46 



Introduction to the Study of Science 



how much water vapor will the air contain? By what two 

 methods may saturation be produced? Do both methods 

 depend upon temperature? The relation of saturation and 

 temperature may be shown in the following manner. A mass 

 of air is saturated at 50 F., and with no change in its water- 

 vapor content is then warmed to 68 F. The 

 absolute humidity of the air is .25 of an ounce 

 per cubic yard. At this temperature the air 

 can contain twice as much moisture before it 

 becomes saturated. Expressing this as a ratio, 

 the air contains .25/.4T of what it can hold, or 

 in percentage, about 53 per cent of its total 

 capacity. The same mass of air may be imag- 

 212 " F L I I ined as cooled to 50 F. It now contains all 



the water vapor possible, without condensation, 

 or .25/.2S, or 100 per cent, its maximum capac- 

 ity. What would be .the ratio of the amount 

 of vapor present to the total capacity of the air, 

 if its temperature were 86 F.? 95 F.? 



Relative humidity. The expression of the 

 actual amount of water vapor in the air as the 

 percentage of the total amount that air can 

 contain at a given temperature is called the 

 relative humidity. It is the relative humidity, 

 that is, the ratio of the vapor present to what 

 the air can contain, that determines our experi- 

 ence of drying air or of damp air. 



The presence of water vapor in the air, although 

 invisible, is detected and measured by an instru- 

 ment called the hygrometer, from two Greek words, hugros } 

 meaning wet, and metron, measure (Fig. 14). This instru- 

 ment really consists of two thermometers, placed upon the 

 same support. The mercury bulb of one is wrapped with a 

 piece of porous cloth which is attached to a short length of 

 lamp wicking. The other end of the wicking is kept in a 



FIG. 14. Sling 

 hygrometer. 



