28 ELEMENTS OF GENERAL SCIENCE 



may be able to estimate roughly the relative moisture of the 

 air and the consequent probability of rain. 



If more accurate information regarding the relative moisture 

 of the air is desired, in order that it may be expressed in 

 figures for record or comparison, a more carefully designed 

 experiment is necessary. In the previous statements about 

 absolute and relative humidity the figures used had been pre- 

 viously determined. If we wish to know the relative humidity 

 of a schoolroom or room at home, we shall need to find out 

 both the absolute and the relative humidity. 



A common experimental procedure for ascertaining the 

 amount of water in the air is as follows : Water is placed in 

 a bright metal cup, ice is added, and the whole is stirred in 

 order that the water may be evenly cooled. In a short time 

 water will be deposited on the outer surface of the cup in 

 a thin film, and as time passes it may accumulate in drops, 

 as it does more familiarly on a pitcher containing ice water. 

 This water has been deposited from the air because the space 

 about the cup is cooled by the cold metal until its capacity 

 for water vapor is less than the amount of such vapor present. 

 Some of the water vapor therefore condenses into the liquid 

 form and is seen on the surface of the cup. At the moment 

 when the first trace of moisture appears upon the surface of 

 the cup the adjacent space is evidently saturated ; that is, the 

 temperature close to the cup is that at which the water vapor 

 present saturates the space. 



We now know the temperature, which for illustration we 

 will suppose is 10 G. (50 F.), to which this particular body 

 of air or any part of it would all need to be reduced in order 

 for it to be at the saturation point. By reference to the table 

 in section 25 we find that at this temperature saturated air 

 contains 9.33 grams of water per cubic meter. This, then, is 

 the absolute humidity of the room. However, the room ther- 

 mometer shows the room temperature to be 20 C. (68 F.), 

 and our table shows that air at this latter temperature at 



