heretics remained in force until the time of Sixtus V. He supported Henry III. against the English barons, and protested against the Pragmatic Sanction of Louis IX. of France. He sent monks to Constantinople to negotiate with the Greeks for church unity, but without result. He canonized Saints Elizabeth of Thuringia, Dominic, Anthony of Padua and Francis of Assisi. He permitted free study of the Aristotelian writings, and issued (1234), through his chaplain, Raymond of Pennaforte, an important new compilation of decretals which he prescribed in the bull _Rex pacificus_ should be the standard text-book in canon law at the universities of Bologna and Paris. Gregory was famed for his learning and eloquence, his blameless life, and his great strength of character. He died on the 22nd of August 1241, while Frederick II. was advancing against him, and was succeeded by Celestine IV. For the life of Gregory IX., consult his Letters in _Monumenta Germaniae historica, Epistolae saeculi XIII. e regestis pontif. Roman. selectae_ (Berlin, 1883); "Les Registres de Gregoire IX," ed. L. Auvray in _Bibliotheque des ecoles francaises d'Athenes et de Rome_ (Paris, 1890-1905); A. Potthast, _Regesta pontif. Roman._ (Berlin, 1875) and "Registri dei Cardinali Ugolino d' Ostia et Ottaviano degli Ubaldini," ed. G. Levi in _Fonti per la storia d' Italia_ (1890). See J. Felten, _Papst Gregor IX._ (Freiburg i. B., 1886); J. Marx, _Die Vita Gregorii IX. quellenkritisch untersucht_ (1889); P. Balan, _Storia di Gregorio IX e dei suoi tempi_ (3 vols., Modena, 1872-1873); F. Gregorovius, _Rome in the Middle Ages_, vol. 5, trans. by Mrs G. W. Hamilton (London, 1900-1902); H. H. Milman, _Latin Christianity_, vol. 5 (London, 1899); R. Honig, _Rapporti tra Federico II e Gregorio IX rispetto alla spedizione in Palestina_ (1896); P. T. Masetti, _I Pontefici Onorio III, Gregorio IX ed Innocenzo IV a fronte dell' Imperatore Federico II nel secolo XIII_ (1884); T. Frantz, _Der grosse Kampf zwischen Kaisertum u. Papsttum zur Zeit des Hohenstaufen Friedrich II._ (Berlin, 1903); W. Norden, _Das Papsttum u. Byzanz_ (Berlin, 1903). An exhaustive bibliography and an excellent article on Gregory by Carl Mirbt are to be found in Hauck's _Realencyklopadie_, 3rd edition.