BURNING OF IRON. 127 



oxygen of the air at ordinary temperatures, yet when 

 strongly heated it rapidly absorbs oxygen, and then 

 becomes converted into a black scaly oxide ; when- 

 ever a piece of iron is heated in the fire, a quantity 

 of a brittle, black oxide is formed on its surface, 

 which easily rubs off the iron — and hence this metal 

 is gradually worn away by exposure to fire ; it is 

 from this reason that the iron pokers, fire-bars, and 

 other things much exposed to the fire, gradually get 

 thinner and thinner, from the constant oxidation of 

 the surface whilst hot, and the removal by rubbing 

 of the brittle coat of Oxide thus formed. 



282. When a piece of iron is very strongly heated, 

 it at last begins to hum ; that is to say, the combi- 

 nation of the external part with oxygen goes on so 

 rapidly, and evolves so much heat, that the whole 

 mass of iron is kept sufficiently hot to continue this 

 process of combination, and in consequence the iron 

 glows brightly, gives oiF abundance of sparks, and 

 runs down in drops of the melted oxide, for some 

 time after it has been removed from the fire in which 

 it was heated. In fact, iron, when thus strongly 

 heated, would catch fire and continue to burn like 

 charcoal, if it were not (that the crust of oxide 

 formed protects the metal from further oxidation, 

 and soon stops its combustion. 



283. One of the most common and abundant of 

 the ores of iron is called pyrites, which is a com- 

 pound of iron and sulphur ; it is not used in the 

 manufacture of iron, because it is very difficult to 



