ABSTRACT
The mean free paths of the thermal particles in interstellar gas are nearly always short compared with the linear sizes of the regions occupied. A given particle (atom, molecule or ion) undergoes many collisions before traversing a significant fraction of the region. The particle velocity distributions are therefore Maxwellian, and we can describe them by a gas kinetic temperature which is usually the same for all species of particle present. The state of the gas can be described in terms of macroscopic properties in addition to temperature, e.g. pressure, density and velocity, which are averages over the properties of many invididual particles contained within regions of extent much greater than the mean free paths. Usually, the interstellar gas is in a state of motion and these bulk properties are functions of both position and time. To understand the nature of the motion, we have to derive equations which govern the flow of gases. The exact solution of these equations is usually difficult and will not be discussed. However, a study of them is essential, firstly, to understand the types of phenomena which occur in gas flow, and secondly, to derive the broad principles used to obtain simple models of various events. One such principle which we shall implicitly use on several occasions follows immediately from the basic assumption that the mean free paths are small: this immediately allows us to conclude that the collision of streams of gas does not result in any appreciable diffusion of one gas into another, i.e. the two component streams retain their identities after the collision and a distinct boundary exists between them.
