24 ELEMENTS OF GENERAL SCIENCE 



however, the room were cooled to 40 F., its capacity would 

 be decreased to less than 3 grains per cubic foot. Some of the 

 water would therefore condense into liquid form and settle 

 upon surrounding objects. In the case supposed, how much 

 would condense ? 



The temperature at which a given space would be saturated 

 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 

 temperature of 70 and there were 3.8 grains of water per 

 cubic foot, what part did the room actually contain of the 

 total amount that it would have been able to contain ? Ex- 

 press 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 ? 

 If it contained no moisture, 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 abso- 

 lute humidity. Whether the atmosphere feels dry or moist or 

 whether clouds are seen (fig. 17) depends upon what propor- 

 tion 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 atmosphere feels and 

 the probability of moisture condensing in the form of rain than 

 we are with the number of grains of water in a cubic foot 



24. Determination of relative humidity. Relative humidity 

 is the relation, expressed in percentage, of the amount of water 

 per unit volume actually constituting part of the air to the 

 amount of water per unit volume necessary to produce 



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. 



