334 PRINCIPLES OF CHEMISTRY 



particularly, ammonia is always found in iron rust ; the ammonia must 

 arise from the action of the hydrogen of the water, at the moment of its. 

 separation, on the nitrogen of the air. Highly -polished steel does not 

 rust nearly so readily, but if moistened with water, it easily becomes 

 coated with rust. As rust depends on the access of moisture, iron may 

 be preserved from rust by coating it with substances which prevent 

 the moisture having access to it. Thus arises the practice of covering 

 iron objects with paraffin, 13 varnish, oil, paints, or enamelling it 

 with a glassy-looking flux possessing the same coefficient of expansion as 

 iron, or with a dense scoria (formed by the heat of superheated steam), 

 or with a compact coating of various metals. Wrought iron (both as 

 sheet iron and in other forms), cast iron, and steel are often coated with 

 tin, copper, lead, nickel, and similar metals, which prevent contact with 

 the air. These metals preserve iron very effectually from rust if they 

 form a completely compact surface, but in those places where the iron 

 becomes exposed, either accidentally or from wear, rust appears much 

 more quickly than on a uniform iron surface, because, towards these 

 metals (and also towards the rust), the iron will then behave as an 

 electro-positive pole in a galvanic couple, and hence will attract 

 oxygen. A coating of zinc does not produce this inconvenience, because 

 iron is electro-negative with reference to zinc, in consequence of which 

 galvanised iron does not easily rust, and even an iron boiler containing 

 some lumps of zinc rusts less than one without zinc. 14 Iron oxidises 

 at a high temperature, forming iron scale, Fe 3 O 4 , composed of ferrous 

 and ferric oxides, and, as has been seen, decomposes water and acids 

 with the evolution of hydrogen. It is also capable of decomposing 

 salts and oxides of other metals, which property is applied in the arts 

 for the extraction of copper, silver, lead, tin, &c. For this reason 

 iron is soluble in the 'solutions of many salts for instance, in cupric 

 sulphate, with precipitation of copper and formation of ferrous sul- 

 phate. 15 When iron acts on acids it always/orms compounds FeX 2 



18 Paraffin is one of the best preservatives for iron against oxidation in the air. I 

 found this "by experiments about 1860, and immediately published the fact. This method 

 is now very generally applied. 



* See Chapter XVIII., Note 84 bis. Based on the rapid oxidation of iron and its 

 increase in volume in the presence of water and salts of ammonium, a packing is used 

 for water mains and steam pipes which is tightly hammered into the socket joints. 

 This packing consists of a mixture of iron filings and a small quantity of sal-ammoniac 

 (and sulphur) moistened with water ; after a certain lapse of time, especially after the 

 pipes have been used, this mass swells to such an extent that it hermetically seals the 

 joints of the pipes. 



16 Here, however, a ferric salt may also be formed (when all the iron has dissolved 

 and the cupric salt is still in excess), because the cupric salts are reduced by ferrous 

 salts. Cast iron is also dissolved. 



