174 GENERAL SCIENCE 



body. If the air is warm and damp we say it is " close " 

 or sultry. Cold, damp air is penetrating and " raw." 



As air is warmed, its capacity for retaining moisture is 

 increased ; and the reverse is true if it be cooled. If we 

 cool air having a relative humidity of 80, it will soon 

 reach a temperature at which it is saturated or has a 

 relative humidity of 100 per cent. If it be cooled further, 

 some of its moisture will be precipitated as dew, rain, or 

 some other form of precipitation. If we heat air, the 

 relative humidity will be reduced. If we cool it, the 

 relative humidity will be increased. 



Dew Point. - - The temperature of air at its point of 

 saturation is the dew point. 



Experiment 49. Fill a polished metal cup with water and 

 stir it with a thermometer as ice is added slowly. The temperature 

 at which a film of moisture forms on the outside of the cup is the 

 dew point of the air surrounding the cup. 



We have often noticed the " sweating " of pitchers con- 

 taining ice water. This simply means that the air sur- 

 rounding the pitcher has been cooled below its dew point, 

 and some of its moisture must be deposited. 



The dew point apparatus may be used to determine the 

 relative humidity of the atmosphere, since the ratio be- 

 tween the amount of moisture present in the air and the 

 amount which would be present if the air were saturated 

 is the same as the ratio between the pressure which the 

 water vapor in the air exerts and the pressure which it 

 would exert if the air were in a saturated condition. For 

 example, let us suppose that the dew point of the air in a 

 room be 12 and the temperature of the air in the room be 

 25. The amount of moisture in the air is enough to 

 saturate it at the temperature of 12. By referring to the 

 Appendix, Table I, we find that the pressure of water 



