552 OXIDATION OF IRON. 



state it can be welded, or two pieces be united by hammering 

 them together. The point of fusion of cast iron is 3479; that 

 of malleable iron is much higher. Iron expands in becoming 

 solid, and therefore takes the impression of a mould with exact- 

 ness. Iron is attracted by the magnet at all temperatures under 

 an orange red heat. It is then itself magnetic by induction, but 

 immediately loses its polarity, if pure, when withdrawn from the 

 magnet. If it contains carbon, as steel and cast iron, it is 

 affected less strongly, but more durably, by the proximity of a 

 magnet, becoming then permanently magnetic. The black 

 oxide, which forms the loadstone, and the corresponding sul- 

 phuret, are the only compounds of iron which share this pro- 

 perty with the metal. A steel magnet loses its polarity at the 

 boiling point of almond oil ; a loadstone, j ust below visible igni- 

 tion (Faraday). 



Iron reduced from the oxide by hydrogen at a heat under 

 redness, forms a spongy mass, which takes fire spontaneously 

 at the usual temperature, when exposed to air, and oxide of 

 iron is reproduced (Magnus). But iron, in mass, appears to un- 

 dergo no change in dry air, and to be incapable of decomposing 

 pure water at that temperature. Nor does it appear to be acted 

 upon by oxygen and water together, but the presence of carbo- 

 nic acid in the water, causes the iron to be rapidly oxidated 

 with evolution of hydrogen gas. In the ordinary rusting of 

 iron, the carbonate of the protoxide appears to be first pro-" 

 duced, but that gradually passes into the hydrated peroxide, 

 and the carbonic acid is evolved. The rust always contains 

 ammonia, of which the hydrogen is imagined to come from the 

 water decomposed ; the native oxides of iron also contain am- 

 monia. Iron remains bright in solutions of the alkalies and in 

 lime-water, which appear^ to protect it from oxidation, but neu- 

 tral and more particularly acid salts have the opposite effect. 

 The corrosion of iron under water appears, in general, to be im- 

 mediately occasioned by the formation of a subsalt of that 

 metal with excess of oxide, of which the acid is supplied by the 

 saline matter in solution. Articles of iron may be completely 

 defended from the injury occasioned in this way, by contact 

 with the more positive metal zinc, as in galvanized iron (page 

 219); while the protecting metal itself wastes away very slowly. 

 Cast iron is converted into a species of graphite^ by many years 



