IKON, COBALT, AND NICKEL 888 



below the melting point of platinum (1750 ). 11 But pure iron becomes 

 soft at a temperature considerably below that at which it melts, and 

 may then be easily forged, welded, and rolled or drawn into sheets and 

 wire iibis Pure iron may be rolled into an exceedingly thin sheet, 

 weighing less than a sheet of ordinary paper of the same size. This 

 ductility is the most important property of iron in all its forms, and is 

 most marked with sheet iron, and least so with cast iron, whose 

 ductility, compared with wrought iron, is small, but it is still very 

 considerable when compared with other substances such, for instance, 

 as rocks. 12 



The chemical properties of iron have been already repeatedly 

 mentioned in preceding chapters. Iron rusts in air at the ordinary 

 temperature that is to say, it becomes covered with a layer of iron 

 oxides. Here, without doubt, the moisture of the air plays a part, 

 because in dry air iron does not oxidise at aH, and also because, more 



11 According to information supplied by A. T. Skinder's experiments at the Oboukoff 

 Steel Works, 140 volumes of liquid molten steel give 128 volumes of solid metal. By 

 means of a galvanic current of great intensity and dense charcoal as one electrode, and 

 iron as the other, Bernadoss. welded iron and fused holes through sheet iron. Soft 

 wrought iron, like steel and soft malleable cast iron, may be melted in Siemens' 

 regenerative furnaces, and in furnaces heated with naphtha. 



11 bu <3. ore (1869), Tait, Barret, Tchernoff, Osmond, and others observed that at a 

 temperature approaching 600 that is, between dark and bright red .heat all kinds of 

 wrought iron undergo a peculiar change called recalescence, i.e. a spontaneous rise of 

 temperature. If iron be considerably heated and allowed to cool, it may be observed 

 that at this temperature the cooling stops that is, latent heat is disengaged, corre- 

 sponding with a change in condition. The specific heat, electrical conductivity, magnetic, 

 and other properties then also change. In tempering, the temperature of recalescence 

 must not be reached, and so also in annealing, <fcc. It is evident that a change of the 

 internal condition is here encountered, exactly similar to the transition from a solid to a 

 liquid, although there is no evident physical change. It is probable that attentive study 

 would lead to the discovery of a similar change in other substances. 



13 The particles of steel are linked together or connected more closely than those of 

 the other metals ; this is shown by the fact that it only breaks with a tensile strain of 

 60-80 kilos per sq. mm., whilst wrought iron only withstands about 80 kilos, cast iron 

 10, copper 85, silver 28, platinum 80, wood 8. The elasticity of iron, steel, and other 

 metals ig expressed by the so-called coefficient of elasticity. Let a rod be taken whose 

 length is L ; if a weight, P, be hung from the extremity of it, it will lengthen to L 

 The less it lengthens under other equal conditions, the more elastic the material, if ife 

 resumes its original length when the weight is removed. It has been shown by experiment 

 that the increase in length I, due to elasticity, is directly proportional to the length L 

 and the weight P, and inversely proportional to the section, but changes with the 

 material. The coefficient of elasticity expresses that weight (in kilos per sq. mm.) 

 under which a rod having a square section taken as 1 (we take 1 sq. mm.) acquires 

 double the length by tension. Naturally in practice materials -do not withstand such a, 

 lengthening, under a certain weight they attain a limit of elasticity, i.e. they stretch 

 permanently (undergo deformation). Neglecting fractions (as the elasticity of metal* 

 varies not only with the temperature, but also with forging, purity, &c.), the coefficient 

 of elasticity of steel and iron is 20,000, copper and brass 10,000, silver 7,000, glass 6,000, 

 lead 2,000, and wood 1,200. 



