  We consider the thermal properties of cold, dense clouds of molecular
hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal
pressure is sufficient to permit the existence of particles of solid or liquid
hydrogen at temperatures above the microwave background temperature. Optically
thin thermal continuum emission by these particles can balance cosmic-ray
heating of the cloud, leading to equilibria which are thermally stable even
though the heating rate is independent of cloud temperature. For the Galaxy,
the known heating rate in the disk sets a minimum mass of order 10^-6 Msun
necessary for survival. Clouds of this type may in principle comprise most of
the dark matter in the Galactic halo. However, we caution that the equilibria
do not exist at redshifts z > 1 when the temperature of the microwave
background was substantially larger than its current value; the formation and
survival of such clouds to the present epoch therefore remain open questions.
