SECTION IV.. _of_ Iron.

Of IRON.

Iron is lighter and less ductile than Copper; but it is much harder, and of more difficult fusion.

It is the only body that has the property of being attracted by the magnet, which therefore serves to discover it wherever it is. But it must be observed, that it hath this property only when in its metalline state, and loses it when converted to an earth or calx. Hence very few Iron-ores are attracted by the load-stone: because, for the most part, they are only sorts of earths, which require a phlogiston to be added before they can be brought to the form of true Iron.

When Iron hath undergone no other preparation but the fusion which is necessary to smelt it from its ore, it is usually quite brittle, and flies to pieces under the hammer: which arises in some measure from its containing a certain portion of unmetallic earth interposed between its parts. This we call Pig Iron.

By melting this a second time it is rendered purer, and more free from heterogeneous matters: but still, as its proper parts are probably not brought sufficiently near, or closely enough united, till the Iron hath undergone some further preparation besides that of fusion, it seldom hath any degree of malleability.

The way to give it this property is to make it just red-hot, and then hammer it for some time in all directions; to the end that its parts may be properly united, incorporated, and welded together, and that the heterogeneous matters which keep them asunder may be separated. Iron made by this means as malleable as possible we call Bar Iron, or Forged Iron.

Bar Iron is still harder to fuse than Pig Iron: to make it flow requires the utmost force of fire.

Iron has the property of imbibing a greater quantity of phlogiston than is necessary to give it the metalline form. It may be made to take in this super-abundant phlogiston two ways: the first is by fusing it again with matters that contain the phlogiston; the second is, by encompassing it with a quantity of such matters, charcoal-dust, for instance, and then exposing it so encompassed, for a certain time, to a degree of fire barely sufficient to keep it red-hot. This second method, whereby one substance is incorporated with another by means of fire, but without fusing either of them, is in general called Cementation.

Iron thus impregnated with an additional quantity of phlogiston is called Steel. The hardness of Steel may be considerably augmented by tempering it; that is, by making it red-hot, and suddenly quenching it in some cold liquor. The hotter the metal, and the colder the liquor in which it is quenched, the harder will the Steel be. By this means tools are made, such as files and sheers, capable of cutting and dividing the hardest bodies, as glass, pebbles, and Iron itself. The colour of Steel is darker than that of Iron, and the facets which appear on breaking it are smaller. It is also less ductile and more brittle, especially when tempered.

As Iron may be impregnated with an additional quantity of phlogiston, and thereby converted into Steel, so may Steel be again deprived of that super-abundant phlogiston, and brought back to the condition of Iron. This is effected by cementing it with poor earths, such as calcined bones and chalk. By the same operation Steel may be untempered; nay, it will lose the hardness it had acquired by tempering, if it be but made red-hot, and left to cool gradually. As Iron and Steel differ only in the respects we have here taken notice of, their properties being in all other respects the same, what follows is equally applicable to both.

Iron being exposed to the action of fire for some time, especially when divided into small particles, such as filings, is calcined and loses its phlogiston. By this means it turns to a kind of reddish yellow earth, which, on account of its colour, is called Crocus Martis, or Saffron of Mars.

This calx of Iron has the singular property of flowing in the fire with somewhat less difficulty than Iron itself; whereas every other metalline calx flows with less ease than the metal that produced it. It has moreover the remarkable property of uniting with the phlogiston, and of being reduced to Iron without fusion; requiring for that purpose only to be made red-hot.

Iron may be incorporated with Silver, and even with Gold, by means of certain operations. Under the article of Lead we shall see how it may be separated from these metals.

The acids produce on it much the same effects as on Copper; every one of them acts upon it. Certain neutral salts, alkalis, and even water itself, are capable of dissolving it; and hence it is also very subject to rust. The vitriolic acid dissolves it with the greater ease: but the circumstances which attend the solution thereof are different from those with which the same Acid dissolves Copper: for, 1. whereas the vitriolic acid must be concentrated to dissolve Copper, it must on the contrary be diluted with water to dissolve Iron, which it will not touch when well dephlegmated. 2. The vapours which rise in this dissolution are inflammable; so that if it be made in a small-necked bottle, and the flame of a candle be applied to the mouth thereof, the vapours in the bottle take fire with such rapidity as to produce a considerable explosion.

This solution is of a beautiful green colour; and from this union of the vitriolic acid with Iron there results a neutral metalline salt, which has the property of shooting into crystals of a rhomboidal figure, and a green colour. These crystals are called Green Vitriol, Vitriol of Mars, and Copperas.

Green Vitriol hath a saltish and astringent taste. As it retains a great deal of water in crystallizing, it quickly flows by the action of fire: but this fluidity is owing to its water only, and is not a real fusion; for, as soon as its moisture is evaporated, it resumes a solid form. Its green transparent colour is now changed into an opaque white: and, if the calcination be continued, its acid also exhales and is dissipated in vapours; and as it loses that, it turns gradually to a yellow colour, which comes so much the nearer to a red the longer the calcination is continued, or the higher the force of the fire is raised; which being driven to the utmost, what remains is of a very deep red. This remainder is nothing but the body of the Iron, which having lost its phlogiston is now no more than an earth, nearly of the same nature with that which is left after calcining the metal itself.

Green Vitriol dissolved in water spontaneously lets fall a yellowish earthy sediment. If this solution be defecated by filtration, it still continues to deposite some of the same substance, till the vitriol be wholly decomposed. This sediment is nothing but the earth of Iron, which is then called Ochre.

The nitrous acid dissolves Iron with great ease. This solution is of a yellow colour, inclining more or less to a russet, or dark-brown, as it is more or less saturated with Iron. Iron dissolved by this acid, also, falls spontaneously in a kind of calx, which is incapable of being dissolved a second time; for the nitrous acid will not act upon Iron that has lost its phlogiston. This solution does not crystallize, and if evaporated to dryness attracts the moisture of the air.

Spirit of salt likewise dissolves Iron, and this solution is green. The vapours which rise during the dissolution are inflammable, like those which ascend when this metal is attacked by the vitriolic acid. Aqua regis makes a solution of Iron, which is of a yellow colour.

Iron hath a greater affinity than either Silver or Copper with the nitrous and vitriolic acids: so that if iron be presented to a solution of either in one of these two acids, the dissolved metal will be precipitated; because the acid quits it for the Iron, with which it has a greater affinity.

On this occasion it must be observed, that if a solution of Copper in the vitriolic acid be precipitated by means of Iron, the precipitate has the form and splendour of a metal, and does not require the addition of a phlogiston to reduce it to true Copper; which is not the case, as has been shewn, when the precipitation is effected by earths or alkaline salts.

The colour of this metalline precipitate hath deceived several persons, who being unacquainted with such phenomena, and with the nature of blue vitriol, imagined that Iron was transmuted into Copper, when they saw a bit of Iron laid in a solution of that vitriol become, in form and external appearance, exactly like Copper: whereas the surface only of the Iron was crusted over with the particles of Copper contained in the vitriol, which had gradually fallen upon and adhered to the Iron, as they were precipitated out of the solution.

Among the solvents of Iron we mentioned fixed alkalis; and that they have such a power is proved by the following phenomenon. If a large proportion of alkaline salts be suddenly mixed with a solution of Iron in an acid, no precipitation ensues, and the liquor remains clear and pellucid; or if at first it look a little turbid, that appearance lasts but a moment, and the liquor presently recovers its transparency. The reason is, that the quantity of alkali is more than sufficient to saturate all the acid of the solution, and the super-abundant portion thereof, meeting with the Iron already finely divided by the acid, dissolves it with ease as fast as it falls, and so prevents its muddying the liquor. To evince that this is so in fact, let the alkali be applied in a quantity that is not sufficient, or but barely sufficient, to saturate the acid, and the Iron will then precipitate like any other metal.

Water also acts upon Iron; and therefore Iron exposed to moisture grows rusty. If iron-filings be exposed to the dew, they turn wholly to a rust, which is called Crocus Martis Aperiens.

Iron exposed to the fire together with nitre makes it detonate pretty briskly, sets it in a flame, and decomposes it with rapidity.

This metal hath a greater affinity than any other metalline substance with sulphur; on which account it is successfully used to precipitate and separate all metalline substances combined with sulphur.

Sulphur uniting with Iron communicates to it such a degree of fusibility, that if a mass of this metal heated red-hot be rubbed with a bit of sulphur, it incessantly runs into as perfect a fusion as a metal exposed to the focus of a large burning-glass.