ABSTRACT
Our knowledge of interstellar space has been revolutionized by many recent observations. This article reviews some of the forces that we now believe control the motions of interstellar gas, and its variations in density and temperature. The observed degree of ionization has led to the cloud-intercloud model of interstellar gas. It also implies that the gas may be in a very dynamic, turbulent state, so that hydrostatic equations of state for this gas must be used with caution. Cosmic rays contribute an important force to interstellar gas dynamics and are an additional source of heat. They are most effective near cosmic-ray sources and at the edge of the galactic disk. Galactic structure affects gas dynamics, not only through the shock that bounds spiral arms, but also by a large-scale, ordered magnetic field in a way that encourages escape of gas, cosmic rays, and magnetic flux out of the galactic disk. But this field amplffication also depends on local "turbulence." Minuet or Rock? Large-scale order or turbulence? You don't have one without the other! Key words: interstellar clouds - cosmic rays - magnetic dynamo