Mingling of Molecules 279 



along with it. More rising air, full of evaporated water 

 from the ocean, joins the cloud and cools, and the water 

 forms into more tiny droplets. The droplets get so 

 close together that they shut out the sun's light from the 

 earth, and people say that the sky is darkening. 



Meanwhile some of the droplets begin to touch each 

 other and to stick together. Little by little the drops 

 grow bigger by joining together. Pretty soon they get 

 so big and heavy that they can no longer float high in 

 the air, and they fall to the ground as rain. 



Part of the rain soaks into the ground. Some of it 

 gradually seeps down through the ground to an under- 

 ground stream. This has its outlet in a spring or well, 

 or in an open lake or the ocean. But the rain does not 

 all soak in. After the storm, some of the water again 

 evaporates from the top of the ground and mixes with 

 the warm air, and it goes through the same round. 

 Other raindrops join on the ground to form rivulets 

 that trickle along until they meet and join other rivulets ; 

 and all go on together as a brook. The brook joins 

 others until the brooks form a river ; and the river flows 

 into a lake or into the ocean. 



Then again the sun warms the surface of the ocean 

 or lake; the water evaporates and mixes with the air, 

 which rises, 'expands, and cools; the droplets form and 

 make clouds; the droplets join, forming big drops, 

 and they fall once more as rain. The rain soaks into the 

 ground or runs off in rivulets, and sooner or later it is 

 once more evaporated. And so the cycle is repeated 

 again and again. 



And all this is accounted for by the simple fact that 



