THE EXPERIMENT 195 



is very simple ; all that is necessary is to cool the air 

 a little. When you squeeze a wet sponge with the 

 hand, you make water ooze out of it. Cold acts on 

 moist air very much as the pressure of the hand on 

 the sponge: it causes the moisture to distil in the 

 form of minute drops. If Claire will go to the pump 

 and fill a bottle with very cold water, I will show 

 you this curious experiment. ' ' 



Claire went to the kitchen and came back with a 

 bottle full of the coldest water possible. Her uncle 

 took the bottle, wiped it well with his handkerchief 

 so that no trace of moisture should remain on the 

 outside, and put it on an equally well-wiped plate. 



Now the bottle, at first perfectly clear, becomes 

 covered with a kind of fog which tarnishes its trans- 

 parency : then little drops appear, run down its sides, 

 and fall into the plate. At the end of a quarter of 

 an hour there was enough water accumulated in the 

 plate to fill a thimble. 



"The drops of water now running down the out- 

 side of the bottle," Uncle Paul explained, "do not 

 come, it is very clear, from the inside, for glass can- 

 not be pierced by water. They come from the sur- 

 rounding air, which cools off on touching the bottle 

 and lets its moisture distil. If the bottle were colder, 

 if full of ice, the deposit of liquid drops would be 

 more abundant." 



"The bottle reminds me of something of the 



same kind," said Claire. "When you fill a perfectly 



clean glass with very cold water, the outside of 



the lass immediately tarnishes and looks as if badly 



" 



