196 THE STORY-BOOK OF SCIENCE 



"That again is the surrounding air depositing its 

 moisture on the cold side of the glass. " 



"Is that invisible moisture contained in the air 

 abundant f " asked Jules. 



' ' The invisible vapor of the air is always a thing 

 so subtle, so disseminated, that it would take enor- 

 mous volumes to make a small quantity of water. 

 During the heat of summer, when the air holds the 

 most vapor, it takes 60,000 liters of moist air to fur- 

 nish one liter of water." 



"That is very little," was Jules 's comment. 



"It is a great deal if one thinks of the immense 

 volume of the atmosphere," replied his uncle, and 

 then added: 



"The experiment of the bottle teaches us two 

 things: first, there is always invisible vapor in the 

 air; in the second place, this vapor becomes visible 

 and changes into fog, then into drops of water, by 

 cooling. This return of invisible vapor to visible 

 vapor or fog, then to a state of water, is called con- 

 densation. Heat reduces water to invisible vapor, 

 and cold condenses this vapor, that is to say brings 

 it back to a liquid state or at least to the state of 

 visible vapor or fog. We will have the rest this 

 evening. ' ' 



