Iron

IRON, a metallic chemical element. Although iron (symbol Fe, atomic number 26, atomic weight 55.84, isotopes 56 and 54) occurs only sparingly in the free state, the abundance of ores from which it may be readily obtained led to its application in the arts at a very remote period. It is generally agreed, however, that the iron age, the period of civilization during which this metal played an all-important part, succeeded the ages of copper and bronze, notwithstanding the fact that the extraction of these metals required greater metallurgical skill. The Assyrians and Egyptians made considerable use of the metal; and in Gen. iv. 22 mention is made of Tubal-cain as the instructor of workers in iron and copper. The earlier sources of the ores appear to have been in India; but it was also obtained by the Greeks from the south coast of the Black sea, and by the Romans from Spain and Elba.

The chief occurrences of metallic iron are as minute spiculae disseminated through basaltic rocks, as at Giant's Causeway and in the Auvergne, and, more particularly, in meteorites (q.v.). In combination it occurs, usually in small quantity, in most natural waters, in plants and as a necessary constituent of blood. The economic sources are treated under IRON AND STEEL below; in the same place will be found accounts of the manufacture, prop erties and uses of the metal, the present article being confined to its chemistry. The principal iron ores are the oxides and carbo nates, and these readily yield the metal by smelting with carbon. The metal so obtained invariably contains a certain amount of carbon, free or combined, and the proportion and condition regu late the properties of the metal, giving origin to the three im portant varieties: cast iron, steel, wrought iron. The perfectly pure metal may be prepared by heating the oxide or oxalate in a current of hydrogen ; when obtained at a low temperature it is a black powder which oxidizes in air with incandescence ; i.e., it is "pyrophoric." It may be obtained electrolytically from solutions of ferrous salts.

has a silvery lustre and takes a high polis

h. Its specific gravity is 7.84; and the average specific heat over the range C is 0.10983; this value increases with tem perature to and then begins to diminish. It is the most tenacious of all the ductile metals at ordinary temperatures with the exception of cobalt and nickel ; it becomes brittle, however, at the temperature of liquid air. It softens at a red heat, and may be readily welded at a white heat; above this point it becomes brittle. Pure iron melts at c. 1525° C and may be distilled in the electric furnace. It is attracted by a magnet and may be magnet ized, but the magnetization is quickly lost. The variation of phys ical properties which attends iron on heating has led to the view that the metal exists in allotropic forms. This has been proved recently by X-ray analysis. (See IRON AND STEEL.) When pure iron is cooled from its melting point, its temperature falls evenly to 1400° and then remains almost stationary for a time; the cooling again becomes regular when 890° is reached, where a similar lag occurs or even a rise of temperature (which may be so pronounced as to cause a flow or recalescence) ; with still further cooling, yet a third lag is found at 76o°. Similar

irregularities occur at nearly the same (but slightly higher) temperatures when iron is slowly heated. Moreover, at these temperatures other physical properties of the metal (such as the coefficient of expansion, electrical resistance, and especially the characteristic magnetic property) undergo sudden changes, which lead to the assumption that there are four allotropic modifications of iron with transition points thus: 769° 9060 a (3 The crystal structure of a, 0 and (5 is the same, viz., body centred cubic (see CRYSTALLOGRAPHY), but that of 7-iron is a face-centre cube, and this difference is held by some metallur gists to show that 0-iron is not a distinct modification, but only a solid solution of a and 7-iron.

It should be pointed out that "pure" iron is almost a curiosity, for the metal, as commonly used, nearly always contains a certain small proportion of carbon which much modifies its properties.

Iron is very reactive chemically. Exposed to atmospheric in fluences it is more or less rapidly corroded, giving the familiar rust (q.v.). For the production of rust, the water and oxygen does not seem to be sufficient, and an "impurity" seems to be essential ; this is provided usually in the iron itself or by the presence of carbon dioxide. The decomposition of steam by passing it through a red-hot gun-barrel, resulting in the libera tion of hydrogen and the production of magnetic iron oxide, is a familiar laboratory method for preparing hydrogen (q.v.). When strongly heated, iron inflames in oxygen and in sulphur vapour; it also combines directly with the halogens. It dissolves in most dilute acids with liberation of hydrogen, the reaction between sulphuric acid and iron-turnings being used for the commercial manufacture of this gas. It dissolves in dilute cold nitric acid with the formation of ferrous and ammonium nitrates, no gases being liberated ; when heated, or with stronger acid, ferric nitrate is formed with evolution of nitrogen oxides.

After having been immersed in strong nitric acid, iron neither dissolves in acids nor precipitates metals from solutions. This "passivity" may be brought about by immersion in other solu tions, especially by those containing such oxidizing anions as less strongly by the anions SO", CN', CNS', OH', while C1', Br' practically inhibit passivity; H. is the only cation which has any effect, and this tends to exclude passivity. It is also occasioned by anodic polarization of iron in sulphuric acid. Several hypotheses have been promoted to explain this behaviour, and, although the question is not definitely settled, the more probable view is that it is caused by the formation of a film of an oxide (see U. R. Evans, Journ. Chem,. Soc., 1927, p. 1020). Other metals also exhibit the phenomenon of passivity under sim ilar conditions.