  The emission line regions of starburst galaxies and active nuclei reveal a
wealth of spectroscopic information. A unified picture of the relationship
between ionized, atomic, and molecular gas makes it possible to better
understand these observations. We performed a series of calculations designed
to determine the equation of state, the relationship between density,
temperature, and pressure, through emission-line diagnostic ratios that form in
the H+ region and PDR. We consider a wide range of physical conditions in the
H+ region. We connect the H+ region to the PDR by considering two constant
pressure cases, one with no magnetic field and one where the magnetic field
overwhelms the thermal pressure. We show that diagnostic ratios can yield the
equation of state for single H+ regions adjacent to single PDRs, with the
results being more ambiguous when considering observations of entire galaxies.
As a test, we apply our calculations to the Orion H+/PDR region behind the
Trapezium. We find the ratio of thermal to magnetic pressure in the PDR to be
\~1.2. If magnetic and turbulent energy are in equipartition, our results mean
the magnetic field is not the cause of the unexplained broadening in M 42, but
may significantly affect line broadening in the PDR. Since Orion is often used
to understand physical processes in extragalactic environments, our
calculations suggest magnetic pressure should be considered in modeling such
regions.
