io8 The Realm of Nature CHAP. 



densation. The change of a pound of water into a 

 pound of vapour requires the same expenditure of energy 

 ( 70), whether it takes place in a kettle boiling on a fire, 

 or over the surface of a freezing pond. The work of 

 evaporation uses up heat, and produces a lowering of 

 temperature. On the other hand, when vapour is condensed 

 to the state of water, the potential energy stored up is 

 reconverted into heat ; thus condensation produces a rise 

 of temperature ( 70-73). When air resting over water is 

 heated by the Sun's rays, evaporation begins actively and 

 diminishes the rate of rise of temperature in the air. 

 On the other hand, when a portion of the atmosphere 

 containing saturated vapour is cooling down by radiation, 

 the vapour begins to condense, giving out heat, and so 

 retarding the rate of fall of temperature. In both cases 

 the tendency is toward moderation and slowness of change. 

 The cooling of air containing unsaturated vapour goes on 

 unchecked until the temperature of saturation is reached. 



1 60. Absorptive Power of Air. The water-vapour of 

 the atmosphere is not transparent to all light ; it absorbs 

 certain rays from sunlight, producing black lines or bands in 

 the spectrum, particularly a set in the yellow known as the 

 rain-band (TT in Fig. 8). The rain-band in the spectrum 

 increases in width and darkness as the amount of vapour in 

 the slice of atmosphere looked through increases, and the 

 probability of rain occurring within a certain time may 

 be judged from the darkness of the band. The heat rays 

 of the Sun pass readily into the atmosphere, but heat 

 does not so readily pass out through the air into space. 

 The atmosphere thus acts toward the Earth as a great 

 blanket, or rather a heat-trap allowing radiant heat to enter 

 freely but greatly retarding its escape. Water-vapour has 

 usually been considered the chief heat-entrapping agent, 

 because the chilling by radiation at night is always 

 greatest when the proportion of water-vapour in the air 

 is least. But there is now reason to believe that condensed 

 water and solid dust-motes are more powerful in producing 

 the effect. 



1 6 1. Dust. Solid dust is always present in the at- 



