The two systems, vacuum and plenum, are commonly used together and when employed in connection with the heating device the very best results are obtained. The amount of air, the temperature and humidity, and the purity of the air may be regulated to a nicety in this way. A complete system of =vacuum and plenum= consists of plenum fans for forcing the air into the rooms, vacuum or suction fans for the removal of vitiated air, the ducts for conducting the air to and from the rooms, the necessary machinery to run the fans, and a proper heating system whereby the air may be warmed in the winter and cooled by ice coils in the summer. It is also necessary to have a device for washing the air. Washing the Air As the air is drawn into the buildings by the plenum fan it is passed through a chamber where it is washed. This is done by forcing the air through a spray of water. A water curtain is formed by forcing water through perforated pipes placed across the chamber from each other and the water thus sprayed made to intercept. As the air passes through this curtain of water many of the impurities such as dust, cinders, bacteria, some of the gases, particles of decomposition, and epithelial cells, are removed. Washing does not remove carbon dioxide or bodily odors. Washing is one of Nature’s methods of cleaning the air as is seen in rain. [Illustration: _Central heating with washed air._] After the air is washed it is passed over tempering coils. In the winter the air is warmed and in the summer it is cooled in this way. The humidity is also controlled. It can thus be seen that this method, although rather expensive, is the only real method of ventilating large buildings satisfactorily. CHAPTER V HEATING HEATING GENERAL CONSIDERATIONS HEAT FROM HYGIENIC STANDPOINT COMBUSTION MOLECULAR THEORY BODY HEAT HOW MAINTAINED AT CONSTANT TEMPERATURE INFLUENCE OF BODY HEAT ON HOUSE AIR INFLUENCE OF TEMPERATURE ON AIR OF BODY PROPER TEMPERATURE OF BUILDINGS REQUIREMENTS FOR HEATING SYSTEM LOCAL HEATING RADIATION PRINCIPLE OF OPEN FIRE OBJECTION TO CONDUCTION HEATING BY STOVE OBJECTION TO CENTRAL HEATING CONVECTION CENTRAL HOT AIR HOT WATER STEAM CHAPTER V HEATING GENERAL CONSIDERATIONS Heat from Hygienic Standpoint The subjects of heating, lighting and ventilating will be treated purely from a hygienic standpoint and in no sense from the standpoint of engineering. The proper manner of heating a building is not within the scope of hygiene, but becomes a question of proper engineering and a subject to be considered from that angle. We are concerned only as the heating of buildings influences the health of the occupants. Combustion