  The second major difficulty stemmed from the time reversibility of the kinetic theory. The time inverse of any process is as allowable as the original within the kinetic theory. This clashes with the time asymmetry of the second law of thermodynamics and the time-directedness of the observed behaviour of gases. Heat flows spontaneously from hot regions to cold regions and gases in contact spontaneously mix rather than separate. It is true that defenders of the kinetic theory such as Maxwell and Boltzmann were able to accommodate the difficulty by stressing the statistical nature of the theory and attributing time asymmetries to asymmetries in initial conditions. But this meant that a fundamental tenet of thermodynamics, the second law, was in fact only statistically true. Violations were improbable rather than impossible.  Defenders of the kinetic theory had no direct experimental evidence for deviations from the second law.