gibson=ras.ucalgary.ca; russ=ras.ucalgary.ca AND

We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21 cm synthesis maps, taken from the Canadian Galactic Plane Survey, show a numerous and diverse population of H I self-absorption (HISA) features in gas outside the solar circle. These objects vary in size, shape, and con-trast against the background H I. All display a high level of angular and velocity structure, and most would appear signiÐcantly diluted, if not invisible, in lower resolution H I surveys. A number of Perseus arm features remain unresolved by the 1 @ beam of our survey, with apparent diameters less than 0.6 pc at 2 kpc distance. The majority of HISA features we detect have no obvious 12CO emission counterparts. This suggests that either HISA is not found predominantly in molecular clouds, as has often been pre-sumed in the past, or that CO is not a good tracer of Some HISA lacking CO shows far-infraredH2.dust emission, though whether this arises from shielded molecular gas or from diuse atomic clouds is not clear. Constraining the gas properties of HISA remains a difficult problem, but we introduce a new method that aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships and should prove powerful if the input variables are sufficiently well known. We explore the current allowed parameter ranges for three sample features of very dierent appearance. We Ðnd spin temperatures K and densities cm~3.[50 Z10