826 PRINCIPLES OF CHEMISTRY 



completely soluble in acids. When treated with acids a residue of 

 graphite remains ; it is known as grey or soft cast iron. This is the 

 general form of the ordinary cast iron used for casting various objects, 

 because in this state it is not so brittle as in the shape of white cast 

 iron, which does not leave particles of graphite when dissolved, but 

 yields its carbon in the form of hydrocarbons. This white cast iron 

 is characterised by its whitish-grey colour, dull lustre, the crystalline 

 structure of its fracture (more homogeneous than that of grey iron), and 

 such hardness that a file will hardly cut it. When white cast iron is 

 produced (from manganese ore) at high temperatures (and with an ex- 

 cess of lime), and containing little sulphur and silica but a considerable 

 amount of carbon (as much as 5 p.c.), it acquires a coarse crystalline 

 structure which increases in proportion to the amount of manganese, 

 and it is then known under the name of ' spiegeleisen ' (and 'ferro- 



9 



simultaneously in these lower portions of the furnace. The carbonic acid formed in 

 these parts rises higher, encounters incandescent carbon, and forms with it carbonic 

 oxide. This heated carbonic oxide acts as a reducing agent on the iron ore, and is re- 

 converted by it into carbonic anhydride ; this gas meets with more carbon, and again 

 forms carbonic oxide, which again acts as a reducing agent. The final transformation 

 of the carbonic anhydride into carbonic oxide is effected in those parts of the furnace 

 where the reduction of the oxides of iron does not take place, but where the temperature^ 

 is still high enough to reduce the carbonic anhydride. The ascending mixture of 

 carbonic oxide and nitrogen, COj, &c., is then withdrawn through special lateral 

 apertures formed in the upper cold parts of the furnace walls, and is conducted through 

 pipes to those stoves which are used for heating the air, and also nometimes into other 

 furnaces used for the further processes of iron manufacture. The fuel of blast furnaces 

 consists of wood charcoal (this is the most expensive material, but the pig iron pro- 

 duced is the purest, because charcoal does not contain any sulphur, while coke does), 

 anthracite (for instance, in Pennsylvania, and in Russia at Pastouhoff's works in the 

 Don district), coke, coal, and even wood and peat. It must be borne in mind that the 

 utilisation of naphtha and naphtha refuse would probably give very profitable results 

 in metallurgical processes. 



The process just described is accompanied by a series of other processes. Thus, for 

 instance, in the blast furnace a considerable quantity of cyanogen compounds are formed. 

 This takes place because the nitrogen of the air blast comes into contact with incan- 

 descent carbon and various alkaline matters contained in the foreign matter of the ores. 

 A considerable quantity of potassium cyanide is formed when wood charcoal is employed 

 for iron smelting, as its ash is rich in potash. 



9 The specific gravity of white cast iron is about 7'5. Grey cast iron has a much lower 

 specific gravity, namely, 7'0. Grey cast iron generally contains less manganese and 

 more silica than white ; but both contain from 2 to 8 p.c. of carbon. The difference 

 between the varieties of cast iron depends on the condition of the carbon which 

 enters into the composition of the iron. In white cast iron the carbon is in combination 

 with the iron in all probability, as the compound CFe 4 (Abel and Osmond and others 

 extracted this compound, which is sometimes called ' carbide,' from tempered steel, 

 which stands to unannealed steel as white cast iron does to grey), but perhaps in the state 

 of an indefinite chemical compound resembling a solution. In any case the compound of 

 the iron and carbon in white cast iron is chemically very unstable, because when slowly 

 cooled it decomposes, with separation of graphite, just as a solution when slowly cooled 



