  The variation of an ionizing flux as a mechanism to stimulate the
condensation of a diffuse gas is considered. To illustrate this effect, two
situations are examined: one on the context of pregalactic conditions, and the
other on the context of the actual interstellar medium. We focus our attention
on flash-like variations; that is, during a ``short'' period of time the
ionizing flux is enhanced in comparison to the pre- and post-flash values. In
both cases the cause of the induced phase change is the same: the enhancement
of the cooling rate by the increase in the electron density caused by the
momentary increase of ionizing flux. After the passing of the flash, the
cooling rate remains enhanced due to the ``inertia of the ionization''. In the
first case (metal free gas) the cooling rate is enhanced due to the fact that
the increase of the electron density makes possible the gas phase formation of
H_2 by the creation of the intermediaries H^- and H^+_2. We show that after the
passing of the photo-ionizing flash a cloud near thermo-chemical equilibrium at
~8000 K may be induced to increase its H_2 content by many orders of magnitude,
causing a rapid decrease of its temperature to values as low as 100 K. In the
second case (solar abundances gas) the dominant cooling mechanism of the warm
neutral gas (the excitation of heavy ions by electron impacts) is proportional
to the electron density. We show that, for the expected states of the warm
interstellar gas, ionizing flashes may induce the phase transition from the
warm to the cool phase. The results indicate that the mechanism of induced
condensation studied here might play a relevant role in the gas evolution of
the diffuse gas in both, the pregalactic and the actual interstellar medium
conditions.
