The introduction to Part 1 (Lighthill 1957) proposed a paper in three parts, in the nature of a broad introductory discussion of the dynamics of a pure dissociating gas, which might act as a source of ideas for later, more comprehensive investigations. Part 1 treated ‘equilibrium flow’, Part 2 was to treat transport properties under ‘quasi-equilibrium’ conditions, and Part 3 flows in which large departures from equilibrium occur. When Part 1 was submitted for publication, much of Part 2 had already been written. At this moment, however, the author suddenly became obliged to abandon the project for two years, in order that his contributions to two collective works, Laminar Boundary Layers and Incompressible Aerodynamics, could be produced in time to avoid serious delay in the appearance of the contributions of other authors.