The walls were sometimes overlaid with costly marbles; and, as if no product of nature could be sufficiently rich for Roman display, even the marble itself was not unfrequently covered with paintings by first-rate artists. Artificial marbles, in the production of which the workmen of Italy at that time excelled, sometimes supplied the place of the real. But a favorite mode of decoration was by painting the walls in panels--either in fresco, distemper, or encaustic. The colors were usually very brilliant. Wreaths of flowers, architectural, historic, and domestic scenes, or copies of still life, were among the usual subjects for such paintings. Elaborate mouldings cornices, and ornaments in relief, were also employed in decorating the walls. The ceilings were formed of polished beams, with their interstices glued; or they were arranged in panels and then decorated. The beams and panels were gilt, richly inlaid with ivory and tortoise-shell, or painted in brilliant colors. As the rooms generally derived their air and light from the large hall and the peristyle, both of which opened upward, there was no great necessity for closed windows. But these existed in such rooms as opened on the street, or directly on the air without. They were fitted with lattice-work and shutters, with plates of tile imported from Cappadocia, and at a later period with glass. The ordinary methods for warming rooms were the same as in Greece, with this important addition, that the use of hot air, conveyed to the various chambers by pipes, was common among the Romans. The hot air was derived from a furnace--either special for the purpose, or that used for heating water supplied to the baths. Moreover, in Rome and northern Italy chimneys were used in dwelling-houses, and probably they were everywhere employed for the baths and bake-houses. It seems to us strange that a contrivance apparently simple should have been so long unknown, and always looked on as a luxury.