Mercury—rare‐gas discharges at low mercury pressures and moderate rare‐gas pressures form one of the simplest mixed‐gas systems. The mercury supplies the ionization while the rare gas controls particle motions. The product ER is expected to depend upon paR, pmR, I∕Rn. E is the positive column gradient, R the tube radius, pm and pa the mercury and rare‐gas pressures, respectively, and n is expected to vary in the vicinity of 1 or 2. The quantity I∕Rn is a sensitive indicator of any processes in the discharge which are not linear in the electron density. It is important in single‐component discharges also. DC gradient measurements were made in mercury—argon discharges and verify the expected behavior. The ranges covered were 1.0 to 2.1 in., 1.0 to 75 μ, 0.7 to 35 mm, and 0.1 to 3.0 A, respectively, for R, pm, pa, and I, respectively. The behavior of n is partly explained by two‐stage ionization involving resonance atoms whose lifetimes are governed by the imprisonment of resonance radiation. It is also shown how to reduce the gradient measurements in one mercury—rare‐gas combination to those in another rare‐gas combination. Comparison of mercury—argon and mercury—krypton gradients on this basis indicate that the average electron collision cross section in krypton is 1.4 times that in argon for energies near one electron volt.