  For the inner region of the cooling flow (e.g. within the radius of
$\sim$50--100 kpc) the Thomson optical depth of the hot gas in a massive
cooling flow can be as large as $\sim 0.01$. Assuming that the cooling time in
the inner region is few times shorter than the life time of the cluster, the
Thomson depth of the accumulated cold gas can be higher accordingly (if most of
the gas remains in the form of clouds). The illumination of the cold clouds by
the X-ray emission of the hot gas should lead to the appearance of a 6.4 keV
iron fluorescent line, with an equivalent width proportional to $\tau_T$. The
equivalent width only weakly depends on the detailed properties of the clouds,
e.g. on the column density of individual clouds, as long as the column density
is less than few $10^{23} cm^{-2}$. Another effect also associated exclusively
with the cold gas is a flux in the Compton shoulder of bright X-ray emission
lines. It also scales linearly with the Thomson optical depth of the cold gas.
With the new generation of X-ray telescopes, combining large effective area and
high spectral resolution, the mass of the cold gas in cooling flows (and it's
distribution) can be measured.
