  We demonstrate a technique for obtaining the density of atomic vapor, by
doing a fit of the resonant absorption spectrum to a density-matrix model. In
order to demonstrate the usefulness of the technique, we apply it to absorption
in the ${\rm D_2}$ line of a Cs vapor cell at room temperature. The lineshape
of the spectrum is asymmetric due to the role of open transitions. This
asymmetry is explained in the model using transit-time relaxation as the atoms
traverse the laser beam. We also obtain the latent heat of evaporation by
studying the number density as a function of temperature close to room
temperature.
