Where do Calvus’s slaves keep themselves? Undoubtedly in the very cramped barracks of the second story, a section of which looks down from an upper tier of columns above the court of the peristylium. Even lordly Romans spend little time in their chambers and need only small bedrooms. For the slaves there is extremely little accommodation; any kind of a sleeping pocket, very truly called a “cell” (_cella_) will answer, where a stool, a blanket, and a thin mat on the floor suffice for all save the upper servants. Under the house there are ordinary cellars for the storage of provisions. Somewhere, too, is a strong room, with barred windows, and heavy door, and inside, fastened upon the floor, a set of stocks and manacles. Lucky is the day when, in a slave-familia of this size, this lock-up has not at least one backsliding occupant. =37. The Floors and Windows.=--Inquiring about certain details of such a mansion we discover that like most other Roman houses, it is built of concrete, faced with brick or coarse stone and stucco, and then with as many interior surfaces as possible, covered with slabs of marble or decorative frescos. The roof is of brick tiles; the floors in the humbler chambers, where mosaic is unnecessary, are partly of concrete and partly of small pieces of stone and tile roughly fitted together and then pounded down by a rammer (_pavimentum_). Two or three rooms most used in winter have a special and very luxurious device--part of their floors are made of hollow tile pipes, and through these hot air from a furnace can be forced to warm them precisely as is done at the baths.[20] Little thus far has been said about the windows. These open mainly upon the courts, and they are so few that very many rooms, especially those used by the slaves, seem disagreeably dark, although in the long, hot season this drawback somewhat vanishes. Most of the windows are closed merely by board shutters swinging in leaves, and rather handsomely paneled; but shutting them results in a state of artificial night. For certain rooms used by the master and mistress there is a much better arrangement. Numbers of small pieces of glass are set in bronze lattices and inserted in the windows. Glass cannot be made that is strictly transparent, but it is highly translucent. Such rooms are delightfully illuminated all day long. Certain other wealthy houses use windows set with translucent talc (soft magnesium silicate), but these openings are hardly as satisfactory. Glass is slowly coming into general use, and the window panes will improve as glass-makers learn how to blow larger sheets and to make their product more transparent.