Polaron radiation is a type of radiation that forces Changelings to revert to their gelatenous states. However, prolonged exposure to polaron radiation can easily kill Changelings and solids alike.
In 2373, the Federation planned to use polaron radiation on a Changeling believed to be impersonating Klingon Chancellor Gowron. The plan failed as a result of tampering by General Martok, who was in fact the true Changeling. (DS9: "Apocalypse Rising")