Whatever the system of ventilation adopted, it is wise to _flush rooms frequently with fresh air_. This is best effected by throwing the windows wide open whenever a room is left unoccupied. In this way a much more thorough and complete purification is effected than by any other means. This is especially important in the case of bedrooms, in which organic impurities are most prone to accumulate. Not only should rooms be ventilated, but likewise _the furniture_ they contain. This again is most important for bedrooms. Beds should not be “made” till sometime after using; and in the interval, should be freely exposed to the air. The same applies to night apparel. It is well to allow _rooms to lie fallow_ at intervals. Organic matter accumulates about a room, and devitalises any air which enters. If the room is vacated, and flushed with air for a continuous period, it becomes sweeter and purer. The importance of this is now well recognised in the case of hospital wards. Such temporary disuse of rooms must not, however, be regarded as sufficient without thorough cleansing of every surface in them, in order effectively to remove all organic and other dust. =An Inlet and Outlet= for air should both be provided. According to some an inlet only is required, while others would only provide an outlet; but a perfect system of ventilation requires both. As heated air expands, the outlets should theoretically be larger than the inlets; but as the average difference of temperature is only 10°-15° Fahr., the expansion is only slight, and may be practically neglected. The necessity for both inlets and outlets may be illustrated by a single apparatus like that shown in Fig. 11. A taper is burning at the bottom of the jar, in the stopper of which two tubes, A and B, are placed. So long as both tubes are kept open the candle will keep alight, but if A be blocked, the candle goes out. [Illustration: FIG. 11. ILLUSTRATING NECESSITY OF INLET AND OUTLET.] =Inlets= should bring air from a pure source, and should be arranged at intervals in large rooms. Externally, inlets should be protected from the wind; and the shorter the inlet tubes the better, as thus a current is ensured, and they can be easily cleaned. The position of inlets should not be too near the outlets, otherwise the fresh air may escape immediately. The best position for inlets is at the floor, but this necessitates warming the entering air, as otherwise it would be intolerable, except in summer time. If the air cannot be warmed, it should he admitted about seven feet above the floor, and directed upwards. For size of inlets, see page 142.