Predictions of H ii, Si ii, Si iii, Si iv, and Mg ii column densities as a function of the gas density (log n_H) for an H i column density of log N(H i)= 15, based on a Cloudy ionization model using the local UV background and assuming absolute and relative solar abundances of all elements. The gray-shaded area indicates the density that is typical for the Si iii/Si iv phase (see Sect. 7.3).