THE REFRIGERATOR 



159 



fraction, but as a per cent. In this case we say the relative 

 humidity is 50 per cent. 



DEW POINT. DEW POINT is the temperature at which air 

 containing a certain amount of moisture per cubic foot, that is, 

 having a certain absolute humidity, becomes saturated. 



Suppose, as before, that the air in a schoolroom is at 70F. 

 and that it contains 4 grains of moisture per cu. ft. Now, 

 if the temperature in that room were lowered, the air would 

 become more nearly saturated, that is, its relative humidity 



FIG. 121. Curve showing weight of water vapor in saturated air at different 



temperatures. 



would become higher, because the actual amount of moisture 

 per cubic foot would remain the same while the amount of 

 moisture it might contain would decrease with the fall in 

 temperature. By examining the table above, or still better 

 by examining the curve, Fig. 121, we see that the air will 

 become saturated when the temperature has fallen a trifle 

 below 50, or about 49F. Its relative humidity would then 

 be 100 per cent. This temperature at which the air becomes 

 completely saturated is called the dew point. If any object 

 whose temperature is at, or below 49F., is taken into this room, 



