Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/297

IRON 281 generally see separate articles. Its therapeutic uses are noticed at p. 359. 3. Relationships between Iron (Malleable and Cast) and Steel. Iron possesses the power of uniting with a number of elements, forming products which either are highly intimate mixtures of more than one substance presenting apparent homogeneity, or else are compounds of an in definite character, i.e., in which the constituents are com bined in proportions which do not come under the usual chemical laws of invariableness of composition and of combination in multiple proportions ; in short, these iron compounds are substances belonging to the same category as alloys generally and solutions, the placing of which inside or outside the class of true chemical compounds depends on the particular definition of a chemical compound adopted. Probably the most accurate view of the constitu tion of such substances is that which regards them as being " solidified solutions " of one substance in another (Mat- thiessen), i.e., when the bodies in question have been fused : the most useful commercial forms of iron are of this class. Thus, for example, iron sulphide and metallic iron fused together in such proportions that the latter greatly pre dominates form a homogeneous mixture (or solution of iron sulphide in molten iron), which on cooling solidifies as a whole, not exhibiting any tendency to separation t>f the iron and iron sulphide ; a product similar but melting more readily is formed if iron sulphide and sulphur be fused together, forming one of the varieties of the so-called " Spence s metal " recently patented ; so that between the extremes of pure iron on the one hand and pure sulphur on the other an apparently homogeneous mass can be obtained containing iron or sulphur in any assignable pro portions, the compound being a solidified solution of iron sulphide in either iron or sulphur, according as the former or the latter is in excess. Silicon and phosphorus can be similarly incorporated with excess of iron, forming analogous solidified solutions ; the same remark is true for nitrogen and other non-metallic elements, as well as for manganese and many other metals, notably nickel, gold, tin, platinum, rhodium, aluminium, zinc, titanium, tungsten, and chromium. With arsenic and tin definite compounds can be produced expressible by simple formulae, e.g., FeAs (Gehlen) and FeSn (Deville and Caron), When carbon is thus incorporated with iron a peculiar phenomenon is (under certain circumstances) observable which has no parallel with the other compounds, except perhaps to some extent in the case of silicon ; this is that, whereas the car bon is in the amorphous condition when first dissolved, yet on long-continued maintenance in the molten state, but more especially on cooling (whilst the substance is still liquid or semisolid), a more or less complete separation of carbon in the crystallized graphitoidal state 1 often ensues ; so that the cooled mass is no longer visibly homogeneous, but consists of granules and crystals, partly of graphite and partly of solidified solution of amorphous carbon (and such other elements as were originally present) in iron. This pheno menon may be compared with a somewhat analogous change undergone by phosphorus : when this element is dissolved in carbon disulphide or certain organic bodies, e.g., ethyl iodide, the phosphorus gradually changes more or less completely into the red variety, which, being insoluble in the menstruum, precipitates in flakes. The amount of carbon which changes during solidification from the amor phous into the graphitoidal variety depends largely on the nature and amount of the substances present along with it dissolved in the iron, and also on the absolute amount of 1 Snelus has shown (Journal Iron and Steel Institute, 1871, i. 28) that it is practicable to remove mechanically from a highly crystalline pig iron graphitoidal scales, which consist so entirely of carbon as to leave little or no appreciable residue on combustion. carbon present and on the rate of cooling ; it appears to be promoted by the presence of silicon, the greyest irons (cseteris paribus) being usually the richest in silicon. On remelting graphitoidal cast iron, the graphite is again dis solved, so that by rapidly chilling the fused mass " white " iron results. Under certain conditions silicon appears to extrude from highly silicious irons in cooling, but not in a difficultly oxidizable form, so that the outside of the pigs becomes covered with silica of a peculiar physical aspect (Lowthian Bell, Journal Iron and Steel Institute, 1871, i. 44) ; under other conditions several parts per cent, of silicon can be permanently retained by the pig without extrusion on cooling, forming a peculiar metal known as " glazy iron," bearing to the silicious pig from which silicon does separate much the same relations as highly carbonized white iron bears to grey pig. When foreign substances are present in but small quan tity (manganese excepted), and the amount of total carbon does not exceed 1 5 to 2 per cent, of the iron, little or no separation of graphitoidal carbon takes place, and the resultant product is tolerably homogeneous, and possesses the properties of steel more or less soft in proportion as the carbon percentage is minute or otherwise. When the carbon amounts to some 2 $ 5 or upwards per cent, of the iron, and especially when the fused substance is rapidly cooled, the metal often solidifies as an almost homogeneous mass, possessing somewhat different properties from those of good steel; it is then known as u<hite cast iron (from its colour after fracture) ; under other conditions, especially when a longer time is allowed for solidification, a more or less complete separation of graphite and conse quent production of a coarse-grained crystalline structure results, the product being then termed grey cast iron, which consequently stands to white cast iron in much the same relation as devitrified glass (Reaumur s porcelain) to ordinary glass. When the amount of manganese present is relatively large (constituting several parts per cent, of the iron present), this separation of graphitoidal carbon takes place to but a small or even inappreciable extent ; the cooled mass is homogeneous and highly crystalline, the fractured surface exhibiting great brilliancy, whence the term spiegeleisen applied to such substances. As a rule cast irons, whether white or grey, contain more than traces of impurities, such as sulphur, phosphorus, and silicon ; but otherwise no absolute line of demarcation between malleable iron and steel on the one hand, and between steel and white iron on the other, can be drawn, based on the chemical com position; so that it cannot be said that a substance contain ing so much carbon is malleable iron, and so much more carbon steel, and so much more still cast iron ; the definition is purely arbitrary ; moreover, the physical qualities of a steel containing a given amount of carbon often differ much, according as the proportion of other substances present varies. The ordinary practical test applied todistinguish iron from steel is the ascertaining whether the substance hardens on heating and quenching in cold water, becoming again softened on reheating and cooling slowly : a substance which does this may fairly be regarded as steel (possibly of very bad quality, but still steel), whilst one which does not may be fairly regarded, as a soft iron. With certain specimens it is difficult thus to classify the substances under either head satisfactorily, whilst such a classification would not be accepted by many who would define a steel as being either the product of the cementation of malleable iron or as a substance that has been fused during manufacture, and who consequently would not admit that a very hard puddled metal was steel, even though it did harden distinctly on heating and quenching in cold water. Although it is impossible to draw a sharp line distin- XIII. 36