We propose to use ISO observations of infrared fine structure lines
 to measure the amounts of line-emitting gas at the centers of
 cooling-flow clusters, the depletions of refractory elements
 in this gas, and the hardness of the photoionizing spectrum that
 fuels the line emission.  If clusters contain additional X-ray
 absorbing cold gas extending to 100 kpc radii, our observing
 program will allow us to study this gas as well.  The clusters
 we wish to observe (Autumn launch: Abell 1795 and Abell 2597;
 Spring launch: 2A0335+096 and PKS0745-191) are among the brightest
 cooling-flow clusters in H-alpha emission.  We have already obtained
 very deep Palomar optical spectra of these clusters.  Complementary
 ISO measurements of the Ne II (12.8 microns), Ne III (15.6 microns),
 Si II (35 microns), and O I (63 microns) lines will provide unique
 information about both the ionized gas and any neutral gas also
 present.  The [Ne III]/[Ne II] ratio responds sensitively to the
 spectral shape of the ionizing radiation, helping to identify the
 currently unknown mechanism powering the nebulae.  The [Si II] flux
 indicates the silicon depletion and thus the dust content and origin
 of the nebular gas.  The [O I] flux reveals the total amount of
 neutral X-ray heated gas and will show whether a large mass of
 hitherto unseen condensed gas lies at the centers of cooling flows.
 Measurements of [O I] with the LWS will also yield deep, sparsely-
 sampled, far-IR continuum spectra of these clusters suitable for
 studying their dust contents.