STUDIES IN NATURE 



and some of the warmth of the room escaped 

 into space outside. The air near the window 

 could not now hold as much water, and so some 

 of it dropped out, or was deposited, on the cold 

 glass, where we found it in the morning. 



This is also the way in which dew forms 

 on the ground on clear autumn evenings. 

 The surface of the earth has been warmed by 

 the sun's rays striking on it, by radiation, as we 

 say, and has in turn warmed the air above it. 

 At night, if there are no clouds to keep the earth 

 warm, the surface radiates its heat back into 

 space and gets cold. Then the air above it is 

 chilled and deposits on the grass the water it 

 can no longer hold. There is never much dew 

 under trees because they act as a protection, just 

 as the clouds do, radiating back to the earth 

 some of the heat it is losing. In the same 

 way, if we put a rug or mat over two chairs, 

 and cover in a piece of ground, the surface 

 will keep much drier than the open ground 

 beside rt. 



Warm air, besides being able to hold more 

 water-vapour than cold air, differs from it in 

 another way that is of importance to us in our 

 every-day life. It is lighter than cold air ; that 

 is to say, in a given volume or definite space, 

 there is less air when it is warm than when it is 

 cold. The warm air seems to spread itself out 

 and require more room, and so there is less of it 

 in a given volume. This makes it lighter, just 



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