This is the third and final volume in the study and publication of James Clerk Maxwell's work in gas theory, molecules, and thermodynamics. The nineteenth-century Scottish physicist derived his ideas on thermodynamics from an interest in theories of matter, not contemporary concerns with heat engines and engineering. The manuscripts and papers presented here reveal the development of his ideas and the uniqueness of his interpretations of mechanics, the necessity of a statistical interpretation of the second law of thermodynamics, and his understanding of the dynamics of rare gases. They also reveal the context of a well-developed discipline and professional community to which Maxwell reacted and to whom he needed to respond. These papers shed light on the formation of Maxwell's ideas and theories within the structure of a professional scientific discipline, physics, that had only recently taken shape. While Maxwell responded to and relied on the work of his colleagues, his interpretations often placed his work apart from theirs, to be exploited by later generations of physicists.