Polaron radiation was a type of radiation that forced Changelings to revert to their gelatinous states. However, prolonged exposure to polaron radiation could easily kill Changelings and solids alike.
In 2373, the Federation planned to use polaron radiation (distributed by a polaron emitter) 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")