employed for heating steel furnaces. In 1832 the waste gases were used for heating the blast at Wasseralfingen, in Bavaria, and a similar apparatus was first erected in this country in 1848 by J. P. Budd, at Ystalyfera, in Glamorganshire, since which time various modifications of the same plan have been adopted to a considerable extent, especially in those furnaces that are obliged to draw their fuel from a distance, but in other districts, as for example in South Staffordshire and Scotland, the old flaming throats still prevail.

Within the last few years, the chief inventions and improvements have been in steel manufacture, and many new processes have been introduced. Prominent among these is that named after its inventor, Henry Bessemer, which, although only of a few years' standing, has already effected important services by the produc tion of a material admirably adapted for use in railway and other engineering work in place of wrought iron. Perhaps the problem of most immediate interest at present is that of the economical substitution of mechanical for manual power in the process of puddling, so as to enable the forge-master to manipulate larger masses of malleable iron at a time, and thus to put him more nearly on an equality with the cast-steel maker than is the case at present.

With the exception of the Weald of Sussex, very little change has taken place in the position of our principal iron-working centres from the earliest time down to the present day. Since the great expansion of railways several new and important localities have been brought into work, the ores being carried to the fuel or the reverse, according as might be most advantageous. In this way the great northern coal field of England, which

is almost absolutely without ironstone, gives rise to the largest production in the kingdom by feeding the Cleveland district with coal and coke, and drawing ironstone for its own furnaces in return. The prevalence of cheap ores in the oolitic districts has brought the blast furnace to within fifty miles of London in Northamptonshire, and the pastoral districts of Wiltshire have been invaded by the same visitor. It need not, therefore, be a matter of much surprise if at some future period the Wealden furnaces were to be re-lighted, as they could be easily supplied with fuel from the western coal fields should the supply of ore be sufficient to warrant the attempt, especially as on the opposite coast of France large furnaces have been established for smelting ores out of the same formation, and which are supplied with fuel from England.

CHAPTER II

OUTLINE OF THE CHEMISTRY OF IRON.

THE chief chemical points involved in the metallurgy of iron will next be briefly noticed under this head, in the order adopted equally by Karsten in his classical Eisenhüttenkunde," and also by Percy, commencing with the pure metal, and proceeding to notice the principal compounds with other elements, metallic and nonmetallic, that are of importance from a metallurgical point of view.

Metallic Iron. This may be obtained in a chemically pure condition by reducing peroxide of iron by hydro

gen at a red heat, or by re-melting the purest varieties of malleable iron with an oxidising flux, in order to remove the last traces of combined carbon. It may also be deposited by electrolysis from a solution of protochloride of iron, in the form of brilliant malleable films, a process that has been employed by engravers to protect the face of engraved copper plates from undue wear during printing, and is known as acierage, or steel facing. It does not appear to be quite certain, however, from the contradictory statements made by different observers, that electro-deposited iron so obtained is absolutely free from nitrogen.

The physical properties of the metal vary very considerably, according to the means adopted for its production. When obtained by reducing peroxide of iron by hydrogen at the lowest possible temperature at which the change can be effected (according to Magnus between 600 and 700° F.), it forms a dark grey powder, which combines energetically with oxygen, taking fire spontaneously when slightly heated and thrown into the air. When, however, the reduction takes place at a higher temperature, the metallic powder agglutinates to a sponge of a filamentous texture, a silvery grey colour, and metallic lustre, which is no longer pyrophoric.

Larger and more compact masses may be obtained by removing the last traces of carbon and other foreign substances from the purest commercial wrought iron in the following manner :-A small quantity, from 300 to 500 grains, of good wrought iron, such as pianoforte wire or Russian black plate, cut up into small pieces, and either rusted by exposure to steam or mixed with about 20 per cent. of pure peroxide of iron, is to be melted

under glass free from metallic oxides, in a refractory crucible, at a strong white heat, the operation requiring about an hour's full heat of a good wind furnace. The small quantity of carbon present in the metal is expended in reducing a portion of the sesquioxide, the remainder passing into the slag; the result being a brilliant well-melted button of metal, which exhibits a decidedly crystalline structure, similar to that observed in meteorites when treated with an etching liquor, and is somewhat softer, but less tenacious, than the iron originally employed. The melting point of pure, or even ordinary, malleable iron has not been determined with certainty. According to Pouillet it lies between 1,500 and 1,600° centigrade, while Scheerer gives it as 2,100° of the same scale. The specific gravity varies from 7.7 to 7-9, the weight of a cubic foot at 0° C. being about 486 lbs. The linear dilatation by heat is between 0° and 100°, and 7 between 0° and 300° C.* The specific gravity and tenacity vary with the method of treatment, and will be considered in connection with the strength of merchant ircn.

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Magnetism. Pure iron is susceptible of being magnetised to a much higher degree than steel, but unlike the latter metal, it does not retain its magnetism when the exciting cause is removed. The so-called magnetic oxide, and some other compounds of iron, are also magnetic, but in a less degree.

The following determinations of the proportional magnetism of different compounds of iron are by Plücker (Müller's "Physik," vol. ii. p. 402) :

* In future, except where otherwise stated, the temperatures will be expressed in centigrade degrees.

Specific Heat.

Magnetic oxide

Native peroxide.

Precipitated peroxide

Solution, nitrate of peroxide

protochloride.

Hydrated peroxide .

0.11379 according to Regnault, or 0.1100 by Dulong and Petit. The conducting power for heat is 374, gold being taken as 1,000 (Despretz). The electrical resistance (as determined by Pouillet) is 5.88 times that of a copper conductor of equal sectional area.

The crystalline forms of iron are most probably to be referred to the cubical system, although there is some difference of opinion on this subject. Fuchs supposed them to be in part rhombohedral, and that the metal is dimorphous; the balance of opinion is, however, in favour of the former view. The observed forms are the cube, octahedron, and tetrahedron. According to Peligot, brilliant cubical crystals are occasionally obtained when protochloride of iron is reduced by hydrogen in a porcelain tube at a red heat. The equivalent or atomic weight of iron is 28 when hydrogen is taken as the unit of the scale, or 350 when oxygen is taken as 100; its symbol is Fe.

Passivity of Iron. When a bright iron wire is immersed in fuming nitric acid, containing a certain amount of nitrous acid, it becomes passive; that is, it is not dissolved, even if placed in acid of the ordinary strength, as long as no great increase of temperature takes place. If, however, the temperature be raised, or the metal be touched by a copper wire, it is immediately attacked and dissolved.