158 REFRIGERATION AND ITS USES 



without removing it from the air current. In like manner determine 

 the temperature of the air as.it leaves the food compartment and 

 enters the ice compartment. 



If possible, repeat these experiments with a second refrigerator. 



The temperature obtained in this experiment will depend 

 upon several things: (1) Construction of the refrigerator; (2) 

 amount of ice in the ice compartment; (3) temperature of 

 the room; (4) amount and kind of food in the food compart- 

 ment. Show how each of these factors affected the tempera- 

 ture in the refrigerator you examined. 



186. Meaning of Absolute Humidity, Saturation, Relative 

 Humidity, and Dew Point. In order to understand how the 

 temperature of the air determines the amount of moisture 

 which the air within a refrigerator contains, it is necessary 

 that we learn some new terms and their exact uses. 



DEFINITIONS. 



ABSOLUTE HUMIDITY. By ABSOLUTE HUMIDITY we mean 

 the weight of moisture, or water vapor, actually contained in 

 one unit volume of air. 



SATURATION. We speak of the air as being SATURATED or as 

 having reached the POINT OF SATURATION when it contains all 

 the moisture, or water vapor, it can possibly contain at that 

 temperature. 



In the table given above (Art. 185), the third column gives 

 the absolute humidity of fully saturated air at the various 

 temperatures given in the first column. For example, at 0F. 

 the absolute humidity of saturated air is 0.56 grain per cu. ft., 

 while at 70F. it is 7.98 grains, or nearly fifteen times as great. 



RELATIVE HUMIDITY. RELATIVE HUMIDITY is the ratio of 

 the absolute humidity of air at any temperature to the absolute 

 humidity of air were it saturated at the same temperature. 



Suppose the air in a schoolroom is found to contain 4 grains 

 of water vapor per cu. ft. when the temperature is 70F. 

 The relative humidity of the air is then expressed as the ratio 

 of 4 grains to 7.98 grains, or 4>0 !^f.98> or very nearly J-. The 

 relative humidity is usually expressed, however, not as a 



