Prop., &c. The properties and uses of iron are too well known to require description. Its applications are almost universal. It is remarkably ductile, and possesses great tenacity, but is less malleable than many of the other metals. Its sp. gr. is 7·844. It is the hardest of all the malleable and ductile metals, and when combined with carbon (steel) admits of being tempered to almost any degree of hardness or elasticity. In dry air it does not oxidise at common temperatures; but at a red heat it soon becomes covered with a scaly coating of black oxide, and at an intense white heat burns brilliantly with the production of the same substance. Pure water, free from air or carbonic acid, does not tarnish the surface of polished iron, but the combined action of air and moisture, especially when a little acid vapour is present, causes its surface to be soon covered with rust, which is hydrated sesquioxide of iron (ferric hydrate). Nearly all acids attack iron; dilute sulphuric and hydrochloric acid do so with considerable energy and the evolution of hydrogen gas. At a red heat iron decomposes water rapidly, hydrogen being evolved, and the black oxide of iron formed. Iron is magnetic up to a dull-red heat, at which point it loses all traces of that property. It melts at about 3300° Fahr. With oxygen, chlorine, iodine, the acids, &c., it forms numerous important compounds. As a remedial agent, when properly exhibited, iron acts as a genial stimulant and tonic, and generally proves beneficial in cases of chronic debility, unaccompanied with organic congestion or inflammation. The carbonate (ferrous carbonate), as it exists in mineral waters, held in solution by carbonic acid in excess, appears to be the form most congenial to the human body; and from its state of dilution is rapidly absorbed by the lacteals, and speedily imparts a ruddy hue to the wan countenance. Iron is undoubtedly one of the most valuable articles of the materia medica, and appears from the antiquity of its introduction into medicine, and the number of its preparations, to have been deservedly appreciated.
Tests. Iron forms two classes of salts, namely, ferrous or proto salts, in which iron exhibits a power of combining with two atoms of any monad element, and the ferric or persalts in which iron has a capacity of uniting with three atoms of any monad element.
The ferrous or proto salts have generally a greenish colour, but yield nearly colourless
solutions, except when concentrated. Their solutions are known by the following reactions:—They are not precipitated by hydrosulphuric acid when acid, and but incompletely when neutral. Sulphide of ammonium produces a black precipitate, becoming brown on exposure to the air, insoluble in alkalies, but easily soluble in the mineral acids. Ammonia and potassa give a greenish-white precipitate, gradually becoming green and then brown in the air. This precipitate occasionally is of a bluish-black if excess of potassa is used. The presence of ammoniacal salts interferes with the action of these tests. Ferrocyanide of potassium gives a nearly white precipitate, becoming gradually blue in the air, and immediately so on the addition of a little weak nitric acid or chlorine water. Ferricyanide of potassium produces a rich deep-blue precipitate, insoluble in hydrochloric acid. In highly dilute solutions the effect is only a deep bluish-green coloration. Phosphate of sodium produces a white precipitate, which after a time becomes green.
The ferric salts, which are also called the sesqui or persalts of iron, have for the most part a reddish-yellow colour, yielding deep-coloured solutions, which exhibit the following reactions:—They redden litmus paper. Hydrosulphuric acid in acid solutions reduces ferric to ferrous salts, giving a white or yellow precipitate of sulphur only. In alkaline solutions it yields a blackish precipitate, consisting of sulphur and ferrous sulphide. Sulphide of ammonium gives similar reaction. Ammonia and potassa produce bulky reddish-brown precipitates insoluble in excess. Ferrocyanide of potassium gives a rich blue precipitate, insoluble in hydrochloric acid, and readily decomposed by potassa. Ferricyanide of potassium deepens the colour, but does not give a blue precipitate, as it does with ferrous salts (proto salts). Sulphocyanide of potassium gives an intense ruby-red colour to neutral or acid solutions. Tincture and infusion of galls strike a black colour. Phosphate of sodium gives a white precipitate, which becomes brown, and finally dissolves on the addition of ammonia.
Estim. The iron may be thrown down in the state of ferric hydrate or hydrated sesquioxide, washed, dried, ignited, and weighed. The weight, in grains, multiplied by ·7, indicates the weight of metallic iron.
Fifty gr. of the ore are reduced to powder, dissolved in aqua regia, with the aid of heat, and the solution filtered in order to separate the silica and a little alumina which sometimes is left in an insoluble state; an excess of ammonia is then added to the filtered liquor, which produces a reddish-brown precipitate of ferric hydrate mixed with alumina which is collected on a filter, washed, and boiled with a solution of potassa, in order to dissolve the alumina; the whole is next thrown upon a filter, washed, dried, carefully ignited, and
weighed. The above is well adapted to determine the quantity of iron in clay ironstone, the most common ferruginous ore in England.
The sulphur in cast and wrought iron, and steel, may be estimated by the following process, invented by M. Koppmayer:—10 grammes of iron, finely produced and sifted, are introduced into a bottle holding from 1⁄2 to 1⁄3rd litre. The stopper has three holes. Through one of these passes a funnel with a ground-glass tap, its neck reaching to the bottom of the bottle. Through the second passes the tube at right angles, fitted with a tap and reaching also to the bottom of the bottle.
Through the third hole passes a delivery tube, connecting the bottle to the condensing apparatus.