It is evident that with the use of hot-air furnaces which take their entire amount of air from outdoors, the extra amount of heat necessary for this improved quality of atmosphere is very well expended. The use of ventilating devices adds only a relatively small amount to the total cost of heating and provides for the well-being of the occupants of the house--in the form of good air--an amount of healthfulness impossible of calculation. The best ventilation is attained where a constant supply of fresh air is admitted to the house at points from which the best circulation may be secured and equal quantities of vitiated air are removed from the different apartments. It is understood that in the process of natural ventilation the desired condition can only be approximated and that the permissible ventilation appliances are so placed as to give results such as to permit the air to follow the natural laws that must prevail. If the house is heated by stoves, the outside air is best admitted near the ceiling, so that the cold air on entering may come into contact and mingle with the warmest air in the room. The circulation will by this method be effected by gravity. In the use of the hot-air furnace, the air supply--as has already been explained in the figures on pages 55 and 58--is brought from the outside, where after being heated it enters the rooms through the registers placed near the floor. Being warmer than the air in the room, it tends to quickly rise. The currents set up by its motion help to produce a uniform temperature and to diffuse the new air through the entire space. The more evenly the air is distributed the more uniform will be the condition of temperature of the room. In hot-water and steam heating, the direct method of heating in Fig. 29 and the indirect method of Fig. 30 show two forms of apparatus for admitting air to buildings that are quite generally employed for ventilation of dwellings. In the use of all such devices for ventilation purposes, there should be provided means of escape of air corresponding in amount to the fresh air admitted. The exhaust air vent should be located near the floor to bring about the best results. The degree of success attending the use of such apparatus will depend on the amount of care taken, to suit the position of the dampers to the prevailing weather. [Illustration: FIG. 165.--The Wolpert air tester; an instrument used to determine the quality of air.] =The Wolpert Air Tester.=--The purity of air is expressed by quantity of carbonic acid gas included in its composition. In order to determine the degree of purity of any atmosphere the amount of contained gas must be determined. This is accomplished by use of simple apparatus that may be successfully operated by those who are unacquainted with chemical analytical methods. The process is due to chemical action but the manipulation of the required apparatus is purely mechanical.