 The force of Perrin's argument for the kinetic theory, and hence for the reality of molecules, stems from the fact that his argument requires only the central assumptions of the theory, the equipartition of energy and the randomness of molecular agitation, without requiring the addition of auxiliary or simplifying assumptions. It is this fact that made his calculations of Avogadro's number qualitatively distinct from, and more telling than, other estimates. Ostwald cited this as the reason for his conversion to belief in molecules (Nye, 1972, 151–152). Interestingly, the derivation of the ratio of the principle specific heats of a gas similarly requires only the basic assumptions of the kinetic theory cited above. That is why the clash of the prediction with measured values spelt serious trouble for the classical kinetic theory. Equipartition of energy breaks down for the vibrational modes of a molecule and for rotational modes also at temperatures sufficiently low, as Perrin (1990, 73) noted.