NITROGEN. THE ATMOSPHERE 291 



Ventilation. In winter, cold air is brought into our rooms. 

 The amount of water vapor contained in this air, even if it is 

 saturated with moisture, is very small (see Appendix IV) . When 

 this air has been heated, therefore, its relative humidity is too low, 

 discomfort is felt because there is too much evaporation, and mois- 

 ture has to be added artificially. Here the moisture afforded by 

 evaporation from our bodies has little effect on the air. In sum- 

 mer, however, the outside air is often already nearly saturated at 

 the temperature of the room. At such times the speed of dis- 

 placement by the ventilating appliances may not be great enough 

 to keep the relative humidity down, and discomfort will arise 

 from the opposite cause. To relieve it, the evaporation may 

 be promoted by electric fans. They do not remove or add any 

 air, but they stir it, and blow away the moist, nearly saturated, 

 layers next to the skin. 



The chief purposes of ventilation are, therefore, to supply fresh 

 air, to keep it in motion, and to maintain a humidity that is 

 neither too low nor too high. 



The Carbon Dioxide. The breathing of animals, the com- 

 bustion of coal and wood, and the decay (oxidation) of vegetable 

 and animal matter produce carbon dioxide CO 2 . The same gas 

 issues from volcanoes, and often in great quantities from the soil 

 in regions which are no longer volcanic. The proportion in the air 

 is therefore greatest in cities and in some volcanic regions, and 

 least in the country and over the sea. It varies from 3.5 parts in 

 10,000 in the country, to 1 per cent in crowded rooms. 



Its presence may be proved in any air, and very quickly in 

 the breath itself, by bubbling the air through calcium hydroxide 

 solution (lime-water). Calcium carbonate is precipitated (p. 

 336). 



Carbon Dioxide and Respiration. We draw about half a 

 liter of air into our lungs at each breath, or hah" a cubic meter per 



