  Roughly speaking, one may divide Boltzmann's work in four periods. The period 1866–1871 is more or less his formative period. In his first paper (1866), Boltzmann set himself the problem of deriving the full second law from mechanics. The notion of probability does not appear in this paper. The following papers, from 1868 and 1871, were written after Boltzmann had read Maxwell's work of 1860 and 1867. Following Maxwell's example, they deal with the characterization of a gas in thermal equilibrium, in terms of a probability distribution. Even then, he was set on obtaining more general results, and extended the discussion to cases where the gas is subject to a static external force, and might consist of poly-atomic molecules. He regularly switched between different conceptions of probability: sometimes this referred to a time average, sometimes a particle average or, in an exceptional paper (1871b), it referred to an ensemble average. The main result of those papers is that from the so-called Stoßzahlansatz (SZA), i.e. an assumption about the number of collisions (or a closely analogous assumption) the Maxwellian distribution function is stationary, and thus an appropriate candidate for the equilibrium state. In some cases Boltzmann also argued it was the unique such state.