290 



tube containing dry calcium chloride. It may be ascertained also 

 by noting the temperature to which the air has to be cooled before 

 it becomes saturated and deposits fog or dew. For example, if 

 air at 18 has to be cooled to 11 before it deposits dew, it contains 

 water vapor at a partial pressure of 9.8 mm. If saturated at 18, 

 it would have contained water vapor under a partial pressure 

 of 15.4 mm. Its relative humidity was therefore 9.8/15.4, or 

 63.6 per cent. 



Moisture and Comfort. The chemical changes occurring 

 in our bodies, and particularly the oxidation of waste and of 

 digested food by oxygen carried by the blood, are accompanied by 

 liberation of heat. Yet our bodies must remain at 98.4 F. 

 (37 C.). A rise of a few tenths of a degree (F.) produces notice- 

 able discomfort. Much of the heat is lost by radiation from 

 the surface. The extent of this loss depends upon the surface, 

 which is invariable, and upon the surrounding temperature, which 

 we can not always control. Non-conducting clothes reduce the 

 radiation, and are increased in thickness in cold weather. The 

 real adjustment, however, is accomplished, independently of 

 radiation, by evaporation of water at the surface of the skin. 

 The evaporation of 1 gram of water requires about 540 calories of 

 heat. Evaporation of a single half-ounce (14 g.) of water will 

 therefore lower the temperature of 76f kilograms (168 pounds) 

 of water (or of flesh, which is largely water) by one-tenth of 

 a degree C. (nearly 0.2 F.). 



Our comfort, then, depends upon the possibility of continual, 

 moderate evaporation from the surface of our bodies. " Much " 

 moisture in the air means, to us, not necessarily a great absolute 

 amount, but a near approach to the maximum possible at the 

 existing temperature. So the ratio of the amount present, to 

 the maximum the humidity is the significant fact for a 

 practical purpose, such as feeling comfortable (or drying the wash 

 quickly). 



