Chapter X: Part 10

IRMIN, or IRMINUS, in Teutonic mythology, a deified eponymic hero of the Herminones. The chief seat of his worship was Irminsal, or Ermensul, in Westphalia, destroyed in 772 by Charlemagne. Huge wooden posts (Irmin pillars) were raised to his honour, and were regarded as sacred by the Saxons.

IRNERIUS (Hirnerius, Hyrnerius, Iernerius, Gernerius, Guarnerius, Warnerius, Wernerius, Yrnerius), Italian jurist, sometimes referred to as "lucerna juris." He taught the "free arts" at Bologna, his native city, during the earlier decades of the 12th century. Of his personal history nothing is known, except that it was at the instance of the countess Matilda, Hildebrand's friend, who died in 1115, that he directed his attention and that of his students to the _Institutes_ and _Code_ of Justinian; that after 1116 he appears to have held some office under the emperor Henry V.; and that he died, perhaps during the reign of the emperor Lothair II., but certainly before 1140. He was the first of the Glossators (see GLOSS), and according to ancient opinion (which, however, has been much controverted) was the author of the epitome of the _Novellae_ of Justinian, called the _Authentica_, arranged according to the titles of the _Code_. His _Formularium tabellionum_ (a directory for notaries) and _Quaestiones_ (a book of decisions) are no longer extant. (See ROMAN LAW.)

See Savigny, _Gesch. d. rom. Rechts im Mittelalter_, iii. 83; Vecchio,
_Notizie di Irnerio e della sua scuola_ (Pisa, 1869); Ficker, _Forsch,
z. Reichs- u. Rechtsgesch. Italiens_, vol. iii. (Innsbruck, 1870); and
Fitting, _Die Anfange der Rechtsschule zu Bologna_ (Berlin, 1888).

IRON [symbol Fe, atomic weight 55.85 (O = 16)], a metallic chemical element. Although iron 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 Genesis 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; the Greeks, however, obtained it from the Chalybes, who dwelt on the south coast of the Black Sea; and the Romans, besides drawing from these deposits, also exploited Spain, Elba and the province of Noricum. (See METAL-WORK.)

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, properties, and uses of the metal, the present article being confined to its chemistry. The principal iron ores are the oxides and carbonates, 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 regulate the properties of the metal, giving origin to the three important 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; produced at higher temperatures the metal is not pyrophoric. Peligot obtained it as minute tetragonal octahedra and cubes by reducing ferrous chloride in hydrogen. It may be obtained electrolytically from solutions of ferrous and magnesium sulphates and sodium bicarbonate, a wrought iron anode and a rotating cathode of copper, thinly silvered and iodized, being employed (S. Maximowitsch, _Zeit. Elektrochem._, 1905, 11, p. 52).

In bulk, the metal has a silvery white lustre and takes a high polish. Its specific gravity is 7.84; and the average specific heat over the range 15 deg.-100 deg. is 0.10983; this value increases with temperature to 850 deg., 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. It fuses at about 1550 deg.-1600 deg., and may be distilled in the electric furnace (H. Moissan, _Compt. rend._, 1906, 142, p. 425). It is attracted by a magnet and may be magnetized, but the magnetization is quickly lost. The variation of physical properties which attends iron on heating has led to the view that the metal exists in allotropic forms (see IRON AND STEEL, below).

Iron is very reactive chemically. Exposed to atmospheric influences it is more or less rapidly corroded, giving the familiar rust (q.v.). S. Burnie (_Abst. J.C.S._, 1907, ii. p. 469) has shown that water is decomposed at all temperatures from 0 deg. to 100 deg. by the finely divided metal with liberation of hydrogen, the action being accelerated when oxides are present. The decomposition of steam by passing it through a red-hot gun-barrel, resulting in the liberation of hydrogen and the production of magnetic iron oxide, Fe3O4, 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.

It was observed by James Keir (_Phil. Trans._, 1790, p. 359) that iron, after having been immersed in strong nitric acid, is insoluble in acids, neither does it precipitate metals from solutions. This "passivity" may be brought about by immersion in other solutions, especially by those containing such oxidizing anions as NO'3, ClO'3, less strongly by the anions SO"4 CN', CNS', C2H3O'2, OH', while Cl', 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. Other metals may be rendered passive; for example, zinc does not precipitate copper from solutions of the double cyanides and sulphocyanides, nickel and cadmium from the nitrates, and iron from the sulphate, but it immediately throws down nickel and cadmium from the sulphates and chlorides, and lead and copper from the nitrates (see O. Sackur, _Zeit. Elektrochem._, 1904, 10, p. 841). Anodic polarization in potassium chloride solution renders molybdenum, niobium, ruthenium, tungsten, and vanadium passive (W. Muthmann and F. Frauenberger, _Sitz. Bayer. Akad. Wiss._, 1904, 34, p. 201), and also gold in commercial potassium cyanide solution (A. Coehn and C. L. Jacobsen, _Abs. J.C.S._, 1907, ii. p. 926). 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, a suggestion made many years ago by Faraday (see P. Krassa, _Zeit. Elektrochem._, 1909, 15, p. 490). Fredenhagen (_Zeit. physik. Chem._, 1903, 43, p. 1), on the other hand, regarded it as due to surface films of a gas; submitting that the difference between iron made passive by nitric acid and by anodic polarization was explained by the film being of nitrogen oxides in the first case and of oxygen in the second case. H. L. Heathcote and others regard the passivity as invariably due to electrolytic action (see papers in the _Zeit. physik. Chem._, 1901 et seq.).

_Compounds of Iron._

_Oxides and Hydroxides._--Iron forms three oxides: ferrous oxide, FeO, ferric oxide, Fe2O3, and ferroso-ferric oxide, Fe3O4. The first two give origin to well-defined series of salts, the ferrous salts, wherein the metal is divalent, and the ferric salts, wherein the metal is trivalent; the former readily pass into the latter on oxidation, and the latter into the former on reduction.

_Ferrous oxide_ is obtained when ferric oxide is reduced in hydrogen at 300 deg. as a black pyrophoric powder. Sabatier and Senderens (_Compt. rend._, 1892, 114, p. 1429) obtained it by acting with nitrous oxide on metallic iron at 200 deg., and Tissandier by heating the metal to 900 deg. in carbon dioxide; Donau (_Monats._, 1904, 25, p. 181), on the other hand, obtained a magnetic and crystalline-ferroso-ferric oxide at 1200 deg. It may also be prepared as a black velvety powder which readily takes up oxygen from the air by adding ferrous oxalate to boiling caustic potash. Ferrous hydrate, Fe(OH)2, when prepared from a pure ferrous salt and caustic soda or potash free from air, is a white powder which may be preserved in an atmosphere of hydrogen. Usually, however, it forms a greenish mass, owing to partial oxidation. It oxidizes on exposure with considerable evolution of heat; it rapidly absorbs carbon dioxide; and readily dissolves in acids to form ferrous salts, which are usually white when anhydrous, but greenish when hydrated.

_Ferric oxide_ or iron sesquioxide, Fe2O3, constitutes the valuable ores red haematite and specular iron; the minerals brown haematite or limonite, and gothite and also iron rust are hydrated forms. It is obtained as a steel-grey crystalline powder by igniting the oxide or any ferric salt containing a volatile acid. Small crystals are formed by passing ferric chloride vapour over heated lime. When finely ground these crystals yield a brownish red powder which dissolves slowly in acids, the most effective solvent being a boiling mixture of 8 parts of sulphuric acid and 3 of water. Ferric oxide is employed as a pigment, as jeweller's rouge, and for polishing metals. It forms several hydrates, the medicinal value of which was recognized in very remote times. Two series of synthetic hydrates were recognized by Muck and Tommasi: the "red" hydrates, obtained by precipitating ferric salts with alkalis, and the "yellow" hydrates, obtained by oxidizing moist ferrous hydroxide or carbonates. J. van Bemmelen has shown that the red hydrates are really colloids, the amount of water retained being such that its vapour pressure equals the pressure of the aqueous vapour in the superincumbent atmosphere. By heating freshly prepared red ferric hydrate with water under 5000 atmospheres pressure Ruff (_Ber._, 1901, 34, p. 3417) obtained definite hydrates corresponding to the minerals limonite (30 deg.-42.5 deg.), gothite (42.5 deg.-62.5 deg.), and hydrohaematite (above 62.5 deg.). Thomas Graham obtained a soluble hydrate by dissolving the freshly prepared hydrate in ferric chloride and dialysing the solution, the soluble hydrate being left in the dialyser. All the chlorine, however, does not appear to be removed by this process, the residue having the composition 82Fe(OH)3.FeCl3; but it may be by electrolysing in a porous cell (Tribot and Chretien, _Compt. rend._, 1905, 140, p. 144). On standing, the solution usually gelatinizes, a process accelerated by the addition of an electrolyte. It is employed in medicine under the name _Liquor ferri dialysati_. The so-called soluble meta-ferric hydroxide, FeO(OH)(?), discovered by Pean de St Gilles in 1856, may be obtained by several methods. By heating solutions of certain iron salts for some time and then adding a little sulphuric acid it is precipitated as a brown powder. Black scales, which dissolve in water to form a red solution, are obtained by adding a trace of hydrochloric acid to a solution of basic ferric nitrate which has been heated to 100 deg. for three days. A similar compound, which, however, dissolves in water to form an orange solution, results by adding salt to a heated solution of ferric chloride. These compounds are insoluble in concentrated, but dissolve readily in dilute acids.

Red ferric hydroxide dissolves in acids to form a well-defined series of salts, the ferric salts, also obtained by oxidizing ferrous salts; they are usually colourless when anhydrous, but yellow or brown when hydrated. It has also feebly acidic properties, forming _ferrites_ with strong bases.

_Magnetite_, Fe3O4, may be regarded as ferrous ferrite, FeO.Fe2O3. This important ore of iron is most celebrated for its magnetic properties (see MAGNETISM and COMPASS), but the mineral is not always magnetic, although invariably attracted by a magnet. It may be obtained artificially by passing steam over red-hot iron. It dissolves in acids to form a mixture of a ferrous and ferric salt,[1] and if an alkali is added to the solution a black precipitate is obtained which dries to a dark brown mass of the composition Fe(OH)2.Fe2O3; this substance is attracted by a magnet, and thus may be separated from the admixed ferric oxide. Calcium ferrite, magnesium ferrite and zinc ferrite, RO.Fe2O3 (R = Ca, Mg, Zn), are obtained by intensely heating mixtures of the oxides; magnesium ferrite occurs in nature as the mineral magnoferrite, and zinc ferrite as franklinite, both forming black octahedra.

_Ferric acid_, H2FeO4. By fusing iron with saltpetre and extracting the melt with water, or by adding a solution of ferric nitrate in nitric acid to strong potash, an amethyst or purple-red solution is obtained which contains potassium ferrate. E. Fremy investigated this discovery, made by Stahl in 1702, and showed that the same solution resulted when chlorine is passed into strong potash solution containing ferric hydrate in suspension. Haber and Pick (_Zeit. Elektrochem._, 1900, 7, p. 215) have prepared potassium ferrate by electrolysing concentrated potash solution, using an iron anode. A temperature of 70 deg., and a reversal of the current (of low density) between two cast iron electrodes every few minutes, are the best working conditions. When concentrated the solution is nearly black, and on heating it yields a yellow solution of potassium ferrite, oxygen being evolved. Barium ferrate, BaFeO4.H2O, obtained as a dark red powder by adding barium chloride to a solution of potassium ferrate, is fairly stable. It dissolves in acetic acid to form a red solution, is not decomposed by cold sulphuric acid, but with hydrochloric or nitric acid it yields barium and ferric salts, with evolution of chlorine or oxygen (Baschieri, _Gazetta_, 1906, 36, ii. p. 282).

_Halogen Compounds._--Ferrous fluoride, FeF2, is obtained as
colourless prisms (with 8H2O) by dissolving iron in hydrofluoric acid,
or as anhydrous colourless rhombic prisms by heating iron or ferric
chloride in dry hydrofluoric acid gas. Ferric fluoride, FeF3, is
obtained as colourless crystals (with 4(1/2)H2O) by evaporating a
solution of the hydroxide in hydrofluoric acid. When heated in air it
yields ferric oxide. Ferrous chloride, FeCl2, is obtained as shining
scales by passing chlorine, or, better, hydrochloric acid gas, over
red-hot iron, or by reducing ferric chloride in a current of hydrogen.
It is very deliquescent, and freely dissolves in water and alcohol.
Heated in air it yields a mixture of ferric oxide and chloride, and in
steam magnetic oxide, hydrochloric acid, and hydrogen. It absorbs
ammonia gas, forming the compound FeCl2.6NH2, which on heating loses
ammonia, and, finally, yields ammonium chloride, nitrogen and iron
nitride. It fuses at a red-heat, and volatilizes at a yellow-heat; its
vapour density at 1300 deg.-1400 deg. corresponds to the formula
FeCl2. By evaporating in vacuo the solution obtained by dissolving
iron in hydrochloric acid, there results bluish, monoclinic crystals
of FeCl2.4H2O, which deliquesce, turning greenish, on exposure to air,
and effloresce in a desiccator. Other hydrates are known. By adding
ammonium chloride to the solution, evaporating in vacuo, and then
volatilizing the ammonium chloride, anhydrous ferrous chloride is
obtained. The solution, in common with those of most ferrous salts,
absorbs nitric oxide with the formation of a brownish solution.

Ferric chloride, FeCl3, known in its aqueous solution to Glauber as
_oleum martis_, may be obtained anhydrous by the action of dry
chlorine on the metal at a moderate red-heat, or by passing
hydrochloric acid gas over heated ferric oxide. It forms iron-black
plates or tablets which appear red by transmitted and a metallic green
by reflected light. It is very deliquescent, and readily dissolves in
water, forming a brown or yellow solution, from which several hydrates
may be separated (see SOLUTION). The solution is best prepared by
dissolving the hydrate in hydrochloric acid and removing the excess of
acid by evaporation, or by passing chlorine into the solution obtained
by dissolving the metal in hydrochloric acid and removing the excess
of chlorine by a current of carbon dioxide. It also dissolves in
alcohol and ether; boiling point determinations of the molecular
weight in these solutions point to the formula FeCl3. Vapour density
determinations at 448 deg. indicate a partial dissociation of the
double molecule Fe2Cl6; on stronger heating it splits into ferrous
chloride and chlorine. It forms red crystalline double salts with the
chlorides of the metals of the alkalis and of the magnesium group. An
aqueous solution of ferric chloride is used in pharmacy under the name
_Liquor ferri perchloridi_; and an alcoholic solution constitutes the
quack medicine known as "Lamotte's golden drops." Many oxychlorides
are known; soluble forms are obtained by dissolving precipitated
ferric hydrate in ferric chloride, whilst insoluble compounds result
when ferrous chloride is oxidized in air, or by boiling for some time
aqueous solutions of ferric chloride.

Ferrous bromide, FeBr2, is obtained as yellowish crystals by the union
of bromine and iron at a dull red-heat, or as bluish-green rhombic
tables of the composition FeBr2.6H2O by crystallizing a solution of
iron in hydrobromic acid. Ferric bromide, FeBr3, is obtained as dark
red crystals by heating iron in an excess of bromine vapour. It
closely resembles the chloride in being deliquescent, dissolving
ferric hydrate, and in yielding basic salts. Ferrous iodide, FeI2, is
obtained as a grey crystalline mass by the direct union of its
components. Ferric iodide does not appear to exist.

_Sulphur Compounds._--Ferrous sulphide, FeS, results from the direct
union of its elements, best by stirring molten sulphur with a
white-hot iron rod, when the sulphide drops to the bottom of the
crucible. It then forms a yellowish crystalline mass, which readily
dissolves in acids with the liberation of sulphuretted hydrogen.
Heated in air it at first partially oxidizes to ferrous sulphate, and
at higher temperatures it yields sulphur dioxide and ferric oxide. It
is unaltered by ignition in hydrogen. An amorphous form results when a
mixture of iron filings and sulphur are triturated with water. This
modification is rapidly oxidized by the air with such an elevation of
temperature that the mass may become incandescent. Another black
amorphous form results when ferrous salts are precipitated by ammonium
sulphide.

Ferric sulphide, Fe2S3, is obtained by gently heating a mixture of its
constituent elements, or by the action of sulphuretted hydrogen on
ferric oxide at temperatures below 100 deg. It is also prepared by
precipitating a ferric salt with ammonium sulphide; unless the alkali
be in excess a mixture of ferrous sulphide and sulphur is obtained. It
combines with other sulphides to form compounds of the type M'2Fe2S4.
Potassium ferric sulphide, K2Fe2S4, obtained by heating a mixture of
iron filings, sulphur and potassium carbonate, forms purple glistening
crystals, which burn when heated in air. Magnetic pyrites or
pyrrhotite has a composition varying between Fe7S8 and Fe8S9, i.e.
5FeS.Fe2S3 and 6FeS.Fe2S3. It has a somewhat brassy colour, and occurs
massive or as hexagonal plates; it is attracted by a magnet and is
sometimes itself magnetic. The mineral is abundant in Canada, where
the presence of about 5% of nickel makes it a valuable ore of this
metal. Iron disulphide, FeS2, constitutes the minerals pyrite and
marcasite (q.v.); copper pyrites is (Cu, Fe)S2. Pyrite may be prepared
artificially by gently heating ferrous sulphide with sulphur, or as
brassy octahedra and cubes by slowly heating an intimate mixture of
ferric oxide, sulphur and sal-ammoniac. It is insoluble in dilute
acids, but dissolves in nitric acid with separation of sulphur.

Ferrous sulphite, FeSO3. Iron dissolves in a solution of sulphur
dioxide in the absence of air to form ferrous sulphite and
thio-sulphate; the former, being less soluble than the latter,
separates out as colourless or greenish crystals on standing.

Ferrous sulphate, green vitriol or copperas, FeSO4.7H2O, was known to,
and used by, the alchemists; it is mentioned in the writings of
Agricola, and its preparation from iron and sulphuric acid occurs in
the _Tractatus chymico-philosophicus_ ascribed to Basil Valentine. It
occurs in nature as the mineral melanterite, either crystalline or
fibrous, but usually massive; it appears to have been formed by the
oxidation of pyrite or marcasite. It is manufactured by piling pyrites
in heaps and exposing to atmospheric oxidation, the ferrous sulphate
thus formed being dissolved in water, and the solution run into tanks,
where any sulphuric acid which may be formed is decomposed by adding
scrap iron. By evaporation the green vitriol is obtained as large
crystals. The chief impurities are copper and ferric sulphates; the
former may be removed by adding scrap iron, which precipitates the
copper; the latter is eliminated by recrystallization. Other
impurities such as zinc and manganese sulphates are more difficult to
remove, and hence to prepare the pure salt it is best to dissolve pure
iron wire in dilute sulphuric acid. Ferrous sulphate forms large green
crystals belonging to the monoclinic system; rhombic crystals,
isomorphous with zinc sulphate, are obtained by inoculating a solution
with a crystal of zinc sulphate, and triclinic crystals of the formula
FeSO4.5H2O by inoculating with copper sulphate. By evaporating a
solution containing free sulphuric acid in a vacuum, the
hepta-hydrated salt first separates, then the penta-, and then a
tetra-hydrate, FeSO4.4H2O, isomorphous with manganese sulphate. By
gently heating in a vacuum to 140 deg., the hepta-hydrate loses 6
molecules of water, and yields a white powder, which on heating in the
absence of air gives the anhydrous salt. The monohydrate also results
as a white precipitate when concentrated sulphuric acid is added to a
saturated solution of ferrous sulphate. Alcohol also throws down the
salt from aqueous solution, the composition of the precipitate varying
with the amount of salt and precipitant employed. The solution absorbs
nitric oxide to form a dark brown solution, which loses the gas on
heating or by placing in a vacuum. Ferrous sulphate forms double salts
with the alkaline sulphates. The most important is ferrous ammonium
sulphate, FeSO4.(NH4)2SO4.6H2O, obtained by dissolving equivalent
amounts of the two salts in water and crystallizing. It is very
stable and is much used in volumetric analysis.

Ferric sulphate, Fe2(SO4)3, is obtained by adding nitric acid to a hot
solution of ferrous sulphate containing sulphuric acid, colourless
crystals being deposited on evaporating the solution. The anhydrous
salt is obtained by heating, or by adding concentrated sulphuric acid
to a solution. It is sparingly soluble in water, and on heating it
yields ferric oxide and sulphur dioxide. The mineral coquimbite is
Fe2(SO4)3.9H2O. Many basic ferric sulphates are known, some of which
occur as minerals; carphosiderite is Fe(FeO)5(SO4)4.10H2O; amarantite
is Fe(FeO)(SO4)2.7H2O; utahite is 3(FeO)2SO4.4H2O; copiapite is
Fe3(FeO)(SO4)5.18H2O; castanite is Fe(FeO)(SO4)2.8H2O; romerite is
FeSO4.Fe2(SO4)3.12H2O. The iron alums are obtained by crystallizing
solutions of equivalent quantities of ferric and an alkaline sulphate.
Ferric potassium sulphate, the common iron alum,
K2SO4.Fe2(SO4)3.24H2O, forms bright violet octahedra.

_Nitrides, Nitrates, &c._--Several nitrides are known. Guntz (_Compt.
rend._, 1902, 135, p. 738) obtained ferrous nitride, Fe3N2, and ferric
nitride, FeN, as black powders by heating lithium nitride with ferrous
potassium chloride and ferric potassium chloride respectively. Fowler
(_Jour. Chem. Soc._, 1901, p. 285) obtained a nitride Fe2N by acting
upon anhydrous ferrous chloride or bromide, finely divided reduced
iron, or iron amalgam with ammonia at 420 deg.; and, also, in a compact
form, by the action of ammonia on red-hot iron wire. It oxidizes on
heating in air, and ignites in chlorine; on solution in mineral acids
it yields ferrous and ammonium salts, hydrogen being liberated. A
nitride appears to be formed when nitrogen is passed over heated iron,
since the metal is rendered brittle. Ferrous nitrate, Fe(NO3)2.6H2O,
is a very unstable salt, and is obtained by mixing solutions of
ferrous sulphate and barium nitrate, filtering, and crystallizing in a
vacuum over sulphuric acid. Ferric nitrate, Fe(NO3)3, is obtained by
dissolving iron in nitric acid (the cold dilute acid leads to the
formation of ferrous and ammonium nitrates) and crystallizing, when
cubes of Fe(NO3)3.6H2O or monoclinic crystals of Fe(NO3)3.9H2O are
obtained. It is used as a mordant.

Ferrous solutions absorb nitric oxide, forming dark green to black
solutions. The coloration is due to the production of unstable
compounds of the ferrous salt and nitric oxide, and it seems that in
neutral solutions the compound is made up of one molecule of salt to
one of gas; the reaction, however, is reversible, the composition
varying with temperature, concentration and nature of the salt.
Ferrous chloride dissolved in strong hydrochloric acid absorbs two
molecules of the gas (Kohlschutter and Kutscheroff, _Ber._, 1907, 40,
p. 873). Ferric chloride also absorbs the gas. Reddish brown amorphous
powders of the formulae 2FeCl3.NO and 4FeCl3.NO are obtained by
passing the gas over anhydrous ferric chloride. By passing the gas
into an ethereal solution of the salt, nitrosyl chloride is produced,
and on evaporating over sulphuric acid, black needles of FeCl2.NO.2H2O
are obtained, which at 60 deg. form the yellow FeCl2.NO. Complicated
compounds, discovered by Roussin in 1858, are obtained by the
interaction of ferrous sulphate and alkaline nitrites and sulphides.
Two classes may be distinguished:--(1) the ferrodinitroso salts, e.g.
K[Fe(NO)2S], potassium ferrodinitrososulphide, and (2) the
ferroheptanitroso salts, e.g. K[Fe4(NO)7S8], potassium
ferroheptanitrososulphide. These salts yield the corresponding acids
with sulphuric acid. The dinitroso acid slowly decomposes into
sulphuretted hydrogen, nitrogen, nitrous oxide, and the heptanitroso
acid. The heptanitroso acid is precipitated as a brown amorphous mass
by dilute sulphuric acid, but if the salt be heated with strong acid
it yields nitrogen, nitric oxide, sulphur, sulphuretted hydrogen, and
ferric, ammonium and potassium sulphates.

_Phosphides, Phosphates._--H. Le Chatelier and S. Wologdine (_Compt.
rend._, 1909, 149, p. 709) have obtained Fe3P, Fe2P, FeP, Fe2P3, but
failed to prepare five other phosphides previously described. Fe3P
occurs as crystals in the product of fusing iron with phosphorus; it
dissolves in strong hydrochloric acid. Fe2P forms crystalline needles
insoluble in acids except aqua regia; it is obtained by fusing copper
phosphide with iron. FeP is obtained by passing phosphorus vapour over
Fe2P at a red-heat. Fe2P3 is prepared by the action of phosphorus
iodide vapour on reduced iron. Ferrous phosphate, Fe3(PO4)2.8H2O,
occurs in nature as the mineral vivianite. It may be obtained
artificially as a white precipitate, which rapidly turns blue or green
on exposure, by mixing solutions of ferrous sulphate and sodium
phosphate. It is employed in medicine. Normal ferric phosphate,
FePO4.2H2O, occurs as the mineral strengite, and is obtained as a
yellowish-white precipitate by mixing solutions of ferric chloride and
sodium phosphate. It is insoluble in dilute acetic acid, but dissolves
in mineral acids. The acid salts Fe(H2PO4)3 and 2FeH3(PO4)2.5H2O have
been described. Basic salts have been prepared, and several occur in
the mineral kingdom; dufrenite is Fe2(OH)3PO4.

_Arsenides, Arsenites, &c._--Several iron arsenides occur as minerals;
lolingite, FeAs2, forms silvery rhombic prisms; mispickel or arsenical
pyrites, Fe2AsS2, is an important commercial source of arsenic. A
basic ferric arsenite, 4Fe2O3.As2O3.5H2O, is obtained as a flocculent
brown precipitate by adding an arsenite to ferric acetate, or by
shaking freshly prepared ferric hydrate with a solution of arsenious
oxide. The last reaction is the basis of the application of ferric
hydrate as an antidote in arsenical poisoning. Normal ferric
arsenate, FeAsO4.2H2O, constitutes the mineral scorodite;
pharmacosiderite is the basic arsenate 2FeAsO4.Fe(OK)3.5H2O. An acid
arsenate, 2Fe2(HAsO4)3.9H2O, is obtained as a white precipitate by
mixing solutions of ferric chloride and ordinary sodium phosphate. It
readily dissolves in hydrochloric acid.

_Carbides, Carbonates._--The carbides of iron play an important part
in determining the properties of the different modifications of the
commercial metal, and are discussed under IRON AND STEEL.

Ferrous carbonate, FeCO3, or spathic iron ore, may be obtained as
microscopic rhombohedra by adding sodium bicarbonate to ferrous
sulphate and heating to 150 deg. for 36 hours. Ferrous sulphate and
sodium carbonate in the cold give a flocculent precipitate, at first
white but rapidly turning green owing to oxidation. A soluble
carbonate and a ferric salt give a precipitate which loses carbon
dioxide on drying. Of great interest are the carbonyl compounds.
Ferropentacarbonyl, Fe(CO)5, obtained by L. Mond, Quincke and Langer
(_Jour. Chem. Soc._, 1891; see also ibid. 1910, p. 798) by treating
iron from ferrous oxalate with carbon monoxide, and heating at 150
deg., is a pale yellow liquid which freezes at about -20 deg., and
boils at 102.5 deg. Air and moisture decompose it. The halogens give
ferrous and ferric haloids and carbon monoxide; hydrochloric and
hydrobromic acids have no action, but hydriodic decomposes it. By
exposure to sunlight, either alone or dissolved in ether or ligroin,
it gives lustrous orange plates of diferrononacarbonyl, Fe2(CO)9. If
this substance be heated in ethereal solution to 50 deg., it deposits
lustrous dark-green tablets of ferrotetracarbonyl, Fe(CO)4, very
stable at ordinary temperatures, but decomposing at 140 deg.-150 deg.
into iron and carbon monoxide (J. Dewar and H. O. Jones, _Abst.
J.C.S._, 1907, ii. 266). For the cyanides see PRUSSIC ACID.

Ferrous salts give a greenish precipitate with an alkali, whilst
ferric give a characteristic red one. Ferrous salts also give a bluish
white precipitate with ferrocyanide, which on exposure turns to a dark
blue; ferric salts are characterized by the intense purple coloration
with a thiocyanate. (See also CHEMISTRY, S _Analytical_). For the
quantitative estimation see ASSAYING.

A recent atomic weight determination by Richards and Baxter (_Zeit.
anorg. Chem._, 1900, 23, p. 245; 1904, 38, p. 232), who found the
amount of silver bromide given by ferrous bromide, gave the value
55.44 [O = 16].

_Pharmacology._

All the official salts and preparations of iron are made directly or
indirectly from the metal. The pharmacopoeial forms of iron are as
follow:--

1. _Ferrum_, annealed iron wire No. 35 or wrought iron nails free from
oxide; from which we have the preparation _Vinum ferri_, iron wine,
iron digested in sherry wine for thirty days. (Strength, 1 in 20.)

2. _Ferrum redactum_, reduced iron, a powder containing at least 75%
of metallic iron and a variable amount of oxide. A preparation of it
is _Trochiscus ferri redacti_ (strength, 1 grain of reduced iron in
each).

3. _Ferri sulphas_, ferrous sulphate, from which is prepared _Mistura
ferri composita_, "Griffiths' mixture," containing ferrous sulphate 25
gr., potassium carbonate 30 gr., myrrh 60 gr., sugar 60 gr., spirit of
nutmeg 50 m., rose water 10 fl. oz.

4. _Ferri sulphas exsiccatus_, which has two subpreparations: (a)
_Pilula ferri_, "Blaud's pill" (exsiccated ferrous sulphate 150,
exsiccated sodium carbonate 95, gum acacia 50, tragacanth 15, glycerin
10, syrup 150, water 20, each to contain about 1 grain of ferrous
carbonate); (b) _Pilula aloes et ferri_ (Barbadoes aloes 2, exsiccated
ferrous sulphate 1, compound powder of cinnamon 3, syrup of glucose
3).

5. _Ferri carbonas saccharatus_, saccharated iron carbonate. The
carbonate forms about one-third and is mixed with sugar into a greyish
powder.

6. _Ferri arsenas_, iron arsenate, ferrous and ferric arsenates with
some iron oxides, a greenish powder.

7. _Ferri phosphas_, a slate-blue powder of ferrous and ferric
phosphates with some oxide. Its preparations are: (a) _Syrupus ferri
phosphatis_ (strength, 1 gr. of ferrous phosphate in each fluid
drachm); (b) _Syrupus ferri phosphatis cum quinina et strychnina_,
"Easton's syrup" (iron wire 75 grs., concentrated phosphoric acid 10
fl. dr., powdered strychnine 5 gr., quinine sulphate 130 gr., syrup 14
fl. oz., water to make 20 fl. oz.), in which each fluid drachm
represents 1 gr. of ferrous phosphate, 4/5 gr. of quinine sulphate,
and 1/32 gr. of strychnine.

8. _Syrupus ferri iodidi_, iron wire, iodine, water and syrup
(strength, 5.5 gr. of ferrous iodide in one fl. dr.).

9. _Liquor ferri perchloridi fortis_, strong solution of ferric
chloride (strength, 22.5% of iron); its preparations only are
prescribed, viz. _Liquor ferri perchloridi_ and _Tinctura ferri
perchloridi_.

10. _Liquor ferri persulphatis_, solution of ferric sulphate.

11. _Liquor ferri pernitratus_, solution of ferric nitrate (strength,
3.3% of iron).

12. _Liquor ferri acetatis_, solution of ferric acetate.

13. The scale preparations of iron, so called because they are dried
to form scales, are three in number, the base of all being ferric
hydrate:

(a) _Ferrum tartaratum_, dark red scales, soluble in water.

(b) _Ferri et quininae citratis_, greenish yellow scales soluble in
water.

(c) _Ferri et ammonii citratis_, red scales soluble in water, from
which is prepared _Vinum ferri citratis_ (ferri et ammonii citratis 1
gr., orange wine 1 fl. dr.).

Substances containing tannic or gallic acid turn black when compounded
with a ferric salt, so it cannot be used in combination with vegetable
astringents except with the infusion of quassia or calumba. Iron may,
however, be prescribed in combination with digitalis by the addition
of dilute phosphoric acid. Alkalis and their carbonates, lime water,
carbonate of calcium, magnesia and its carbonate give green
precipitates with ferrous and brown with ferric salts.

Unofficial preparations of iron are numberless, and some of them are
very useful. Ferri hydroxidum (U.S.P.), the hydrated oxide of iron,
made by precipitating ferric sulphate with ammonia, is used solely as
an antidote in arsenical poisoning. The Syrupus ferri phosphatis Co.
is well known as "Parrish's" syrup or chemical food, and the Pilulae
ferri phosphatis cum quinina et strychnina, known as Easton's pills,
form a solid equivalent to Easton's syrup.

There are numerous organic preparations of iron. Ferratin is a reddish
brown substance which claims to be identical with the iron substance
found in pig's liver. Carniferrin is another tasteless powder
containing iron in combination with the phosphocarnic acid of muscle
preparations, and contains 35% of iron. Ferratogen is prepared from
ferric nuclein. Triferrin is a paranucleinate of iron, and contains
22% of iron and 2(1/2)% of organically combined phosphorus, prepared
from the casein of cow's milk. Haemoglobin is extracted from the blood
of an ox and may be administered in bolus form. Dieterich's solution
of peptonated iron contains about 2 gr. of iron per oz. Vachetta has
used the albuminate of iron with striking success in grave cases of
anaemia. Succinate of iron has been prepared by Hausmann. Haematogen,
introduced by Hommel, claims to contain the albuminous constituents of
the blood serum and all the blood salts as well as pure haemoglobin.
Sicco, the name given to dry haematogen, is a tasteless powder.
Haemalbumen, introduced by Dahmen, is soluble in warm water.

_Therapeutics._

Iron is a metal which is used both as a food and as a medicine and has
also a definite local action. Externally, it is not absorbed by the
unbroken skin, but when applied to the broken skin, sores, ulcers and
mucous surfaces, the ferric salts are powerful astringents, because
they coagulate the albuminous fluids in the tissues themselves. The
salts of iron quickly cause coagulation of the blood, and the clot
plugs the bleeding vessels. They thus act locally as haemostatics or
styptics, and will often arrest severe haemorrhage from parts which
are accessible, such as the nose. They were formerly used in the
treatment of _post partum_ haemorrhage. The perchloride, sulphate and
pernitrate are strongly astringent; less extensively they are used in
chronic discharges from the vagina, rectum and nose, while injected
into the rectum they destroy worms.

Internally, a large proportion of the various articles of ordinary
diet contains iron. When given medicinally preparations of iron have
an astringent taste, and the teeth and tongue are blackened owing to
the formation of sulphide of iron. It is therefore advisable to take
liquid iron preparations through a glass tube or a quill.

In the stomach all salts of iron, whatever their nature, are converted
into ferric chloride. If iron be given in excess, or if the
hydrochloric acid in the gastric juice be deficient, iron acts
directly as an astringent upon the mucous membrane of the stomach
wall. Iron, therefore, may disorder the digestion even in healthy
subjects. Acid preparations are more likely to do this, and the acid
set free after the formation of the chloride may act as an irritant.
Iron, therefore, must not be given to subjects in whom the gastric
functions are disturbed, and it should always be given after meals.
Preparations which are not acid, or are only slightly acid, such as
reduced iron, dialysed iron, the carbonate and scale preparations, do
not disturb the digestion. If the sulphate is prescribed in the form
of a pill, it may be so coated as only to be soluble in the intestinal
digestive fluid. In the intestine the ferric chloride becomes changed
into an oxide of iron; the sub-chloride is converted into a ferrous
carbonate, which is soluble. Lower down in the bowel these compounds
are converted into ferrous sulphide and tannate, and are eliminated
with the faeces, turning them black. Iron in the intestine causes an
astringent or constipating effect. The astringent salts are therefore
useful occasionally to check diarrhoea and dysentery. Thus most salts
of iron are distinctly constipating, and are best used in combination
with a purgative. The pill of iron and aloes (B.P.) is designed for
this purpose. Iron is certainly absorbed from the intestinal canal. As
the iron in the food supplies all the iron in the body of a healthy
person, there is no doubt that it is absorbed in the organic form.
Whether inorganic salts are directly absorbed has been a matter of
much discussion; it has, however, been directly proved by the
experiments of Kunkel (_Archiv fur die gesamte Physiologie des
Menschen und der Tiere_, lxi.) and Gaule. The amount of iron existing
in the human blood is only 38 gr.; therefore, when an excess of iron
is absorbed, part is excreted immediately by the bowel and kidneys,
and part is stored in the liver and spleen.

Iron being a constituent part of the blood itself, there is a direct
indication for the physician to prescribe it when the amount of
haemoglobin in the blood is lowered or the red corpuscles are
diminished. In certain forms of anaemia the administration of iron
rapidly improves the blood in both respects. The exact method in which
the prescribed iron acts is still a matter of dispute. Ralph Stockman
points out that there are three chief theories as to the action of
iron in anaemia. The first is based on the fact that the iron in the
haemoglobin of the blood must be derived from the food, therefore iron
medicinally administered is absorbed. The second theory is that there
is no absorption of iron given by the mouth, but it acts as a local
stimulant to the mucous membrane, and so improves anaemia by
increasing the digestion of the food. The third theory is that of
Bunge, who says that in chlorotic conditions there is an excess of
sulphuretted hydrogen in the bowel, changing the food iron into
sulphide of iron, which Bunge states cannot be absorbed. He believes
that inorganic iron saves the organic iron of the food by combining
with the sulphur, and improves anaemia by protecting the organic food
iron. Stockman's own experiments are, however, directly opposed to
Bunge's view. Wharfinger states that in chlorosis the specific action
of iron is only obtained by administering those inorganic preparations
which give a reaction with the ordinary reagents; the iron ions in a
state of dissociation act as a catalytic agent, destroying the
hypothetical toxin which is the cause of chlorosis. Practical
experience teaches every clinician that, whatever the mode of action,
iron is most valuable in anaemia, though in many cases, where there is
well-marked toxaemia from absorption of the intestinal products, not
only laxatives in combination with iron but intestinal antiseptics are
necessary. That form of neuralgia which is associated with anaemia
usually yields to iron.

FOOTNOTE:

[1] By solution in concentrated hydrochloric acid, a yellow liquid is
obtained, which on concentration over sulphuric acid gives yellow
deliquescent crusts of ferroso-ferric chloride, Fe3Cl8.18H2O.

IRON AGE, the third of the three periods, Stone, Bronze and Iron Ages, into which archaeologists divide prehistoric time; the weapons, utensils and implements being as a general rule made of iron (see ARCHAEOLOGY). The term has no real chronological value, for there has been no universal synchronous sequence of the three epochs in all quarters of the world. Some countries, such as the islands of the South Pacific, the interior of Africa, and parts of North and South America, have passed direct from the Stone to the Iron Age. In Europe the Iron Age may be said to cover the last years of the prehistoric and the early years of the historic periods. In Egypt, Chaldaea, Assyria, China, it reaches far back, to perhaps 4000 years before the Christian era. In Africa, where there has been no Bronze Age, the use of iron succeeded immediately the use of stone. In the Black Pyramid of Abusir (VIth Dynasty), at least 3000 B.C., Gaston Maspero found some pieces of iron, and in the funeral text of Pepi I. (about 3400 B.C.) the metal is mentioned. The use of iron in northern Europe would seem to have been fairly general long before the invasion of Caesar. But iron was not in common use in Denmark until the end of the 1st century A.D. In the north of Russia and Siberia its introduction was even as late as A.D. 800, while Ireland enters upon her Iron Age about the beginning of the 1st century. In Gaul, on the other hand, the Iron Age dates back some 800 years B.C.; while in Etruria the metal was known some six centuries earlier. Homer represents Greece as beginning her Iron Age twelve hundred years before our era. The knowledge of iron spread from the south to the north of Europe. In approaching the East from the north of Siberia or from the south of Greece and the Troad, the history of iron in each country eastward is relatively later; while a review of European countries from the north towards the south shows the latter becoming acquainted with the metal earlier than the former It is suggested that these facts support the theory that it is from Africa that iron first came into use. The finding of worked iron in the Great Pyramids seems to corroborate this view. The metal, however, is singularly scarce in collections of Egyptian antiquities. The explanation of this would seem to lie in the fact that the relics are in most cases the paraphernalia of tombs, the funereal vessels and vases, and iron being considered an impure metal by the ancient Egyptians it was never used in their manufacture of these or for any religious purposes. This idea of impurity would seem a further proof of the African origin of iron. It was attributed to Seth, the spirit of evil who according to Egyptian tradition governed the central deserts of Africa. The Iron Age in Europe is characterized by an elaboration of designs in weapons, implements and utensils. These are no longer cast but hammered into shape, and decoration is elaborate curvilinear rather than simple rectilinear, the forms and character of the ornamentation of the northern European weapons resembling in some respects Roman arms, while in others they are peculiar and evidently representative of northern art. The dead were buried in an extended position, while in the preceding Bronze Age cremation had been the rule.

See Lord Avebury, _Prehistoric Times_ (1865; 1900); Sir J. Evans,
_Ancient Stone Implements_ (1897); _Horae Ferales, or Studies in the
Archaeology of Northern Nations_, by Kemble (1863); Gaston C. C.
Maspero, _Guide du Musee de Boulaq_, 296; _Scotland in Pagan
Times--The Iron Age_, by Joseph Anderson (1883).

IRON AND STEEL.[1] 1. Iron, the most abundant and the cheapest of the heavy metals, the strongest and most magnetic of known substances, is perhaps also the most indispensable of all save the air we breathe and the water we drink. For one kind of meat we could substitute another; wool could be replaced by cotton, silk or fur; were our common silicate glass gone, we could probably perfect and cheapen some other of the transparent solids; but even if the earth could be made to yield any substitute for the forty or fifty million tons of iron which we use each year for rails, wire, machinery, and structural purposes of many kinds, we could not replace either the steel of our cutting tools or the iron of our magnets, the basis of all commercial electricity. This usefulness iron owes in part, indeed, to its abundance, through which it has led us in the last few thousands of years to adapt our ways to its properties; but still in chief part first to the single qualities in which it excels, such as its strength, its magnetism, and the property which it alone has of being made at will extremely hard by sudden cooling and soft and extremely pliable by slow cooling; second, to the special combinations of useful properties in which it excels, such as its strength with its ready welding and shaping both hot and cold; and third, to the great variety of its properties. It is a very Proteus. It is extremely hard in our files and razors, and extremely soft in our horse-shoe nails, which in some countries the smith rejects unless he can bend them on his forehead; with iron we cut and shape iron. It is extremely magnetic and almost non-magnetic; as brittle as glass and almost as pliable and ductile as copper; extremely springy, and springless and dead; wonderfully strong, and very weak; conducting heat and electricity easily, and again offering great resistance to their passage; here welding readily, there incapable of welding; here very infusible, there melting with relative ease. The coincidence that so indispensable a thing should also be so abundant, that an iron-needing man should be set on an iron-cored globe, certainly suggests design. The indispensableness of such abundant things as air, water and light is readily explained by saying that their very abundance has evolved a creature dependent on them. But the indispensable qualities of iron did not shape man's evolution, because its great usefulness did not arise until historic times, or even, as in case of magnetism, until modern times.

These variations in the properties of iron are brought about in part by corresponding variations in mechanical and thermal treatment, by which it is influenced profoundly, and in part by variations in the proportions of certain foreign elements which it contains; for, unlike most of the other metals, it is never used in the pure state. Indeed pure iron is a rare curiosity. Foremost among these elements is carbon, which iron inevitably absorbs from the fuel used in extracting it from its ores. So strong is the effect of carbon that the use to which the metal is put, and indeed its division into its two great classes, the malleable one, comprising steel and wrought iron, with less than 2.20% of carbon, and the unmalleable one, cast iron, with more than this quantity, are based on carbon-content. (See Table I.)

TABLE 1.--_General Classification of Iron and Steel according (1) to Carbon-Content and (2) to Presence or Absence of Inclosed Slag._

+---------------------+------------------------+----------------------+-----------------------+
| | Containing very little | Containing an Inter- |Containing much Carbon |
| | Carbon (say, less than | mediate Quantity of | Carbon (say, from 2.2 |
| | 0.30%). | Carbon (say, between | 2.2 to 5%). |
| | | 0.30 and 2.2%). | |
+---------------------+------------------------+----------------------+-----------------------|
| Slag-bearing or | WROUGHT IRON. | WELD STEEL. | |
| "Weld-metal" Series.| Puddled and bloomary, | Puddled and blister | |
| | or Charcoal-hearth | steel belong here. | |
| | iron belong here. | | |
+---------------------+------------------------+----------------------+-----------------------+
| | LOW-CARBON or MILD | HALF-HARD and HIGH- | CAST IRON. |
| |STEEL, sometimes called | CARBON STEELS, some- | |
| | "ingot-iron." | times called "ingot- | |
| | | steel." | |
| | It may be either | They may be either | Normal cast iron, |
| | Bessemer, open-hearth, | Bessemer, open- | "washed" metal, and |
| Slagless or "Ingot- | or crucible steel. | hearth, or crucible | and most "malleable |
| Metal" Series. | | steel. Malleable | cast iron" belong |
| | | cast iron also often| here. |
| | | belongs here. | |
| +------------------------+----------------------+-----------------------+
| | | ALLOY STEELS. | ALLOY CAST IRONS.* |
| | | Nickel, manganese, | Spiegeleisen, ferro- |
| | | tungsten, and chrome | manganese, and silico-|
| | | steels belong here. | spiegel belong here. |
+---------------------+------------------------+------------------ ---+-----------------------+
* The term "Alloy Cast Irons" is not actually in frequent use, not
because of any question as to its fitness or meaning, but because
the need of such a generic term rarely arises in the industry.

2. _Nomenclature._--Until about 1860 there were only three important classes of iron--wrought iron, steel and cast iron. The essential characteristic of wrought iron was its nearly complete freedom from carbon; that of steel was its moderate carbon-content (say between 0.30 and 2.2%), which, though great enough to confer the property of being rendered intensely hard and brittle by sudden cooling, yet was not so great but that the metal was malleable when cooled slowly; while that of cast iron was that it contained so much carbon as to be very brittle whether cooled quickly or slowly. This classification was based on carbon-content, or on the properties which it gave. Beyond this, wrought iron, and certain classes of steel which then were important, necessarily contained much slag or "cinder," because they were made by welding together pasty particles of metal in a bath of slag, without subsequent fusion. But the best class of steel, crucible steel, was freed from slag by fusion in crucibles; hence its name, "cast steel." Between 1860 and 1870 the invention of the Bessemer and open-hearth processes introduced a new class of iron to-day called "mild" or "low-carbon steel," which lacked the essential property of steel, the hardening power, yet differed from the existing forms of wrought iron in freedom from slag, and from cast iron in being very malleable. Logically it was wrought iron, the essence of which was that it was (1) "iron" as distinguished from steel, and (2) malleable, i.e. capable of being "wrought." This name did not please those interested in the new product, because existing wrought iron was a low-priced material. Instead of inventing a wholly new name for the wholly new product, they appropriated the name "steel," because this was associated in the public mind with superiority. This they did with the excuse that the new product resembled one class of steel--cast steel--in being free from slag; and, after a period of protest, all acquiesced in calling it "steel," which is now its firmly established name. The old varieties of wrought iron, steel and cast iron preserve their old names; the new class is called steel by main force. As a result, certain varieties, such as blister steel, are called "steel" solely because they have the hardening power, and others, such as low-carbon steel, solely because they are free from slag. But the former lack the essential quality, slaglessness, which makes the latter steel, and the latter lack the essential quality, the hardening power, which makes the former steel. "Steel" has come gradually to stand rather for excellence than for any specific quality. These anomalies, however confusing to the general reader, in fact cause no appreciable trouble to important makers or users of iron and steel, beyond forming an occasional side-issue in litigation.

3. _Definitions._--_Wrought iron_ is slag-bearing malleable iron, containing so little carbon (0.30% or less), or its equivalent, that it does not harden greatly when cooled suddenly.

_Steel_ is iron which is malleable at least in some one range of temperature, and also is either (a) cast into an initially malleable mass, or (b) is capable of hardening greatly by sudden cooling, or (c) is both so cast and so capable of hardening. (Tungsten steel and certain classes of manganese steel are malleable only when red-hot.) Normal or carbon steel contains between 0.30 and 2.20% of carbon, enough to make it harden greatly when cooled suddenly, but not enough to prevent it from being usefully malleable when hot.

_Cast iron_ is, generically, iron containing so much carbon (2.20% or more) or its equivalent that it is not usefully malleable at any temperature. Specifically, it is cast iron in the form of castings other than pigs, or remelted cast iron suitable for such castings, as distinguished from pig iron, i.e. the molten cast iron as it issues from the blast furnace, or the pigs into which it is cast.

_Malleable cast iron_ is iron which has been cast in the condition of cast iron, and made malleable by subsequent treatment without fusion.

_Alloy steels_ and _cast irons_ are those which owe their properties chiefly to the presence of one or more elements other than carbon.

_Ingot iron_ is slagless steel with less than 0.30% of carbon.

_Ingot steel_ is slagless steel containing more than 0.30% of carbon.

_Weld steel_ is slag-bearing iron malleable at least at some one temperature, and containing more than 0.30% of carbon.