218 THE STORY-BOOK OF SCIENCE 



steam that puts it in motion. Let us, then, first 

 learn what steam is and what its power. 



' ' When water is put on the fire, it first gets hot, 

 then begins to boil, sending off vapor, which is dis- 

 sipated in the air. If the boiling continues some 

 time, it ends with there being nothing in the pot; 

 all the water has disappeared. " 



1 1 That is what happened to Mother Ambroisine 

 day before yesterday, " put in Emile. "She was 

 boiling some potatoes, and having neglected to look 

 into the pot for some time, she found them without 

 a drop of water, half burnt. She had to begin all 

 over again. Mother Ambroisine was not pleased. 7 ' 



"By heat," continued Uncle Paul, "water be- 

 comes invisible, intangible, as subtle as air. That is 

 what is called vapor. ' ' 



"You told us that the moisture in the air, the 

 cause of fogs and clouds, is also vapor." This from 

 Claire. 



"Yes, that is vapor, but vapor formed only by 

 the heat of the sun. Now, you must know that the 

 stronger the heat, the more abundant is the vapor. 

 If you put a pot full of water on the fire, the burning' 

 heat of the grate sets free incomparably more vapor 

 than the temperature of a hot summer sun could. 

 As long as it escapes freely from the pot, the vapor 

 thus formed has nothing remarkable about it; so 

 your attention has never been arrested by the fumes 

 of a boiling pot. But if the pot is covered, covered 

 tight, so as not to leave the slightest opening, then 

 the steam, which tends to expand to an enormous 

 volume, is furious to get out of its prison ; it pushes 



