  X-ray spectra of a significant fraction of cooling flow (CF) clusters of
galaxies indicate the presence of a large column of ``cold'' absorbing gas. The
physical nature of the absorbing medium remains a mystery. Searches for H I
absorption using the 21 cm hyperfine structure line yielded null results in
most cases. The purpose of this contribution is to point out that the Lyman
$\alpha$ absorption cross section is ~10^7 times larger than for the 21 cm
line, it can therefore be used as a very sensitive probe of the H I column in
clusters, and can thus place stringent constraints on the nature of the X-ray
absorber. This method is applied to the Perseus CF cluster where a medium
resolution (~250 km/s) UV spectrum is available. The upper limit on the H I
column obtained using Lyman $\alpha$ is at least ~50 times smaller than the 21
cm detection, and ~5,000 smaller than implied by X-ray spectra, indicating that
the X-ray absorber is exceedingly devoid of H I. Higher resolution UV spectra
with HST may improve the H I column limits by an additional factor of ~4,000.
This method can be applied to strongly constrain the nature of the X-ray
absorbing medium in a significant fraction of CF clusters .
