322 PRINCIPLES OF CHEMISTRY 



difficult by the fact that the iron does not melt at the heat developed 

 by the combustion of the charcoal, and therefore it does not separate 

 from those mechanically mixed impurities which are found in the iron 

 ore. This is obviated by the following very remarkable property of 

 iron : at a high temperature it is capable of combining with a small 

 quantity (from 2 to 5 p.c ) of carbon, and then forms cast iron, which 

 easily melts in the heat developed by the combustion of charcoal in air. 

 For this reason metallic iron is not obtained directly from the ore, but 

 is only formed after the further treatment of the cast iron } the first 

 product extracted from the ore being cast iron. The fused mass dis- 

 poses itself in the furnace below the slag that is, the impurities of the 

 ore fused by the heat of the furnace. If these impurities did not fuse 

 they would block up the furnace in which the ore was being smelted, 

 and the continuous smelting of the cast iron would not be possible ; 6 

 it would be necessary periodically to cool the furnace and heat it up 

 again, which means a wasteful expenditure of fuel, and hence in the 

 production of cast iron, the object in view is to obtain all the earthy 

 impurities of the ore in the shape of a fused mass or slag. Only 

 in rare cases does the ore itself form a mass which fuses at the 

 temperature employed, and these cases are objectionable if much iron 

 oxide is carried away in the slag. The impurities of the ores most 

 often consist of certain mixtures for instance, a mixture of clay and 

 sand, or a mixture of limestone and clay, or quartz, &c. These 



precipitated ferric oxide is reduced by hydrogen at 85, that obtained by oxidising the 

 metal or from its nitrate at 175 ; (b) when other conditions are the same the reduction 

 by carbonic oxide commences earlier than that by hydrogen, and the reduction by 

 hydrogen still earlier than that by charcoal ; (c) the reduction is effected with greater 

 facility when a greater quantity of heat is evolved during the reaction. Ferric oxide 

 obtained by heating ferrous sulphate to a red heat begins to be reduced by carbonic 

 oxide at 202, by hydrogen at 260, by charcoal at 480, whilst for magnetic oxide, Fe 3 C>4, 

 the temperatures are 200, 290, and 450 respectively. 



c The primitive methods of iron manufacture were conducted by intermittent pro- 

 cesses in hearths resembling smiths' fires. As evidenced by the uninterrupted action 

 of the steam boiler, or the process of lime burning, and the continuous preparation and 

 condensation of sulphuric acid or the uninterrupted smelting of iron, every industrial 

 process becomes increasingly profitable and complete under the condition of the con- 

 tinuous action, as far as possible, of all agencies concerned in the production. This 

 continuous method of production is the first condition for the profitable production 

 on the large scale of nearly all industrial products. This method lessens the cost of 

 labour, simplifies the supervision of the work, renders the product uniform, and fre- 

 quently introduces a very great economy in the expenditure of fuel and at the same time 

 presents the simplicity and perfection of an equilibrated system. Hence every manu- 

 facturing operation should be a continuous one, and the manufacture of pig iron and 

 sulphuric acid, which have long since become so, may be taken as examples in many 

 respects. A study of these two manufactures should form the commencement of an 

 acquaintance with all the contemporary methods of manufacturing both from a tech* 

 nical and economical point of view. 



