26 ELEMENTS OF GENERAL SCIENCE 



26. Saturation and the dew point. If the temperature falls 

 below that at which the contained vapor saturates the space, 

 some of the water will condense into the liquid form. For 

 instance, if the space in a room is at a temperature of 22 C. 

 (71.6 F.) and contains 10 grams of water vapor to the cubic 

 meter, the air will appear rather dry, for it would be able 

 tQ contain over 9 grams more without being fully saturated. 

 If, however, the room were cooled to 5 C. (41 F.) its 

 capacity would be decreased to less than 7 grams per cubic 

 meter. Some of the water would therefore condense into the 

 liquid form and settle upon surrounding objects. In the 

 case supposed how much would condense ? (See sect. 25.) 



The temperature at which a given space would be satu- 

 rated by the water that it contains and below which the 

 water will begin to condense is called the dew point. What 

 was the dew point in the above example ? 



In the example used above, when the air was at the tem- 

 perature of 22 C. and there were 10 grams of water per 

 cubic meter, what part did the room actually contain of 

 the total amount that it would have been able to contain ? 

 Express your result in the form of a fraction ; also in the 

 form of a percentage. 1 Is this relative or absolute humidity ? 

 If the room contained all the water that it could contain 

 without a change in temperature, what would be the relative 

 humidity ? What is the relative humidity at the dew point ? 



Relative humidity is of much more interest to us than 

 absolute humidity. Whether the atmosphere feels dry or 

 moist or whether clouds are seen (fig. 19) depends upon 

 what proportion it has of the water it is able to contain 

 (that is, upon how near the relative humidity is to 100 per 

 cent), and we are much more concerned with the way the 



1 There is a slight error in this method, since no allowance is made for 

 contraction due to cooling. Exact work would require that the proper cor- 

 rection be applied, but the error is so slight that it is immaterial for present 

 purposes and will be disregarded. 



