  It is not easy to pinpoint this transition in the course of history, except to say that in Maxwell's work in the 1860s definitely belong to the first category, and Gibbs' book of 1902 to the second. Boltzmann's own works fall somewhere in the middle. His earlier contributions clearly belong to the kinetic theory of gases (although his 1868 paper already applies probability to an entire gas system), while his work of 1877 is usually seen as belonging to statistical mechanics. However, Boltzmann himself never indicated a clear distinction between these two different theories, and any attempt to draw a demarcation at an exact location in his work seems somewhat arbitrary.