  We compare the radio and soft X-ray brightness as a function of position
within the young supernova remnant Cassiopeia A. A moderately strong
correlation (r = 0.7) was found between the X-ray emission (corrected for
interstellar absorption) and radio emission, showing that the thermal and
relativistic plasmas occupy the same volumes and are regulated by common
underlying parameters. The logarithmic slope of the relationship, ln(Sx-ray) =
1.2 x Sradio + ln(k) implies that the variations in brightness are primarily
due to path length differences. The X-ray and radio emissivities are both high
in the same general locations, but their more detailed relationship is poorly
constrained and probably shows significant scatter. The strongest radio and
X-ray absorption is found at the western boundary of Cas A. Based on the
properties of Cas A and the absorbing molecular cloud, we argue that they are
physically interacting. We also compare ASCA derived column densities with 21
cm H I and 18 cm OH optical depths in the direction of Cas A, in order to
provide an independent estimate of ISM properties. We derive an average value
for the H I spin temperature of about 40 K and measure the ratio of OH to
molecular hydrogen to be nominally larger than previous estimates.
  Keywords: Cas A, Cassiopeia A, interstellar medium, molecular clouds, radio
astronomy, supernova remnants, X-ray astronomy
