Those who live in warm, close, ill-ventilated rooms are much more subject to colds from exposure to draughts and cold air than those who dwell in a pure atmosphere of moderate temperature. This being the case, persons should not accustom themselves to a higher temperature than is barely necessary for comfort. Some persons are most comfortable with a room temperature in winter of 68º or 69º Fahrenheit. Others require a temperature of 70º or 71º. Invalids, infants, and old persons, whose vitality is low, require a higher temperature than those in the full vigor of life. =Ventilation.=--The importance of breathing pure air was fully discussed in a previous chapter. The best methods of securing it will be considered here. Air, when heated, becomes lighter and rises. Cool air, when it enters a warm room, sinks to the bottom. The cooler and purer air of a room is, therefore, always found nearest the floor. Although the carbonic acid given out by the lungs is heavier than an equal quantity of atmospheric air, yet, by the operation of the law of diffusion, it commingles with the other gases, and is found in greatest quantity near the ceiling. Doors and windows are the means commonly employed for ventilation; transoms and special ventilating flues are used to some extent. Where large numbers of people are congregated together for several hours at a time, as in churches, theatres, and public halls, proper ventilation becomes a matter of extreme importance. If, in such cases, doors and windows alone are depended upon, the results are never satisfactory. Those nearest the windows are made uncomfortable by the chilling drafts, while the persons in the middle of the room experience very little relief from the stifling atmosphere. In the construction of such buildings suitable provision should be made in floor, side-wall, and ceiling for an ample supply of pure air, without a conscious current or other annoyance to the audience. In school houses, where children are confined for long periods, and where their physical growth and mental activity demand the purest air, neglect of proper means of ventilation on the part of school directors and trustees is little short of criminal. In the home the subject of ventilation during the day time is a simple matter. The frequent opening of outer doors and of inner doors, with the occasional lowering of the upper sash of the window, will furnish an abundant supply of pure air.