Petavius (Dogmat. Theolog. tom. v. l. v. c. 2, 3, 4, p. 217 - 225) and Tillemont (Mem. Eccles. tom. xiv. p. 713, etc., 799) represent the history and doctrine of the Trisagion. In the twelve centuries between Isaiah and St. Proculs's boy, who was taken up into heaven before the bishop and people of Constantinople, the song was considerably improved. The boy heard the angels sing, "Holy God! Holy strong! Holy immortal!"