This incomparable circular edifice, originally intended by Agrippa to form the conclusion of his thermæ,[14] with which it is intimately connected, is one of the noblest and most perfect productions of that style of architecture specifically denominated Roman. When the first wonderful creation of this species came into existence, the founder of this glorious dome appears to have himself shrunk back from it, and to have felt that it was not adapted to be the every-day residence of men, but to be a habitation for the gods.

The Church of S. Maria ad Martyres was originally the sudatorium, or sweating-room, of the baths of Agrippa, being similar in construction to all the sweating-rooms now existing, notably one in the Villa of Hadrian at Tivoli. It exactly answers Vetruvius's description of this department of the baths. It seems afterwards to have been dedicated as a temple of the gods, or Pantheon of the Julian line, according to Dion Cassius (liii. 27), when the portico was added in the third consulship of Agrippa.

M. AGRIPPA . L. F. COS . TERTIUM . FECIT.

The straight vertical joint where the Greek portico has been built up to the Roman body can be distinctly seen, and the pediment and entablature can be observed behind the portico. It was burned in the fire under Titus; and was restored, as the inscription on the architrave tells us, by Septimius Severus and Caracalla—

PANTHEUM VETUSTATE CORRUPTUM CUM OMNI CULTU RESTITVERUNT.

Recent explorations have shown that in front of the Pantheon was a large enclosure surrounded by a covered arcade, somewhat after the manner of the colonnade at S. Peter's, and entered by an arch of triumph. Remains of this arch exist under the houses in front of the Pantheon, which are to be pulled down.

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When Agrippa dedicated the Pantheon as a temple, it was consecrated to Jupiter the Avenger. "Some of the finest works that the world has ever beheld ... the roofing of the Pantheon of Jupiter Ultor that was built by Agrippa" (Pliny, "N. H." xxxvi. 24). The repairs commenced by Septimius Severus and Caracalla were completed by Alexander Severus, who built his baths close by. We call attention to a coin of this emperor, which represents the temple and its enclosure on the reverse; on the obverse is the emperor's portrait, and the legend IMP . C . M . AVR . SEV . ALEXANDER . AUG. On the coin the columns are placed close on either flank, and two are omitted, to show the seated statue of Jupiter in the temple, which statue is now in the Hall of Busts in the Vatican Museum, and is a copy of the celebrated Jupiter of Phidias.