AIR 



changing, and there is never the same amount of 

 it present two days running. When water boils 

 in a kettle, we know it conies out of the spout 

 as steam or vapour, and is lost in the air ; if we 

 leave a saucer full of water in the sun the water 

 also gradually disappears, dries up or evaporates ; 

 and if we put the same saucer full of water out 

 of doors on a hot day and again on a cold day, 

 the water will disappear faster on the hot day 

 than on the cold day, or faster in a hot room 

 than in a cold room. Let us think for a few 

 minutes if we have noticed anything which ought 

 to have told us this. 



The rooms of a house are in winter time 

 warmer than the air outside, and if we wake up 

 on a cold frosty morning and go to our windows, 

 what shall we find on the window panes inside 

 the room ? Water, in drops on the glass, in 

 little pools on the sills, or perhaps even in tiny 

 crystals of ice all over the window. Where then 

 did this water come from ? Not surely through 

 the glass or the window frames ? Even when 

 it rains outside we do not expect the water to 

 come through, unless unfortunately there are 

 actually holes somewhere that we know about 

 and can see. The water has certainly come from 

 somewhere inside the room. This is what has 

 happened. The air was warm in the evening and 

 held a certain quantity of water- vapour. During 

 the night, the air near the glass got cold because 

 the cold air outside chilled the window-panes 



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