24 ELEMENTS OF GENERAL SCIENCE 



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 

 it contains a large amount of water. For instance, a space 

 to-day may contain half as much moisture as would be 

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

 contain only a third as much as would be needed to satu- 

 rate 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 usu- 

 ally expressed in percentages. When saturated, the space 

 contains 100 per cent of the moisture that it is able to con- 

 tain without change of temperature ; if only half the possible 

 amount is present, the percentage is fifty ; 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 

 grams per cubic meter. Absolute humidity therefore refers 

 to the amount of water actually in the air, and relative 

 humidity refers to the relation between tne amount of water 

 actually contained and the total amount which might be 

 contained at the same temperature. 



25. Reference table of the amount of water in one cubic 

 meter of saturated air at given temperatures. In this table 

 the temperatures are given in both centigrade and Fahrenheit 

 scales. In this and succeeding work students should practice 

 thinking in both systems of temperature measurements, since 

 the Fahrenheit system is in common public use, and the centi- 

 grade system in general use in science and likely to become 

 more common in public use. 



Absolute humidity and relative humidity can be the same 

 only when the amount of water actually hi the air is the full 



