  The randomness of the motion rules out causes, such as convection currents in the liquid, which operate on a scale larger than the dimensions of the particles. Causes of that kind would lead to correlations between the motions of neighbouring particles and that is precisely what is ruled out by a truly random motion of particles. The permanence of the motion is a puzzle because the particles, moving through a viscous liquid, will be slowed down, losing heat to the liquid, suggesting that the whole motion should come to a halt just as a sizeable object such as a cricket ball, projected into a liquid, will be brought to rest. General, quantitative features of Perrin’s results made life difficult for the anti-atomists, but there was yet more to his case.