  In his (1868), Boltzmann set out to apply this argument to a variety of other models (including gases in a static external force field). However, Boltzmann started out with a somewhat different interpretation of probability in mind than Maxwell. For him, \(f(\vec{v})d^3\vec{v}\) is introduced firstly as the relative time during which a (given) particle has a velocity between \(\vec{v}\) and \(\vec{v} + d^3\vec{v}\) (WA I, 50).  But, in the same breath, he identifies this with the relative number of particles with this velocity. This equivocation between different meanings of probability returned again and again in Boltzmann's writing.[7] Either way, of course, whether we average over time or particles, probabilities are defined here in strictly mechanical terms, and therefore objective properties of the gas. Yet apart from this striking difference in interpretation, the first section of the paper is a straightforward continuation of the ideas Maxwell had developed in his 1867. In particular, the main ingredient is always played by the SZA, or a version of that assumption suitably modified for the case discussed.