A wood or grate fire is an excellent ventilator. A heating-system which introduces warmed new air is better than one acting by direct radiation, provided the furnace is well constructed and gas-proof. [Sidenote: Cool Air] The importance of coolness is almost as little appreciated as the importance of motion. Most people enervate themselves by heat, especially in winter. The temperature of living-rooms and work-rooms should not be above 70 degrees, and, for people who have not already lost largely in vigor, a temperature of 5 to 10 degrees lower is preferable. Heat is depressing. It lessens both mental and muscular efficiency. Among the employes of a large commercial organization in New York who were examined by the Life Extension Institute, some of the men in one particular room were suffering from an increase of body temperature and a skin rash. On investigation it was found that the room in which they worked was overheated. There was no special provision for ventilation. A window-board was installed, with the result that the men recovered and no other cases of skin rash occurred in that room. [Sidenote: Dry air] As to dryness of air, there is little which the individual can do except to choose a dry climate in which to live or spend his vacations. Unfortunately, there is not as yet any simple and cheap way of drying house air which is too moist, as is often the case in warm weather. [Sidenote: Humidity] In the cold season, indoor air is often too dry and may be moistened with advantage. This may be done, to some extent, by heating water in large pans or open vessels. But for efficient moistening of the air, either a very large evaporating-surface or steam jets are required. The small open vessels or saucers on which some people rely, even when located in the air-passages of a hot-air furnace, have only an infinitesimal influence. Vertical wicks of felt with their lower ends in water kept hot by the heating apparatus yield a rapid supply of moisture. Evaporation is greatly facilitated if the water or wicks are placed in the current of heated air entering the room. By a suitable construction, the water may be replenished automatically. In very cold dry weather, the air-supply of an ordinary medium-sized house requires the addition of not less than 10 gallons of moisture every 24 hours, and sometimes much more. Some authorities doubt any ill effects from extreme dryness. This is a subject yet to be cleared by experimental research. [Sidenote: Freshness] It is obvious that fresh pure air is preferable to impure air. Air may be vitiated by poisonous gases, by dust and smoke, or by germs. Dust and smoke often go together. Lighting by electricity is preferable to lighting by gas, as some of the gas is liable to escape and vitiate the air. [Sidenote: Tobacco Smoke]