THE BAPTISTERY,
said to have been erected by Constantine. Eight columns of porphyry support a cornice, upon which are eight smaller columns; these sustain the cupola. The font is of green basalt. A tradition says Constantine was baptized here, though Socrates says he received Christian baptism at Nicomedia just before his death. Gibbon says Rienzi bathed in the font on the night before he was made a knight. The two side chapels, dedicated respectively to John the Baptist and John the Evangelist, are said to have been made out of the house of Constantine. The mosaics are of the fifth century, after the arabesque paintings in the Baths of Titus. Adjoining is the Oratory of S. Venantius, in which is a mosaic of the seventh century—our Saviour in the act of giving his blessing. Two grand porphyry columns, supporting an entablature, formed the portico of the baptistery, opposite side to where we entered. There is a mosaic vault of the sixteenth century in the left chapel of this portico, and in the opposite one a good S. Philip Neri by Guido.
On our left of the obelisk is