The interaction of supernova blast waves with the inhomogenous interstellar
medium leads to a wide variety of morphologies for middle-aged supernova
remnants.  For many remnants, the expected shell morphology is missing in
X-ray and/or radio, and sometimes X-rays are center-filled while the optical
morphology is a partial shell.  It has been speculated that some of the
unusual morphologies are due to interaction with denser regions in the
interstellar medium. Because the cooling rate is much higher in dense
regions, the shock rapidly becomes radiative and X-rays would be absent.
One of the main coolants is the [OI] 63-micron line, which can be
observed with unprecedented sensitivity by ISO.  We already found that the
line is very bright in two of our sample remnants (Reach and Rho 1996).  We
propose observations of 7 SNR (W 28, W 44, CTB 109, IC 443, PKS1209-52,
3C391, and Kes 79) that are known or suspected to be interacting with dense
clouds, and which we also have imaged in X-rays with ROSAT or ASCA.  We
propose new observations that build on our previous proposal WREACH.SNR_OI,
in which we have mapped (at priority 1 or 2) the [OI] line emission for 5
of the remnants.  The new observations consist of complete three parts:
(1) LWS grating spectra to obtain the continuum energy distribution (and
search for other spectral lines) for each of the remnants; (2) LWS
Fabry-Perot observations to measure the line profile of the [OI] line and
confirm the large shifts in line centroid already detected in our grating
spectra of 3C391; and (3) SWS spectra to detect other lines, due to H2,
[SiII], and [FeII], that are characteristic of molecular shocks.  The
brightness of the rotational H2 lines in particular is unknown; these lines
are predicted to be brighter (by orders of magnitude) in slow, magnetized
shocks into dense clouds compared to strong shocks in lower-density clouds.