IRAS emission typical of cool dust is detected from the inner
100-200 kpc in 10% of rich clusters.  The far IR luminosities are
1E44-1E45 ergs/s, comparable to the entire blue luminosity of
the central cD galaxy and ten times greater than the X-ray
emission from the cluster core.  The dust is probably heated by
collisions with the hot cluster gas, which has the profound
implication that far IR radiation and not X-ray emission is the
primary coolant in the cores of these clusters.  We propose 60-
180 um ISOPHOT mapping observations to determine the
origin of the dust (whether this material was stripped from a
galaxy or is due to stellar mass loss in the cD), the temperature
and mass of the dust, and whether this phenomenon is very
common to clusters, but at flux levels inaccessible to IRAS.