We are going to talk about gases in terms of molecules, and molecules in terms of gases. On the one hand, we have a scheme of representation for the system behaviour of a gas which deals in such things as pressure, volume, temperature, entropy and internal energy. On the other hand, we have some notions about the microscopic particles which in some way aggregate together to form the gaseous system. In order to bring these two worlds, the macroscopic and microscopic, into some kind of correspondence with each other, we have to find appropriate analogues of behaviour. The point is to work towards an understanding of how molecules can coalesce into a system.