ROMAN ARCHITECTURE. IIQ Stuccoes (" Opus albarium ") were frequently used for the coverings of walls both internal and external, and the final coat was polished. Mural paintings were executed on the prepared stucco, and may be classified as follows : — (a.) Fresco painting, (b.) Tempera painting, (c.) Varnish painting, and (d.) Caustic painting. Marble, alabaster, porphyry and jasper as linings to the walls have been already referred to. They were usually attached by iron or bronze cramps to the walls upon a thick cement backing. Mosaics were also much used for ornamenting walls, vaults and floors. They are divided by Middleton into : — (a.) " Opus tesselatum," or " vermiculatum," formed of squared tesserae of stone, marble, or glass to form patterns. (b.) " Opus sectile " or " Opus scutulatum," of tesserae of marble, porphyry, or glass cut into shapes to form the pattern of which the " Opus Alexandrinum " was a very rich variety. (c.) " Opus Spicatum," made of paving bricks in herring-bone fashion. The glass mosaics sometimes forming elaborate figure pictures, were mostly used to decorate the walls and vaults only, and not the floors. Gilded bronze was employed as a roofing material to important buddings, as employed at the Pantheon (page 134). The abundant use of statues, many of them brought from Greece, led to the adoption of niches for their reception within the thickness of the walls. These were either semicircular, crowned with a semi-dome, or rectangular, and they occasionally had columns supporting a pediment, thus forming a frame. 3. EXAMPLES. Etruscan Architecture. — In dealing with Roman Architec- ture mention must be made of the Etruscans or early inhabitants of central Italy, who were great builders, and whose methods of construction had a marked effect on that of the Romans. The style dates from about b. c. 750, and from their buildmgs it is known that they were aware of the value of the true or radiating arch for constructive purposes, and used it extensively in their buildings. The architectural remains consist chiefly of tombs, city walls, gateways (as at Perugia), bridges and aqueducts, and their character is similar to the early Pelasgic work at Tiryns and Mycenae (page 54). The walls are remarkable for their great solidity of construction, and for the cyclopean masonry, where huge masses of stone are piled up without the use of cement, or mortar of any kind. The " Cloaca Maxima " (c. b.c. 578) (No. 47), or great drain of Rome,
