  We assume a distribution of clouds optically thick for electron scattering
(OTCM) which are moderately optically thin for absorption and we consider them
as a model of the mean quasar spectrum of Laor et al. (1997). We show that the
model is particularly sensitive to the value of the ionization parameter $\xi$
and that for $\xi \sim 500$ the model well reproduces the optical/UV/X-ray mean
quasar spectrum, in agreement with the estimates of the ionization parameter
based on the energy of the iron $K_{\alpha}$ line. We cannot definitively
reject synchrotron emission as a source of primary radiation but we favor the
model in which the hard X-ray emission is produced by Compton scattering of
soft photons in a central hot medium surrounded by cool clouds. In such a model
clouds are located typically at the distance of $\sim 12 R_{Schw}$, with the
covering factor about 0.88 and the radius of hot plasma is $\sim 9 R_{Schw}$.
The model explains optical/UV emission as predominantly due to the dark sides
of the clouds and the soft X-ray emission as due to the reflection by the
irradiated sides of the clouds. Therefore, atomic features are expected in
these bands although they are hardly present in the observational data. The
kinematical effects connected with the cloud motion affect those features but
do not remove them. The level of the primary emission required to model the
mean quasar spectrum is too low to reproduce the equivalent width of the iron
K$_{\alpha}$ line correctly but more detailed computations may resolve this
problem.
