22 ELEMENTS OF GENERAL SCIENCE 



warmed, however, an additional amount of water will evapo- 

 rate, while if it is cooled, some of the water vapor will return 

 to the liquid form and may be seen upon the walls in the form 

 of drops. 



It thus appears that water evaporates into the adjacent space 

 until the space is saturated, and that the amount of water 

 which is needed to saturate a given amount of space depends 

 upon the temperature. The exact amounts of water in grains 

 per cubic foot of space are given in the table (sect. 22). 



It should be added that the amount of air in the space has 

 no influence on the amount of water vapor which the space 

 is able to contain. 



22. Absolute humidity and relative humidity. The space 

 about us is not often saturated with water vapor. Among 

 the reasons for this may be mentioned the fact that in many 

 parts of the world there is not much water exposed, and 

 therefore not much opportunity for water to evaporate. Even 

 where there is much water exposed, as at the surface of lakes 

 and oceans, the space above the water is not usually saturated, 

 though containing a large amount of water. For instance, a 

 space to-day may contain one half as much moisture as would 

 be possible at the present temperature, while to-morrow it may 

 contain only one third as much as would be needed to saturate 

 it without changing temperature ; that is, in the first case the 

 space may be said to be one-half saturated, and in the second 

 case one-third saturated. These relations are usually expressed 

 in percentages. When saturated, the space contains 100 per 

 cent of the moisture that it is able to contain without change 

 of temperature ; if only half the possible amount is present, 

 the percentage is 50 ; and similarly for other values. This 

 percentage expresses the relation between the water actually 

 present and the amount which the space would be able to 

 contain. This relation is called the relative humidity. 



The actual amount of water present in a given space is 

 known as its absolute humidity and may be measured in grains 



