The body quickly becomes chilled in a cold damp air because the increase in the water vapor increases the conductivity of the air for heat. It can thus be seen that the heat producing processes of the body must be increased in order to maintain the normal temperature of the body. As we know, all activity of the body requires an expenditure of energy, so if the body is exposed to a cold damp air for an abnormal length of time it will necessitate an increased expenditure of internal energy to meet the increased demands for heat and thus dissipate forces that should and would, under normal conditions, be utilized in the metabolism of the body. If the heat production is at a minimum in the body, which it may be, due to several factors such as old age, infancy, or dis-ease, this exposure to lowered temperature and high humidity will result in injury to the body. An interference with the transmission of mental impulses to the kidneys resulting in a decrease in their functional activity may mean a retention of poisons in the body and produce a condition diagnosed as rheumatism, for example; or an interference with the serous circulation may result in the tissue cells being under-nourished. Both of these conditions will interfere with the processes of intellectual adaptation. Educationally, man may help in the adaptation by proper clothing and by giving attention to proper exercise. Warm Dry Air By far the most desirable air is the warm, relatively dry air; but as has already been noted, it is possible to have an atmosphere with a relative humidity that is too low. If the air is abnormally dry, and at the same time warm, there will be a great loss of body moisture due to the increased evaporation. When the loss of water from the body reaches 21% death ensues. In an experiment performed by Rubner and Lewaschew it was found that a man weighing about 127 pounds gave off about 54.1 grams of water in an hour in a temperature of 68° F. with a relative humidity of 82%; the same individual in a temperature of 68° F., but with a relative humidity of 82%, gave off only 15.3 grams. Proper Temperature and Humidity The most desirable atmospheric conditions are obtained at a temperature of 68° F. to 70° F., with the relative humidity from 40% to 60%. This is given only as a general average and will necessarily vary with many conditions, such as seasons of the year and occupation. Moderately cool and relatively dry air increases the activities of the body, makes breathing easy and more frequent, and hence increases the circulation of the blood. Innate is thus enabled, in such an atmosphere, to bring about more perfect metabolism. Proper temperature and humidity of the air in houses, impurities found therein, source of impurities and their effect upon health, will all be considered thoroughly under Hygienic Housing. CHAPTER IV VENTILATION VENTILATION