  where the proportionality constant depends on the geometry of the collision and the relative velocity. For Maxwell, and Boltzmann later, this assumption seemed almost self-evident. One ought to note, however, that by choosing the initial, rather than the final velocities of the collision, the assumption introduced an explicit time-asymmetric element. This, however, was not noticed until 1895. Maxwell showed that, under the SZA, the distribution (1) is indeed stationary. He also argued, but much less convincingly, that it should be the only stationary distribution.