  The first occasion on which Boltzmann returned to the reversibility objection is in (1887b). This paper delves into a discussion between Tait and Burbury about the approach to equilibrium for a system consisting of gas particles of two different kinds. The details of the debate need not concern us, except to note that Tait raised the reversibility objection to show that taking any evolution approaching equilibrium one may construct, by reversal of the velocities, another evolution moving away from equilibrium. At this point Boltzmann entered the discussion: