layer and the total scale height of the system. Without gravity, the neutral (mostly molecular) layer would simply extend to infinity, as can be readily seen in Figure \[figConstantP\].

The incident radiation field
----------------------------

![ Incident continuum given by the [[table ISM]{}]{} command (dashed line), absorption of ionizing photons by the [[extinguish]{}]{} command with a Hydrogen column density of $10^{21}$ cm$^{-2}$ (solid line), and propagation through a uniform gas slab with constant density of $1$ cm$^{-3}$ and the same Hydrogen column density (dotted line). []{data-label="figContinua"}](figs/Fig4.eps){width="50.00000%"}

![ Effects of the incident continuum on the vertical structure of a model with $n_0=1$ cm$^{-3}$. The [[table ISM]{}]{} case is depicted in Figures \[figN\] and \[figP\]. The two top panels show the molecular, neutral and ionized Hydrogen densities for the incident radiation fields obtained by applying the [[extinguish]{}]{} command and the transmission through a uniform slab. The third and fourth panels show the gas temperature and the total pressure in each case. []{data-label="figExtTrans"}](figs/Fig5.eps){width="50.00000%"}

As discussed in the documentation of the [[table ISM]{}]{} command, ionizing radiation between 1 and 4 Rydberg is heavily absorbed by the neutral Hydrogen in the ISM so that few such photons exist, at least in the Galactic plane. In order 