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_