  We present two ROSAT PSPC observations of the radio-loud, lobe-dominated
quasar 3C 351, which shows an `ionized absorber' in its X-ray spectrum. The
factor 1.7 change in flux in the $\sim$2~years between the observations allows
a test of models for this ionized absorber.
  The absorption feature at ~0.7 keV (quasar frame) is present in both spectra
but with a lower optical depth when the source intensity - and hence the
ionizing flux at the absorber - is higher, in accordance with a simple,
single-zone, equilibrium photoionization model. Detailed modeling confirms this
agrement quantitatively. The maximum response time of 2 years allows us to
limit the gas density: n_e > 2 x 10^4 cm^{-3}; and the distance of the ionized
gas from the central source R < 19 pc. This produces a strong test for a
photoionized absorber in 3C~351: a factor 2 flux change in ~1 week in this
source must show non-equilibrium effects in the ionized absorber.
