Аннотация:When a phase transition occurs from a thermally stable state at (5-10) x 10 to the 4th K to another stable state at (1.5-2) x 10 to the 4th K in a medium heated by X rays whose spectrum contains no soft photons with E less than 200 eV, the volume emission coefficients of optical spectral lines should undergo nonmonotonic changes. These will take the form of abrupt flares rising about the steady level by a factor of 5-20 within an interval of (1-2) x 10 to the 12th/nH sec, where nH is the number density of H atoms. This effect could prove decisive for explaining the rapid variability of the spectral lines observed in Seyfert-galaxy nuclei and quasars.