  Brownian motion is the fluctuating motion of particles of an emulsion visible through a microscope. Two features of it led physicists in the late nineteenth century to suspect that it was caused by the molecular motions assumed in the kinetic theory. Those two features were its permanence and its random character. Perrin’s experiments of 1908 were able to give precision to those suspicions. He was able to show that the motions of the particles are indeed random, in a technical sense, and he showed that the general features of the motion are permanent, once equilibrium has been reached. The density distribution and mean free path of the particles remain constant at constant temperature. The kinetic theory had a ready explanation of these features, attributing the randomness to the randomness of the motions of the molecules making up the liquid in which the emulsion was suspended, and the equilibrium conditions as a dynamic equilibrium corresponding to the distribution of velocities formalised by Maxwell. Those wishing to resist the conclusion that Brownian motion constituted strong evidence for the kinetic theory needed to offer some alternative explanation for the two features.