Mingling of Molecules 275 



There is water vapor in the air all around us 

 invisible water vapor, its molecules mingling with those 

 of the air water that has evaporated from the oceans 

 and lakes and all wet places. ' 



This water vapor changes into droplets of water when 

 it gets cool enough. And those droplets of water make 

 up our clouds and fogs ; they join together to form our 

 rain and snow high in the air, or gather as dew or frost 

 on the grass at night. 



If the water vapor should suddenly lose its power 

 of changing into droplets of water when it cooled, 

 well, let us pretend it has lost this power but that 

 any amount of water can evaporate, and see what 

 happens : 



What fine weather it is ! There is not a cloud in the 

 sky. As evening closes in, the stars come out with 

 intense brightness. The whole sky is gleaming with 

 stars more than we have ever seen at night before. 



The next morning we find no dew or frost on the grass. 

 All the green things look dry. As the day goes on, 

 they begin to wilt and wither. We all wish the day 

 were not quite so fine a little rain would help things 

 wonderfully. Not a cloud appears, however, and we 

 water as much of our gardens as we can. They drink 

 the water greedily, and that night, again no dew or fog 

 and not the faintest cloud or mist to dim the stars. 

 And the new day once more brings the blazing sun further 

 to parch the land and plants. Day after day and night 

 after night the drought gets worse. The rivers sink 

 low; brooks run dry; the edges of the lakes become 

 marshes. The marshes dry out to hardened mud. 



