reached the fuel might be useful. It would, in my opinion, be difficult to justify the reasonableness of this expectation, with all the light which the previous and subsequent labours of physicists have thrown on the subjects of heat and combustion. The idea, however, possessed a certain attraction, and it was speedily put to the test of experiment. The result was that, within four years of its introduction, the furnaces at the Clyde works, in Scotland, were running more than double their former make, without requiring more fuel for the larger than for the smaller quantity. In other words, the consumption of fuel per ton of iron, when burnt with air heated to 600 deg. F. (315 deg. C.), was less than one half of what it was when using the blast at atmospheric temperatures.

The utmost surprise was excited among scientific men by these results, the examination of which will be best deferred until the action of the blast furnace, chemically and otherwise, forms the topic of consideration. I will then endeavour to explain the precise mode of action of the hot blast, the introduction of which was, at the time, of such immense value as to constitute undoubtedly one of the most marked epochs in the advancement of the iron trade.

At the period to which we have brought this history, the comparative cheapness of iron had considerably extended its application not only to those objects to which it had been long adapted, but to certain new inventions, many of which, but for its assistance, would have remained unthought of. Chain cables for mooring ships had taken the place of hempen ropes; pipes of iron, instead of wood, conveyed the water required in our towns; and before long all great centres of population were illuminated by gas, distilled in iron retorts, and led in metal piping to the different points of consumption.

For the economical transport of great weights of any material, water communication was, at the time at which we have arrived, deemed indispensable. Where natural facilities did not permit this, artificial means were resorted to; and from many of our more considerable manufacturing towns the produce was carried away, towards the sea coast or elsewhere, on canals of excellent design and capacity. At the same time, however, in some of our colliery districts (probably from the almost insuperable difficulty of cutting canals), the coal was

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conveyed from the mines to the ship on railways—an invention which has been in use for this purpose for something like 250 years. For about 120 years after their first introduction the sole material used in the construction of these primitive railways was wood; and it says something for the enterprise of the early coal owners that, at a period (about 1740) when the make of pig in the United Kingdom was only 17,340 tons, one of their body suggested and tried rails of cast iron.

Owing to various reasons-the high price of the metal probably underlying them all—it was 40 years later, or about 1780, when cast iron rails came into frequent use. But, long after this date, wood continued to be often employed; and I remember, so late as about 1840, seeing the old beech-wood rails in use on a colliery railway in the County of Durham.

The economy effected in the manufacture of malleable iron by the puddling furnace began, in due course, to bear fruit; and exactly 20 years after Cort's patent-viz., in 1804—we come upon the use of wrought iron rails, made in the shape of square bars, and only two feet long. These were found unsuitable; and it was not until 1820, when Birkenshaw succeeded in rolling bars of a convenient section, that malleable iron commenced to be the material employed, at first sparingly and afterwards generally, for railway purposes.

This humble form of transport, so long employed in the North of England, was destined to prove the nursery not only for railway communication itself, but for those steam engines without which it would have been useless to contemplate the building of tracks of iron upon anything approaching their present scale.

After driving the stage-wagon from the high-road, iron gradually took the place of wood as a material for naval constructions, in the propulsion of which the wind plays but a secondary part: the furnace and the forge again providing, in the steam engine, the means of locomotion.

About a quarter of a century after the pretty general adoption of Neilson's great discovery, it was becoming apparent that a metal, hitherto considered as typical of strength and endurance, was unequal to the heavy demands made upon it by the ever-growing magnitude of our railway traffic. From this apprehension we were relieved in 1855

by a further important invention, viz., that of the Bessemer process, made by the eminent man whose name it bears.

In the present section of this work, it will not be inappropriate to recall the various steps which may be regarded as having been the forerunners of this discovery-a discovery which will probably for some time, if not permanently, remain as a culminating point in the progress of the manufacture of iron.

The blast furnace furnished the old forge master with crude iron, so that in his low and expensively conducted hearth he had now to deal with a raw material containing nearly 95 per cent. of metal, instead of an ore often of only half this richness. When Cort proposed to dispense with the costly mode of treatment just mentioned, considerable difficulty was at first encountered from the corrosive action on the furnace, due to the silicon, which is always found in greater or less quantity in all pig iron. It was, in consequence, found more economical to submit the crude metal to the partial action of the old hearth, or refinery as it came to be called, so as to expel a considerable portion of this objectionable constituent. Since the earlier days of Cort's furnace, the process of refining has been generally superseded by the device of saturating the silica, produced by the oxidation of the silicon, with oxide of iron. This takes place in the operation of puddling; and it is only exceptionally that a refinery is now found among the appliances of a modern forge.

Those who have watched with attention the action of refining, will remember that towards its close a violent ebullition and evolution of flame manifest themselves, indicative of a great elevation of temperature. This is partly due to combustion of some of the carbon, but mainly to the oxidation of the silicon contained in the pig. If the operation be continued long enough, these two substances, which confer fusibility on the iron, are gradually reduced in quantity; and the temperature evolved by their action, even when assisted by burning coke, being insufficient to maintain the altered metal in a fluid state, it assumes a semi-pasty condition. In Sir Henry Bessemer's ingeniously contrived apparatus, the circumstances connected with the evolution of heat are greatly modified, by pouring a vast volume of air very quickly through the molten iron. The combustion of the

carbon and silicon becomes so rapid, that the temperature of the mass rises to a pitch of intensity sufficient to maintain even malleable iron in a perfectly liquid state.

It was soon found that the plan thus proposed by Sir Henry Bessemer for burning off these foreign substances, the presence of which constitutes the difference between pig and malleable iron, was beset with practical difficulties; which at one time even threatened to arrest the progress of a discovery that has already wrought such changes in the manufacture of the metal we are considering. For reasons referred to in the previous section, it was speedily demonstrated that the ordinary run of British iron was entirely unsuited for treatment in the converter; and for some time even the use of those brands, which did not contain above one thousandth part of their weight of phosphorus, appeared to be hopeless. Recourse was had at length, on the suggestion of Mr. R. F. Mushet, to the known influence of manganese on the quality of steel; and this removed all impediment to the manufacture by the new method, from metal containing not more than the above mentioned proportion of the hurtful ingredient in question. So complete has been the success which has attended this modification of Sir Henry Bessemer's great discovery, that no one, who has studied the subject, will be surprised if, by it or other means having the same object in view, nearly all the old landmarks and dogmas relating to the manufacture of iron, although founded on long experience, are ultimately swept away.

The violent heat excited in the Bessemer converter depends, as we have seen, on the oxidation or combustion of the silicon and carbon contained in the original pig. This fact, therefore, necessarily limits the use of this process to iron containing a sufficient quantity of these two elements, or of some suitable substitute, the burning of which serves to melt the steel. If a system of producing steel is to be pursued, in which, from the nature of the materials employed, it is impossible to generate the heat in the manner of the Bessemer converter, then the use of ordinary fuel becomes unavoidable. In the reverberatory furnace, as it is commonly constructed, there would be very great difficulty in maintaining a steady temperature of the proper intensity. Each time fresh coal is thrown on to the fire, the vaporizing

of the hydro-carbons, and the admission of a large volume of air, greatly tend to cool the contents of the hearth. Besides this inconvenience, there is an immense loss due to the arrival of the products of combustion at the chimney long before they have parted with more than a fractional portion of their heat, although they are already cooled to a temperature below that to which it would be advantageous to expose the substance under treatment. These difficulties are admirably met in the "regenerative furnace" of Messrs. Siemens. The fuel is employed in a gaseous condition, and the heat that would otherwise escape is imparted to the incoming air and gas; by which means the interior of the structure and its contents are raised to a temperature capable of maintaining any quantity of steel, or even of wrought iron, in a state of perfect fusion.

Exception has been taken to the extent of the service rendered by these gentlemen to metallurgic science, founded on the fact that the idea upon which their furnace is constructed had been described by a previous inventor. Whatever may have been the merits of the latter in connection with the subject, certain it is that Messrs. Siemens, with or without the knowledge of what had been previously done— it matters little-have introduced to the world an invention, invaluable as a means of commanding a most intense temperature for purposes never contemplated, so far as I know, by any former discoverer.

In respect to the immense loss of heat which, thirty or forty years ago, was uniformly permitted to take place from all puddling or mill reheating furnaces, it may be here parenthetically observed, that this serious waste is now avoided to a considerable extent by using the escaping flames for the purpose of driving the steam engines employed in the works.

To some extent the principle of the regenerative furnace has been introduced into the so-called retort furnace of Mr. Price. In this the heat otherwise lost is made to surround an upright retort, where the coal parts with a considerable portion of its gaseous constituents before it reaches the fire-place. The volatile portion passes through the furnace in the usual way, while the fixed carbon, more or less in a state of bright incandescence, descends to the grate bars, the cooling effect already spoken of in connection with reverberatory furnaces