of kinetic reasoning. The relation of the friction constant to the diffusion coefficient is obtained. The Langevin equation, expressing the foregoing ideas in dynamical terms is discussed.
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