HOW CHILLS ARE CAUSED 43 



When ice water is poured into a glass, a mist forms on the 

 outside of ";he glass. This is because the water vapor which 

 comes in contact with the glass is chilled and condenses. 

 Often leaves and grass and sidewalks are so cold that the 

 water vapor in the atmosphere condenses on them, and we 

 say a heavy dew has formed. If the temperature of the air 

 falls to the freezing point while the dew is forming, the vapor 

 is frozen and frost is seen instead of dew. 



The daily evaporation of moisture into the atmosphere 

 keeps the atmosphere more or less full of water vapor; but 

 the atmosphere can hold only a definite amount of vapor at 

 a given temperature, and as soon as it contains the maximum 

 amount for that temperature, further evaporation ceases. If 

 clothes are hung out on a damp, murky day, they do not dry, 

 because the air contains all the moisture it can hold, and the 

 moisture in the clothes has no chance to evaporate. When 

 the air contains all the moisture it can hold, it is said to be 

 saturated, and if a slight fall in temperature occurs when the 

 air is saturated, condensation immediately begins in the form 

 of rain, snow, or fog. If, however, the air is not saturated, a 

 fall in temperature may occur without producing precipitation. 

 The temperature at which air is saturated and condensation 

 of water vapor begins is called the dew point. 



How chills are caused. The discomfort we feel in an 

 overcrowded room is partly due to an excess of moisture in 

 the air, resulting from the breathing and perspiration of many 

 persons. The air is saturated with vapor and cannot take 

 away the perspiration from our bodies, and our clothing 

 becomes moist and our skin tender. When we leave the 

 crowded " tea M or lecture and pass into the colder, drier 

 outside air, clothes and skin give up their load of moisture 

 through sudden evaporation. But evaporation requires heat! 

 This heat is taken from our bodies, and a chill results. 



