The average person uses about four bushels of air a minute. Consequently, rooms that are occupied must be constantly replenished with fresh air; otherwise the point is quickly reached where the occupants are breathing an atmosphere that is not only poor in oxygen but saturated with carbonic acid gas and other impurities conveyed by the breath. Foul air such as this causes headache, dizziness, faintness, nausea, and occasionally even more serious disturbances. Those who live in "close" rooms day after day grow pale and languid; their appetite fails and some of their natural power of resistance against illness is lost. Many people are unhealthy simply because they neglect to supply their living quarters with a steady stream of air from the outside. While it is impossible to keep the air in any room as pure as the outside atmosphere, perfectly satisfactory ventilation can be easily arranged. Some of the impure air in a house is always escaping of its own accord and its place is taken by air from the outside. Thus, the cracks around the windows and doors let bad air out and good air in; and, besides, most building materials are porous. These natural paths, however, must be supplemented. The simplest device for ventilation, which is also the best, consists in opening a window at the top and bottom. The width of the opening may be regulated so as to permit the air in the room to change without occasioning disagreeable drafts; if necessary the current may be broken by a screen of some pervious material placed in the opening. The bed-room should always be supplied with plenty, of fresh air, which "quiets the nerves" and helps one to sleep soundly. Furthermore, the temperature of the bed-room should be lower than the temperature of rooms occupied during the day. Both these requisites will be properly met by leaving a window open at night, which may be done throughout the year in most climates, if one puts on enough covering. There is no danger of catching cold from sleeping with the window open; on the contrary, breathing fresh air day and night is one of the best ways to prevent colds.