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.