44 Introduction to the Study of Science 



Water vapor or gas is invisible, but when condensed or 

 made liquid, is visible as cloud, fog, mist, or rain. Upon what 

 does its form of existence in the atmosphere depend? Con- 

 sider these familiar instances : The warm, moist air of the 

 house or kitchen is invisible, but as it escapes through an open 

 window into the outer cold it forms a cloud. Observe the 

 condensation on cold windows or woodwork. The air exhaled 

 from our lungs is heavily laden with moisture, which is, how- 

 ever, invisible unless the outer temperature is below a certain 

 point, or one breathes upon a cold surface, such as glass or 

 bright metal. At what time of the day does a mist appear? 

 Why does it disappear soon after sunrise? During what part 

 of the day is dew deposited? What causes it to disappear? 

 What becomes of mist and dew after the sun begins to shine upon 

 them? The air in clear, warm weather contains a relatively 

 large amount of invisible moisture, but lowering the tempera- 

 ture below a certain point makes the moisture visible as cloud, 

 mist, or dew. Thus it appears from these observations that 

 the form of moisture in the air depends upon the temperature. 

 It also is seen that the amount of water vapor that air may 

 contain depends upon the temperature ; the warmer the air, 

 the greater the amount of moisture ; and the lower the tempera- 

 ture, the less the amount it can hold. 



The amount of moisture that the atmosphere may contain can 

 be considered in the following way. Suppose a mass of air at 

 a definite temperature, as 32 F., is supplied with water vapor 

 through evaporation until it contains as much as it can hold. 

 Evaporation proceeds rapidly at first, but gradually becomes 

 slower until it ceases altogether. When evaporation ceases, the 

 air is said to be saturated at the given temperature. If the 

 temperature of the air is raised, its capacity for moisture is 

 increased, and evaporation is resumed. Suppose the tempera- 

 ture is raised to 68 F., the air can then hold approximately four 

 times as much moisture as it could at 32 F. (Table, Fig. 13). 

 Evaporation will continue until the air is saturated at that 



