PASSIVE CONDITION OF IRON. 553 



immersion in sea-water, the greater part of the iron being dis- 

 solved while the carbon remains*. In open air, iron burns at a 

 high temperature with vivacity, and its surface becomes covered 

 with a fused oxide, which may afterwards be detached from it in 

 scales, and forms smithy ashes. Iron also decomposes steam 

 at a red heat, and the same oxide is formed as by the combus- 

 tion of the metal in air, namely the magnetic or black oxide, 

 FeO + Fe 2 O 3 . 



Iron dissolves readily in diluted acids, by substitution for hy- 

 drogen, which is evolved as gas. Strong nitric acts violently 

 upon iron, yielding oxygen to it, and undergoing decomposi- 

 tion. But the relations of iron to that acid, when slightly di- 

 luted, are exceedingly singular. They have been particularly 

 studied by Professor Schcenbein. 



Passive condition of iron. Pure malleable iron, such as a 

 piece of clean stocking wire, usually dissolves in nitric acid of 

 sp.gr. 1.3 to 1.35, with effervescence, but it may be thrown into 

 a condition in which it is said by Schcenbein to be passive, as it 

 is no longer dissolved by that acid, and may be preserved in it 

 for any length of time without change: 1. By oxidating the 

 extremity of the wire slightly, by holding it for a few seconds in 

 the flame of a lamp, and after it is cool, dipping it gradually in 

 the nitric acid, introducing the oxidated end first. 2. By dip- 

 ping the extremity of the wire once or twice in concentrated 

 nitric acid, and washing it with water. 3. By placing a plati- 

 num wire first in the acid, and introducing the iron wire, pre- 

 serving it in contact with the former, which may afterwards be 

 withdrawn. 4. A fresh iron wire may be introduced in the 

 same manner into the nitric acid, in contact with a wire already 

 passive ; this may render passive a third wire, and so on. 5. By 

 making the wire the positive pole or zincoid of a voltaic battery, 

 introducing it after the negative pole or chloroid has been placed 

 in the acid. Oxygen gas is then evolved from the surface of 

 the iron wire, without combining with it, as if the wire were of 

 platinum. As the passive state can be communicated by con- 

 tact of passive iron, so it may be destroyed by contact with 

 active iron or zinc undergoing, at the moment, solution in the 



* Mr. Mallet has collected much information respecting the corrosion of 

 iron, in his First Report to the British Association, on the action of sea and 

 river water upon cast and wrought iron. 1839. 



