The property of hardening is not possessed by pure malleable iron; but I have now to explain a rapid and partial process of cementation, by which wrought-iron is first converted exteriorly into steel, and is subsequently hardened to that particular depth; leaving the central parts in their original condition of soft fibrous iron. The process is very consistently called case-hardening, and is of great importance in the mechanical arts, as the pieces combine the economy, strength, and internal flexibility of iron, with a thin casing of steel; which although admirable as an armour , of defence from wear or deterioration as regards the surface, is unfit for the formation of cutting edges or tools, owing to the entire absence of hammering, subsequent to the cementation with the carbon. Cast-iron obtains in like manner a coating of steel, which surrounds the peculiar shape the metal may have. assumed in the iron-foundry and workshop.

The principal agents used for case-hardening are animal matters, as the hoofs, horns, bones, and skins of animals; these are nearly alike in chemical constitution; they are mostly charred and coarsely pounded: some persons also mix a little common salt with some of the above; the works should be surrounded on all sides with a layer from half an inch to one inch thick.

The methods pursued by different individuals do not greatly differ; for example, the gunsmith inserts the iron work of the gun-lock, in a sheet-iron case in the midst of partially charred hone-dust; the lid of the box is tied on with iron wire, and the joint is luted with clay; it is then slowly heated to redness, and retained at that heat from half an hour to an hour, and the contents are immersed in cold water as quickly as possible, to prevent the access of air. The objects sought are a steely exterior, and a clean surface covered with the pretty mottled tints, apparently caused by oxidation from the partial admission of air.

Some of the malleable iron castings, such as snuffers, are case-hardened to admit of a better polish; it is usually done with burnt bone-dust, and at a dull red heat; they remain in the fire about two or three hours, and should be immersed in oil, as it does not render them quite so brittle as when plunged into water. It must be remembered they are sometimes changed throughout their substance into an inferior kind of steel, by a process that should in such instances be called cementation, and not case-hardening, consequently they will not endure violence.

The mechanician and engineer, use horns, hoofs, bone-dust, and leather, and allow the period to extend from two to eight hours, most generally four or five; sometimes for its greater penetration, the process is repeated a second time with new carbonaceous materials. Some open the box and immerse the work in water direct from the furnace; others, with the view to presserve a better surface, allow the box to cool without being opened, and harden the pieces with the open fire as a subsequent operation; the carbon once added, the work may be annealed and hardened much the same as ordinary steel.

When the case-hardening is required to terminate at any particular part, as a shoulder, the object is left with a band or projection; the work is allowed to cool without being immersed in water, the band is turned off, and the work when hardened in the open fire is only affected so far as the original cemented surface remains. This ingenious method was introduced by Mr. Roberts, of Manchester, who considers the success of the case-hardening process to depend on the gentle application of the heat; and that, by proper management not to overheat the work, it may be made to penetrate three-eighths of an inch in four or five hours.

A new substance for the case-hardening process, but containing the same elements as those more commonly employed, has of late years been added, namely, the prussiate of potash, (a salt consisting of two atoms of carbon and one of nitrogen,) which is made from a variety of animal matters.

It is a new application without any change of principle; the time occupied in this steelifying process, is sometimes only minutes instead of hours and days, as for example when iron is heated in the open fire to a dull red, and the prussiate is either sprinkled upon it or rubbed on in the lump, it is returned to the fire for a few minutes and immersed in water; but the process is then exceedingly superficial, and it may if needful be limited to any particular part upon which alone the prussiate is applied. The effect by many is thought to be partial or in spots, as if the salt refused to act uniformly; in the same manner that water only moistens a greasy surface in places.

The prussiate of potash has been used for case-hardening the bearings of wrought-iron shafts, but this seems scarcely worth the doing: it has been also employed with the view of giving an additional and extreme, although superficial hardness to steel, as in Jones's axletrees, Perkins's engraved steel plates, etc.; but I have only heard of one individual who has encased work with this salt; it was for case-hardening the iron rollers and side plates of glaziers' vices employed for milling window-lead.

In the general way, the conversion of the iron into steel, by case-hardening, is quite superficial, and does not exceed the sixteenth of an inch; if made to extend to one quarter or three-eighths of an inch in depth, to say the least it would be generally useless, as the object is to obtain durability of surface, with strength of interior, and this would disproportionately encroach on the strong iron within. The steel obtained in this adventitious manner is not equal in strength to that converted and hammered in the usual way, and if sent in so deeply, the provision for wear would far exceed that which is required.

Let us compare the case-hardening process with the usual conversion of steel. The latter requires a period of about seven days, and a very pure carbon, namely wood charcoal, of which a minute portion only is absorbed; and it being a simple body, when the access of air is prevented by the proper security of the troughs, the bulk of the charcoal remains uuconsumed, and is reserved for future use, as it has undergone no change. The hasty and partial process of cementaations is produced in a period commonly less than as many hours with the animal charcoal, or than as many minutes with the prussiate of potash; but all these are compound bodies, (which contain cyanogen, a body insisting of carbon and nitrogen,) and are never used a second time, but on the contrary the process is often repeated with another dose. It would be therefore, an interesting inquiry for the chemist, as to whether the cyanogen is absorbed after the same manner as carbon in ordinary steel, (and which in Mackintosh's patent process was driven through the crucible in the form of carbonic acid gas, and is stated to be absorbed at the rate of one-thirtieth of an inch in depth, each hour;) or whether the nitrogen assists in any way in hastening the admission of the carbon, by some as yet untraced affinity or decomposition.*

This hasty supposition will apply less easily to cast-iron, which contains from three to seven times as much carbon as steel, and although not always hardened by simple immersion, is con-stantly under the influence of the case-hardening process; unless we adopt the supposition, that the carbon in cast-iron which is mixed with the metal in the shape of cinder in the blast furnace, when all is in a fluid state, is in a less refined union than that instilled in a more aeriform condition in the acts of comentation and case-hardening.†

* It may happen that the carbon is not essential, as the Indian steel or wootz is stated to contain alumine and silex; and manganese is used in Heath's and

Vicker's patents.

† It would hare been an easy task to hare multiplied the examples and remarks upon these curious subjects of hardening and tempering, which have already for, very far, exceeded the intended limits. The reader will find much useful matter upon the same in Lardner's Cyclopedia in the volume on Iron and Steel, and in Gill's