58 THE STORY-BOOK OF SCIENCE 



time, it scarcely tarnishes. And then the rust, which 

 forms in small quantities, is innocuous, like iron rust. 

 To prevent copper from covering itself with poison- 

 ous green spots, to preserve it from rust, it must be 

 kept from contact with damp air and also with cer- 

 tain alimentary suhstances such as vinegar, oil, 

 grease substances that provoke the rapid forma- 

 tion of rust. For this reason the copper saucepan 

 is coated over with tin inside. Under the thin bed 

 of tin which covers it, the copper cannot rust, be- 

 cause it is no longer in contact with the air. The 

 tin remains; but this metal changes with difficulty, 

 and, besides, its rust, if it forms any, is harmless. 

 So they plate copper, that is to say they cover it 

 with a thin bed of tin, to prevent its rusting, and 

 thus to prevent the formation of the dangerous 

 poison that might, some day or other, be mixed with 

 our food. 



4 ' They also tin iron, not to prevent the formation 

 of poison, for the rust of this metal is harmless, but 

 simply to preserve it from changing and covering 

 itself with ugly red spots. This tinned iron is called 

 tin-plate. Lids, coffee-pots, dripping-pans, graters, 

 lanterns, and innumerable other things, are of tin- 

 plate ; that is to say, thin sheets of iron covered on 

 both sides with a coating of tin. ' ' 



