56 THE STORY-BOOK OF SCIENCE 



apples and bread, looked on at this curious work 

 without saying a word. They promised themselves 

 to ask their uncle the reason for whitening the in- 

 side of the copper saucepans with tin. In the eve- 

 ning, accordingly, they spoke of the tinning and 

 plating. 



"Highly cleaned and polished iron is very bril- 

 liant," explained their uncle. "The blade of a new 

 knife, Claire's scissors, carefully kept in their case, 

 are examples. But, if exposed to damp air, iron 

 tarnishes quickly and covers itself with an earthy 

 and red crust called " 



"Bust," interposed Claire. 



"Yes, it is called rust." 



"The big nails that hold the iron wires where 

 the bell-flowers climb up the garden wall are covered 

 with that red crust," remarked Jules; and Emile 

 added : 



"The old knife I found in the ground is covered 

 with it too." 



"Those large nails and the old knife are encrusted 

 with rust because they have remained for a long 

 time exposed to the air and dampness. Damp air 

 corrodes iron; it becomes incorporated with the 

 metal and makes it unrecognizable. When rusty, 

 iron no longer has the properties that make it so use- 

 ful to us ; it is a kind of red or yellow earth, in which, 

 without looking attentively, it would be impossible to 

 suspect a metal." 



"I can well believe it," said Jules. "For my 

 part, I should never have taken rust for iron with 

 which air and moisture had become incorporated." 



