126 RUSTING OF IRON. 



of salts by combining with acids ; but the salts formed 

 by the protoxide have always a tendency to absorb 

 oxygen from the air, and thus become converted into 

 the salts of the peroxide. 



278. The color of a great many stones and soils is 

 principally caused by the presence of a small quan- 

 tity of either the peroxide of iron, or of a mixture 

 of both its oxides. 



279. The rusting of iron, which proceeds so rap- 

 idly when iron is exposed to damp air, is caused by 

 the attraction which the metal has for oxygen. It 

 is very remarkable that iron is unable to combine 

 with the free oxygen always in the air, but is able to 

 take it from water, its compound with hydrogen ; for 

 we find that in dry air, iron remains clean and bright 

 for a long time, but very rapidly rusts when exposed 

 to the joint action of air, carbonic acid gas, and 

 moisture, under which circumstances water is de- 

 composed, and oxide of iron formed. 



280. The rust of iron is not a pure oxide, but 

 commonly a mixture of both oxides with a portion of 

 carbonate, or compound of the protoxide with car- 

 bonic acid. Rust generally contains a considerable 

 quantity of ammonia, for which substance oxide of 

 iron has a strong attraction ; when oxide of iron is 

 thrown down from a solution which contains it, by 

 ammonia, it is very difiicult to expel the whole of 

 the ammonia from the precipitated oxide even by 

 strongly heating it. 



281. Although iron cannot combine with the free 



