SMELTING IRON BY HOT-BLAST

SMELTING IRON BY HOT-BLAST. Since the article on RECENT CHANGES IN THE MANUFACTURE OF IRON was published in the usual order of the progress of this work, certain changes have been introduced in the process of making iron, of so extensive and important a character, that we cannot consider them in any other light than as effecting an entire revolution in those branches of industry and of commerce connected with the manufacture and application of that most valuable of metals. The change has been comparatively unnoticed, extending itself during the last ten years, from the little spot in a remote part of this island whence it took its origin, and has now found its way over a great part of the old world, and been adopted extensively in the new. Wherever it has found its way, wealth and prosperity have attended it, poverty and sterility have fled before it; and there have been opened up vast stores of mineral wealth to assist in the advancement of civilization and industry, which our former impotence and want of skill had marked with the stigma of utter unproductiveness. It is therefore necessary that we give in this article, as supplementary to our former treatise, an account of the new process, now well known over the whole manufacturing world as the hot-blast process for smelting iron.

The hot-blast process was invented in Scotland by Mr. James Beaumont Neilson, a practical engineer at Glasgow, and was by him made the subject of a patent, dated September 1828, being entitled, an "improved application of air, to produce heat in fires, forges, and furnaces, where bellows and other blowing apparatus are required."

The invention of Mr. Neilson, like a great many other inventions, and indeed some of the most valuable, is characterized by extreme simplicity; and his invention so perfectly accords with all the known laws of physics, that we at once apply to it the well known exclamation of wonder, "how strange! that a thing so obvious should not have been done long before!" an exclamation in which we too often find ourselves doing an act of injustice to the very inventors to whom we lie under the weightiest obligations. Mr. Neilson's hot blast is as simple a contrivance as was the steam-engine condenser of his countryman, Mr. Watt, and indeed bears a very close analogy to its character as an invention. Before the time of Mr. Watt, steam was introduced into that part of the steam-engine where it performed its labour, and was then condensed into water in the same place where it had done its duty, thereby causing great waste of fuel and loss of power; and to remedy this defect, Watt introduced his new principle on which his fame rests; he used a separate condenser, that is, cooled the steam, and reconverted it into water, not in the cylinder, where it was detrimental, but in a separate vessel communicating with it. In like manner, Mr. Neilson's invention consists in separately heating that which was formerly done so as to be injurious to the process intended. Cold air was blown into a furnace designed to produce intense heat for the smelting of iron from ore, so that the cold blast of air itself required to be heated by the fuel in the furnace intended to smelt the iron, and so its temperature was cooled down by the very intensity of the blast required for the desired combustion. To remedy this defect, Mr. Neilson introduced his new principle of hot blast; and instead of allowing this blast of cold air to enter at once from the blowing machine or bellows into the furnace, to be heated there, he provided a separate heating apparatus, by which the air of the blast was to be heated by a separate furnace and fire, previously to its entrance into the smelting furnace. This is the whole invention. A hot blast of air, instead of a cold one, is thus introduced for the generation of heat. Could any thing be more simple, more natural, more appropriate, or more likely to prove effectual? This is the hot blast, which has already in this and other countries created many millions of valuable property out of what was formerly worthless, because unavailable by any of the processes formerly known.

Like most other inventions, the progress of this was at first very slow. Retarded by practical difficulties, which beset all new processes in their first use, by men who have every thing to learn,—stopped every now and then by the prejudices of custom and ignorance, which cling with inveterate tenacity to maxims of established practice, and repel equally the innovations which improve and those which merely alter without improvement,—opposed also by the change of interests which such a revolution must necessarily involve; the invention, tardy in its first steps, and feeble in its early efforts, was more than once at the point of being altogether abandoned. Like the invention of Watt, a great part of the interest in its possible remuneration was transferred by the inventor to strangers, whose combined efforts and influence it was necessary to obtain on the side of the innovation. To Mr. Dunlop of Clyde iron-works Mr. Neilson had to give up three-tenths of his patent rights; to Mr. Mackintosh three-tenths; and one-tenth to Mr. Wilson of Dundivan, retaining to himself only three-tenths of this valuable monopoly. But the transfer was judicious; it was necessary. Mr. Mackintosh is distinguished as a man of much practical science; Mr. Dunlop was one of the most sagacious iron-masters of his

time; and Mr. Wilson was a man of tried practical talent. The co-operation of these gentlemen was essential to the speedy and successful trial of the novel though simple process.

The following is the specification of Mr. Neilson's patent: "I, James Beaumont Neilson, do hereby declare, that the nature of my said invention for the improved application of air to produce heat in fires, forges, and furnaces, where bellows or other blowing apparatus are required, and the manner in which the same is to be performed are particularly described and ascertained as follows: That is to say, a blast or current of air must be produced by bellows or other blowing apparatus in the ordinary way, to which mode of producing the blast or current of air this patent is not intended to extend. The blast or current of air so produced is to be passed from the bellows or blowing apparatus into an air vessel or receptacle, made sufficiently strong to endure the blast, and through and from that vessel or receptacle, by means of a tube, pipe, or aperture into the fire, forge, or furnace. The air vessel or receptacle must be air-tight, or nearly so, except the apertures for the admission and emission of the air; and at the commencement, and during the continuance of the blast, it must be kept artificially heated to a considerable temperature.

"It is better that the temperature be kept to a red heat, or nearly so; but so high a temperature is not absolutely necessary to produce a beneficial effect. The air-vessel or receptacle may be conveniently made of iron; but as the effect does not depend upon the nature of the material, other metals or convenient materials may be used. The size of the air vessel must depend upon the blast, and on the heat necessary to be produced. For an ordinary smith's fire or forge, an air vessel or receptacle, capable of containing 1200 cubic inches, will be of proper dimensions; and for a cupola of the usual size for cast-iron founders, an air-vessel capable of containing 10,000 cubic inches will be of a proper size. For fires, forges, or furnaces, upon a greater scale, such as blast-furnaces for smelting iron, and large cast-iron founders' cupolas, air-vessels of proportionably increased dimensions and numbers are to be employed. The form or shape of the vessel or receptacle is immaterial to the effect, and may be adapted to the local circumstances or situation. The air-vessel may generally be conveniently heated by a fire, distinct from the fire to be affected by the blast or current of air, and generally it will be better that the air-vessels and the fire by which it is heated should be inclosed in brick-work or masonry, through which the pipes or tubes connected with the air-vessel should pass. The manner of applying the heat to the air-vessel is, however, immaterial to the effect if it be kept at a proper temperature. In witness whereof, I, the said James Beaumont Neilson, have hereunto set my hand and seal, the 28th day of February, in the year of our Lord 1829."

The only part of the process of smelting the iron which is at all affected by this novelty, is the transit of the air from the blowing machinery into the smelting furnace. The bellows, steam-engine, water-wheel, blowing cylinders, equalizing reservoirs, or other apparatus of whatever sort, for producing the current of air to blow the fire, are left unchanged, as we have already described and figured them in our article on IRON. The furnace may be left of its former shape and dimensions in every respect as used in the old process, and as we formerly described them; all that is required for the introduction of the new process, is to interpose between the smelting furnace and the blowing machinery an oven, heated by a separate fire, through which, in appropriately shaped vessels or pipes, the blast of air on its way to the furnace may be heated to a considerable temperature,—400°, 600°, or any temperature found most suitable to the purpose, and so blown upon the incandescent fuel in the furnace in a hot blast, as distinguished from the ordinary cold blast of atmospheric air.

The consequences of this hot blast system in an econo-

Smelting mic point of view, may be illustrated by the following experimental facts.

In a given furnace in Scotland, previous to the introduction of the new process, the following was the average result obtained:

Advantages of the hot-blast system.
20 tons of coal (coked),
with $3\backslash frac\{1\}\{2\}$ tons of limestone,
smelted $4\backslash frac\{1\}\{2\}$ tons of iron,
being $3\backslash frac\{1\}\{2\}$ tons of fuel to each ton of iron.

In the same furnace, by the introduction of the new process, the result was as follows:

14 tons of coal (raw),
with $2\backslash frac\{1\}\{2\}$ tons of lime,
smelted 8 tons of iron,
being $1\backslash frac\{1\}\{2\}$ tons of fuel to each ton of iron.

Here, then, we see that, in the first place, one half of the fuel only is required; that coking, a very expensive process, is avoided, the coal being used raw, which was formerly impossible; and that only about one-third of the quantity of limestone is required to flux the iron, and all the cost of these materials, and all the labour employed in their transport, and in the process of melting, are clearly saved. But these were not the only benefits conferred on the iron-master. The same furnace, blowing apparatus, and establishment, that formerly produced fifty tons a-week, yielded under the hot blast more than double that quantity.

Peculiar advantage of the hot-blast in Scotland.
As it was in Scotland that these advantages were first obtained, so it is there that they are still possessed in the highest degree. Previously to the introduction of the hot blast, it had become an almost hopeless competition to produce iron there at so cheap a rate as the Welsh iron-masters could import it into Scotland. The hot blast at once turned the scale; and Scotland now transmits annually many thousand tons of iron to all the markets of England, and even into the very heart of the iron districts of the principality itself.

The cause of this difference between the advantage which Wales has derived from the hot-blast process, and that conferred on Scotland, is simply this: the coal fields of Wales are a rich, strong, and highly carbonaceous fuel, which loses not more than thirty to forty per cent. in the process of coking. The coal basin of Scotland in the iron district is, on the contrary, of a poor, earthy, light description, so as to lose from fifty to sixty per cent. in the process of coking, and giving, for every two tons of coal, not more, and frequently much less, than one ton of coked fuel fit for the cold-blast smelting-furnace. The expense and loss in the Scottish coal by this preparatory process of coking, was therefore excessive, when compared with the Welsh coal; and hence the adoption of this new plan, which did not require this preliminary process of coking, became a much greater boon to the Scotchman, in exact proportion to his former disadvantage.

Result in Staffordshire.
Great as these advantages have been found in Scotland, they have by no means been limited to the country of the inventor. In Staffordshire, in the wonderfully rich mineral basins of Dudley, and in Derbyshire, the invention was very early introduced, and most successfully practised. The following are the general results of the hot-blast system, as now practised in Staffordshire, in comparison with the old system:

By the cold blast,
8 tons of coal (coked),
with $1\backslash frac\{1\}\{2\}$ ton of lime,
yielded 3 tons of iron,
being $2\backslash frac\{1\}\{2\}$ tons of fuel to each ton of iron.

By the hot blast, in 1840.
$2\backslash frac\{1\}\{2\}$ tons of coal (raw),
with $\backslash frac\{1\}\{3\}$ths of a ton of limestone,
yield $2\backslash frac\{1\}\{2\}$ tons of iron,
being 1 ton of fuel to each ton of iron.

The saving thus obtained in the article of fuel, is accompanied, it will be noticed, in every instance, with a corresponding

saving in the limestone employed to flux the metal. But even here the whole amount of advantage does not meet the eye. It was formerly important to use the very best quality of coal that could be obtained,—light, bituminous, or earthy coal, being least fit for the purpose, and most unprofitable. Now, although it is still desirable to have a better rather than a worse fuel, the importance of this point is so greatly diminished, that almost all the inferior descriptions of poorer and more bituminous coal may be employed with economy and advantage. Limestone of the best quality was essential to the produce of good iron by the old method; by the new method, it is obvious, from inspection of the examples given, that the saving in limestone varies from a third to two-thirds of the quantity employed; but, in addition to this, it has become practicable to employ limestone of a very inferior quality, where better may be scarce, without sensible deterioration of the metal.

For the production of good iron, the hot-blast system has thus opened vast resources, which formerly could not be rendered available, in consequence of the inferior quality of some one of the raw materials which could be brought together in a given locality. In one place, abundant in excellent carbonate of lime and rich ore, it became impossible to realise the mineral wealth, on account of the inferior quality of the coal; while, in another, an abundant supply of good limestone, and of good coal, could not be used with advantage, from the poverty of the ore, which happened to be a clay stone, containing a small proportion of iron. Perhaps, also, a portion of sulphur mixed with the materials in such a quantity, as to ruin the iron. These all are counterbalanced by the hot blast, and every one of these impracticable cases is now exemplified in daily use.

But there is no sphere in which the hot blast has exerted a more beneficial influence in promoting the success of industry, and extending the resources of civilization, than in its application to those sterile districts of the mineral world, known as the region of the stone coal, blind coal, or anthracite formation. In the blind or stone coal strata, we have that most valuable of all combustibles, carbon, in a condition of high density and purity, amounting to as much as ninety or ninety-five per cent. of the stone coal, with a very small per centage of earthy or hydrogenous matter, yet so contracted, and, as it were, iron-bound, so hard and obdurate, that, instead of forming a good combustible, it seems to partake of the nature of that wonderful and brilliant substance, so hard of combustion as to have been reckoned one of the incombustibles until the glass of the accomplished analyst resolved the glittering brilliant into its primitive carbon. The anthracite seems, in fact, truly to deserve the name of the "black diamond," having been rendered, by its dense and close structure, nearly as incombustible. Although America, and many wide districts of Europe, abound with this rich carbon, it has failed to furnish, even in the hands of many accomplished chemists and mechanicians, an available fuel, either for the generation of steam, or for the ordinary wants of social life, and has resisted even the intense heat of the blast furnace. But even its consummate obstinacy has yielded to the power of the hot blast. In Wales, and in America, hot-blast iron of the very finest quality is now made from stone coal; and thus stores of the richest fuel which the world contains, appear only to have been reserved, by their wonderful obduracy, from the rapid destruction to which all other fuel has been so wastefully subjected, to supply the civilization and science of the nineteenth century with the means of rewarding and extending its astonishing discoveries. So kindly has Providence placed at the surface of the earth one mineral fuel to replace the forests which supplied the fires of our forefathers, and deposited another still richer, but more remote in the bowels of the earth, to await and to reward the industry and research of

their children and our own. What geologist will henceforth call in question the abundance of that supply, which now seems to augment with the very means that are invented for increasing its consumption?

The Welsh coal-field is perhaps the richest in the world. Rich with bitumen on one side, it gradually falls off by insensible gradations into a more purely carbonaceous, but still free-burning coal, and, passing through every intermediate gradation, is found at last in the state of a rich, resplendent refractory anthracite, or stony coal. This, mixed in the proportion of two to three, three to two, or half and half, smelts iron readily under the hot blast, and produces a rich powerful pig, possessing the qualities of a rich charcoal iron, and also capable of being converted into a most tenacious and malleable bar iron. In the anthracite district of Wales, new furnaces are rapidly rising into use; and Mr. Crane is distinguished by his extensive use of Mr. Neilson's hot-blast in that application of it.

But it is probably in America that the hot-blast system will most extensively contribute to the development of mineral wealth. The anthracite is there the staple of fuel. It covers a vast extent of country, and constitutes the only substitute with which our transatlantic brethren can replace those forests that are so rapidly receding before the advances of civilization. Here the hot-blast is most successfully employed.

We have thus given our readers the general specification of the hot-blast improvement; a specification so general as to include all possible forms of apparatus, applicable, not only to the smelting of iron, but to all cases of manufacture in which a blast is employed to produce heat, and which touches only the process of heating the air, without regard either to the mode of producing the blast, the mode of applying the blast, or the means of heating air; and the patent only provides for the carrying into effect the process of heating the air, that the blast shall be passed through a heated vessel or receptacle of any form or dimensions, such as a cast or malleable iron retort, kept to a dull red heat of some 1000° of temperature as the most desirable. It will readily be imagined that this apparatus, described in terms of so great generality, would assume various forms, according to the different views which the various practical men employed to construct the apparatus took of the most effective mode of heating the air. This is accordingly what occurred with the progress of time, and the increasing experience of those who used the apparatus; practical difficulties gradually diminished with the increased experience which was every day suggesting new and better expedients. It was also soon found that the advantage gained was increased in a rapid proportion with the degree of heat communicated to the air, so that the apparatus which gave at first some 300° of temperature to the air, and produced one measure of benefit, was soon superseded by a second form of apparatus giving 400°, and conferring still higher advantages; and this in its turn gave way to another and another, each giving a higher temperature to the air, and promising a higher degree of economy to the user. And thus at last a temperature between 600° and 700° was given to the air; a temperature higher than that of melting lead, which is indeed the criterion now generally applied to test the working heat of the blast.

We shall probably give our readers the clearest views of these general improvements, and of the progress which has generally taken place in carrying them into effect, by taking a single example. For this purpose we have selected the various modes of heating the air that have necessarily suggested themselves to, or been put in practice by the ingenious proprietors of the Butterly iron-works in Derbyshire, who were the first to introduce the hot-blast into that part of the country, and who have been among the most successful in carrying it into effect, and producing one of the most superior qualities of hot-blast iron.

Plate CCCCLXII. fig. 1, shows the construction of the

earliest and simplest plan by which the inventors of hot-blast first brought it into use. An iron vessel hhh was formed of malleable plates, rivetted together like a common steam-boiler, something near three feet in diameter, and eight or ten feet long, cylindrical, and fitted at the ends to two pipes Bb, communicating with the bellows, and Ss with the smelting furnace. Below this large pipe or tube a fire is placed, which is fed from the door D, and the whole is enclosed in an oven of brickwork OOOO, leaving a clear space all round the pipe hhh, so that the flame and hot air rising up and enveloping the receptacle, should keep it so hot as to communicate through the sides of the vessel a higher temperature to the air rushing through it, on its way from the bellows, B, towards the smelting furnace S. And in order to communicate this temperature more uniformly and completely to all the air passing through the vessel hhh, lunular partitions ppp proceeding alternately from opposite sides of the tube on its interior surface, cause the stream of air to impinge first on one side, and then on the other side of the heated iron pipe, as shown in the figure. By this apparatus a moderate current of air has been heated to three or four hundred degrees.

Fig. 2 shows the manner in which the method of heating the air was employed at first in the Butterly iron-works. In the oven OO are placed the two cast-iron retorts hh, about thirty-two inches in diameter, and nine feet long, being of metal an inch and an eighth part in thickness. They are laid parallel to each other; one of them is at one end connected with the pipe Bb from the blowing apparatus, and this conducts the air from its other extremity through a curved junction pipe JJ, into the second receptacle hh; where, on its way becoming hot, it passes through the hot-air pipe Ss, from the heating furnace or oven, and is blown by the orifice T through the twyre T into the hearth of the smelting furnace at F. In this, as is usual, more than one blast, as at T', was blown into the same smelting furnace. The fire dd, is placed at one corner of the oven on the bars rr, and is fed from the door D, and the smoke and flame rising up and surrounding hh, pass through a partition by the flue f, and fill the second compartment of the oven, clear spaces being left quite round the retorts on every side for the communication of the heat; only at the points h and h', they are supported by contact on the brick pillars kk. Finally, the products of combustion pass through a flue f into the short chimney C. In this way the cold air from the blast pipe Bb passing into the vessel hh, is partially heated, and is conducted into a second chamber, which, being placed immediately over the fire, is much more intensely hot, and from this second chamber at once discharged into the smelting furnace.

This apparatus was first tried in November 1830. The air was only heated to 240°. Yet so admirable is the process, that it was attended with the following effect:

With cold blast there were previously used at Butterly,
5½ tons coal (coked),
to 1 ton of iron.

By the new apparatus there were required
only 3 tons coal (raw),
to 1 ton of iron.

So admirable an effect as this was not to be neglected on account of the practical difficulties attending it, which indeed only served as an inducement to further exertion; and in about a year after the following improved apparatus, with an increased number of tubular vessels, having increased dimensions of length, was successfully introduced.

Fig. 3. In the transverse section h¹h²h³, are three pipes of east-iron 1½ inch thick, being about 17 feet long, and about 22 inches in diameter. They are similarly denoted in the ground plan, and in the longitudinal section by the letters h¹h²h³. The cold air pipe from the blowing apparatus B, enters the heating oven at b and traverses the pipe h¹h²h³ to the opposite end of the furnace: here a

Smelting by hot-blast. bent pipe $h^1\; h^2$ conveys the air into a second longitudinal pipe $h^2\; h^3$, by which the stream of air returns back to the end of the oven at which it entered, and is conducted by a second bent pipe $h^2\; h^3$ into the third tubular vessel $h^3\; h^4$ in which it is for the last time exposed to the heated metal, and passes off from the heating oven into the smelting furnace, by the hot air pipe S, whence, as formerly, it is discharged through the twyre into the hearth of the furnace. The fuel is placed through the door D upon the fire of the oven on the bars r, and by means of a number of small pigeon holes $ccc,\; ccc,\; ccc$, is spread over the whole length of the pipes with some measure of uniformity, whereas, if allowed to act directly on the retorts, it would irregularly and less effectually heat their surface. The manner of protecting the retorts from the direct action of the fire, is well shown in the transverse section: a brick arch $aaa$ is thrown over the fire, and the flame is only permitted to ascend through the aforesaid openings $cc,\; cc$. It will be noticed that the retorts are every where clear of the building and of each other, that they are wholly enclosed by the oven $OOO$; and it will also be seen in the transverse section, and in the longitudinal section, that the products of combustion are carried off by a descending flue $ff$, regulated by a damper $e\; e$, into the short vertical chimney C. This damper is very useful in retarding the draught of air, so as to leave the products of combustion a longer time in contact with the heaters.

Its effects. This apparatus is more effective and more durable than the last. It raised the heat of the air as high as $400^\backslash circ$ to $500^\backslash circ$, being $200^\backslash circ$ higher than the former plan, as indeed we should expect from the greater number and length of the vessels, and the greater surface in contact with the air. The result, compared with the last, is as follows:

By the former plan with air at $240^\backslash circ$,
3 tons of coal were required
to melt 1 ton of iron.

By the improved plan, with air at $400^\backslash circ$ to $500^\backslash circ$,
$2\backslash frac\{1\}\{2\}$ tons of coal were required
to melt 1 ton of iron.

The reader will not fail to be struck with the fact, that a much greater gain was obtained by the original ruder plan of using the air, when compared with the old system of hot-blast, than was afterwards got from the introduction of the improved apparatus over the former one. The higher temperature does not appear to have conferred an advantage at all proportionable to the increased temperature.

Third apparatus. Fig. 4 is a very superior and durable heating apparatus, which continues in efficient use to the present time. Its form is well adapted to prevent all the evils of expansion and contraction; and it is our opinion that by multiplying the vessels, a more effective apparatus might be made than any now in use. Its form is very ingenious, and its functions may be easily learned by studying the figure, in which the letters describe the same parts with those of the previous figures. B is the pipe coming from the blowing apparatus to the heating furnace $OOOO$; S is the hot air pipe, proceeding from the heating oven to the smelting furnace; $h^1\; h^1\; h^2$ is a large retort into which the air is at first carried, and from the further end of which a malleable iron tube $i^1\; i^1$, concentric with the retort, carries it back again to that end where it first entered, and through a bent pipe II conducts it along by a second interior tube $i^2\; i^2$ to the further end of the second heater $h^2\; h^2\; h^3$, along the hot surface of which it returns in the manner shown by the arrows to the base of the second retort: now, this second retort communicates at the base with a third $h^3\; h^3$, and after traversing its heated surface to the end, is brought back by a tube concentric with the axis to the base of the retort. This process might be continued with great advantage, and make one of the best series of heating vessels. The fire is covered by arches $aaa$, with small apertures $ccc$, as in the former case, to prevent the vessels from being injured by the local action of the intense fires.

This apparatus raised the temperature to between $500^\backslash circ$ and $600^\backslash circ$, and produced a further saving in coals. By the former plan $2\backslash frac\{1\}\{2\}$ tons of coal were required to smelt one ton of iron. By this improved apparatus, $2\backslash frac\{1\}\{2\}$ tons of coal melted a ton of iron. Smelted by hot-blast.

Figs. 5 and 6 are the general forms of apparatus now most extensively used. Fig. 5 is a form first given to the apparatus, we believe, by Mr. Dixon at Calder, and hence generally called the Calder pipes. Similar pipes were also early introduced at Gartscherrie, and at Wedensbury; and are very generally approved of from their extensive heating surface, although they are liable to the inconvenience of cracking, from unequal expansion. The form, as erected at Butterly, consists of two parallel horizontal pipes L L called technically "the lying pipes," one forming a communication immediately with the cold-blast pipe B, and the other with the hot-blast pipe S. Into sockets formed in the lying pipe, are inserted a series of smaller pipes springing up at right angles with the one lying pipe, and after forming an arch, returning down to the second lying pipe, being inserted in like manner into sockets in it. The air, therefore, on entering the first lying pipe, is forced through these transverse heating pipes, or "A pipes" as they are called from their figure, and thus exposed to the heat of a very extensive surface, is delivered into the hot-air pipe. Sometimes, as at Gartscherrie and many other works, there are several times as many pipes as in this example, and the air is made to cross and recross the fire several times. Its effects.

The figures of the transverse pipes are as various as the taste of the parties who use them. Sometimes they rise up and form a large semicircular arch over the fire, which is placed in the centre; sometimes a double tier of such arches is employed. Sometimes they cross the fire in the form of a pointed arch variously acuminate. Then again by some, the pipes are carried up four, six, eight, or ten feet, like columns, united at the top by a semicircular arch.

The cross section of the heating pipe is as various as the form into which the pipe itself is made to bend. A circular pipe was used at first, then a flattened elliptical one for the purpose of exposing more surface in proportion to volume; next a pipe, flat at one part, and semicircular on the other, was introduced; next a pipe of a cardioid or heart-like section was employed. At Butterly Mr. Terrop used a circular pipe cast with a solid core, to keep the air near the hot surface, as shown in the figure; and finally, he employed the rectangular section as shown in our figure 6th. All of these appear to answer their purpose of heating the air sufficiently well, and all of them cause trouble and expense by cracking now and then.

The result of all these forms is to produce a hot-blast of more than $600^\backslash circ$ temperature, and at Butterly, figs. 5 and 6 yield as follows: $2\backslash frac\{1\}\{2\}$ tons of coal melt one ton of iron. It is found, however, at Butterly, that pigs, for the forge, require about three cwt. less of coal than this average quantity, and that No. 1 iron requires about three cwt. more than this, as we should expect.

The Scotch use less coal than the Butterly Company, principally on this account, that the former use calcined ore, of which thirty-two cwt. with seven cwt. of lime, makes a ton of iron, while at Butterly, the quantity used for a ton of iron is $2\backslash frac\{1\}\{2\}$ tons, with a ton of lime, being $3\backslash frac\{1\}\{2\}$ tons of materials in one case, and less than two tons in the other.

The explanation of the principles from which hot-blast derives its efficacy as the means of producing elevated temperature, is very easy and plain to any one acquainted with the elementary principles of chemistry and mechanics. All that we think it necessary to add on this subject, we shall state in the words of Mr. Babbage and Dr. Ure.

"The increased effect," says Babbage, "produced by thus heating the air is by no means an obvious result; and an analysis of its bag's action will lead to some curious views respecting the future application of machinery for blowing furnaces."

"Every cubic foot of atmospheric air, driven into a furnace, con-

sists of two gases; ¹ about one-fifth being oxygen, and four-fifths azote. According to the present state of chemical knowledge, the oxygen alone is effective in producing heat; and the operation of blowing a furnace may be thus analyzed.

"1. The air is forced into the furnace in a condensed state, and, immediately expanding, abstracts heat from the surrounding bodies.

"2. Being itself of moderate temperature, it would, even without expansion, still require heat to raise it to the temperature of the hot substances to which it is to be applied.

"3. On coming into contact with the ignited substances in the furnace, the oxygen unites with them, parting at the same moment with a large portion of its latent heat, and forming compounds which have less specific heat than their separate constituents. Some of these pass up the chimney in a gaseous state, whilst others remain in the form of melted slags, floating on the surface of the iron, which is fused by the heat thus set at liberty.

"4. The effects of the azote are precisely similar to the first and second of those above described; it seems to form no combinations, and contributes nothing, in any stage, to augment the heat.

"The plan, therefore, of heating the air before driving it into the furnace, saves obviously the whole of that heat which the fuel must have supplied in raising it from the temperature of the external air up to that of 600° Fahrenheit; thus rendering the fire more intense, and the glassy slags more fusible, and perhaps also more effectually decomposing the iron ore. The same quantity of fuel, applied at once to the furnace, would only prolong the duration of its heat, not augment its intensity.

"The circumstance of so large a portion of the air driven into furnaces being not merely useless, but acting really as a cooling, instead of a heating, cause, added to so great a waste of mechanical power in condensing it, amounting, in fact, to four-fifths of the whole, clearly shews the defects of the present method, and the want of some better mode of exciting combustion on a large scale."

The reader will find it interesting to compare this account with the following by Dr. Ure.

"Wherever a forced stream of air is employed for combustion, the resulting temperature must evidently be impaired by the coldness of the air injected upon the fuel. The heat developed in combustion is distributed into three portions; one is communicated to the remaining fuel, another is communicated to the azote of the atmosphere, and to the volatile products of combustion, and a third to the iron and fluxes, or other surrounding matter to be afterwards dissipated by wider diffusion. This inevitable distribution takes place in such a way, that there is a nearly equal temperature over the whole extent of a fire-place, in which an equal degree of combustion exists.

"We thus perceive that if the air and the coal be very cold, the portions of heat absorbed by them might be very considerable, and sufficient to prevent the resulting temperature from rising to a proper pitch; but if they were very hot they would absorb less caloric, and would leave more to elevate the common temperature. Let us suppose two furnaces charged with burning fuel, into one of which cold air is blown, and into the other hot air, in the same quantity. In the same time, nearly equal quantities of fuel will be consumed with a nearly equal production of heat; but notwithstanding of this, there will not be the same degree of heat in the two furnaces, for the one which receives the hot air will be hotter by all the excess of heat in its air above that of the other, since the former air adds to the heat while the latter abstracts from it. Nor are we to imagine that by injecting a little more cold air into the one furnace, we can raise its temperature to that of the other. With more air indeed we should burn more coals in the same time, and we should produce a greater quantity of heat, but this heat being diffused proportionally among more considerable masses of matter, would not produce a greater temperature; we should have a larger space heated, but not a greater intensity of heat in the same space.

"Thus, according to the physical principles of the production and distribution of heat, fires fed with hot air should, with the same fuel, rise to a higher pitch of temperature than fires fed with common cold air. This consequence is independent of the masses, being as true for a small stove which burns only an ounce of charcoal in a minute, as for a furnace which burns a hundred weight; but the excess of temperature produced by hot air cannot be the same in small fires as in great; because the waste of heat is usually less the more fuel is burned.

"This principle may be rendered still more evident by a numeri-

cal illustration. Let us take, for example, a blast furnace, into which 600 cubic feet of air are blown per minute; suppose it to contain no ore but merely coal or coke, and that it has been burning long enough to have arrived at the equilibrium of temperature, and let us see what excess of temperature it would have if blown with air at 300° C. (572° F.) instead of being blown with air at 0° C.

"Six hundred cubic feet of air under the mean temperature and pressure, weigh a little more than 45 pounds avoirdupois; they contain 10.4 pounds of oxygen, which would burn very nearly 4 pounds of carbon, and disengage 16,000 times as much heat as would raise by one degree cent. the temperature of two pounds of water. These 16,000 portions of heat, produced every minute, will replace 16,000 other portions of heat, dissipated by the sides of the furnace and employed in heating the gases, which escape from its mouth. This must take place in order to establish the assumed equilibrium of caloric.

"If the 45 pounds of air be heated beforehand up to 300° C., they will contain about the eighth part of the heat of the 16,000 disengaged by the combustion, and there will be therefore in the same space one-eighth of heat more, which will be ready to operate upon any bodies within its range, and to heat them one-eighth more. Thus the blast of 300° C. gives a temperature which is nine-eighths of the blast at zero C., or at even the ordinary atmospheric temperature; and as we may reckon at from 2200° to 2700° F. (from 1200° to 1500° C.) the temperature of blast furnaces worked in the common way, we perceive that the hot-air blast produces an increase of temperature equal to from 270° to 360° F.

"Now in order to appreciate the immense effects which this excess of temperature may produce in metallurgic operations, we must consider that often only a few degrees more temperature are required to modify the state of a fusible body, or to determine the play of affinities dormant at lower degrees of heat. Water is solid at 1° under 32° F.; it is liquid at 1° above. Every fusible body has a determinate melting point, a very few degrees above which it is quite fluid, though it may be pasty below it. The same observation applies to ordinary chemical affinities: charcoal, for example, which reduces the greater part of metallic oxides, begins to do so only at a determinate pitch of temperature, under which it is inoperative, but a few degrees above, it is in general lively and complete. It is unnecessary, in this article, to enter into any more details to show the influence of a few degrees of heat, more or less, in a furnace, upon chemical operations, or merely upon physical changes of state.

"These consequences might have been deduced long ago, and industry might thus have been enriched with a new application of science; but philosophers have been and still are too much estranged from the study of the useful arts, and content themselves too much with the minutiae of the laboratory or theoretic abstractions. Within the space of seven years, the use of the hot-blast has been so much extended in Great Britain, as to have enabled many proprietors of iron-works to add 50 per cent. to their weekly production of metal, to diminish the weekly expenses of smelting by 50 per cent., and, in many cases, to produce a better sort of cast-iron from indifferent materials."

The following table is from a report of M. Dufresnoy, made to the director-general of mines in France in 1834.

Since that period the cost has been rendered much lower, and those who have made good arrangements for mineral property, can manufacture iron at a cost of little more than L.2 per. ton.

At the request of the British Association, Dr. Thomson examined the chemical constitution of hot-blast iron, and he gives the following as the result of his inquiry:

"(1.) The specific gravity of hot-blast iron is greater than that of cold-blast.

"The following are the specific gravities of eight specimens of cold blast iron:

"The specific gravity of the Muirkirk iron is considerably less than of that smelted at Carron and the Clyde Iron-Works; the mean of the eight specimens is 6.7034.

"It has been hitherto supposed that the difference between cast-iron and malleable iron consists in the presence of carbon in the former, and its absence from the latter; in other words, that cast iron is a carburet of iron. But in all the specimens of cast iron which we analyzed we constantly found several other ingredients besides iron and carbon. Manganese is pretty generally present in minute quantity, though in one specimen it amounted to no less a quantity than 7 per cent.; its average amount is 2 per cent. Silicon is never wanting, though its amount is exceedingly variable, the average quantity is about 1½ per cent.; some specimens contained 3½ per cent. of it, while others contain less than a half per cent. Aluminum is very rarely altogether absent, though its amount is more variable than that of silicon. Its average amount is 2 per cent.; sometimes it exceeds 4½ per cent., and sometimes it is not quite $\backslash frac\{1\}\{2\}$ part of the weight of the iron.

"Calcium and magnesium are sometimes present, but very rarely, and the quantity does not much exceed 1th per cent. In a specimen of cast iron which I got from Mr. Neilson, and which he had smelted from pyrites, there was a trace of copper, showing that the pyrites employed was not quite free from copper; and in a specimen from the Clyde Iron-Works there was a trace of sulphur. The following table exhibits the composition of six different specimens of cast iron, No. 1, analyzed in my laboratory, either by myself or by Mr. John Tennent.

"The constant constituents of cold-blast cast-iron, No. 1, are iron, manganese, carbon, silicon, and aluminum. The occasional constituents are copper, sulphur, calcium, and magnesium. These occur so rarely, and in such minute quantity, that we may overlook them altogether.

"The constant constituents occur in the following mean atomic proportions:

"2. I examined only one specimen of cast-iron, No. 2. It was an old specimen, said to have come from Sweden, but I have no evidence of the correctness of this statement. Its specific gravity was 7.1633 higher than any specimens of cold-blast iron, No. 1. Its constituents were,

"The presence of sulphur in this specimen leads to the suspicion that it is not a Swedish specimen; for as the Swedish ore is magnetic iron, and the fuel charcoal, the presence of sulphur in the iron is very unlikely.¹

"In this specimen, the atoms of iron and manganese are to those of carbon, silicon, and aluminum, in the proportion of 4½ to one, instead of 3½ to one, as in cast-iron No. 1.

"The atoms of carbon, silicon, and aluminum, approach the proportions of 7, 2, and 1, so that in cast-iron, No. 2, judging from one specimen, there is a greater proportion of carbon, compared with the silicon and aluminum, than in cast-iron, No. 1.

"Mr. Tennent analyzed a specimen of hot blast-iron, No. 2, from Gartsherry. Its specific gravity was 6.9156, and its constituents,

So that it resembles cast-iron, No. 1, in the proportion of its constituents. The carbon is almost the same as in cold-blast iron, No. 2, but the proportion of aluminum is four times as great, while the silicon is little more than half as much. The atomic ratios are, carbon, 4½; silicon, 0.67; aluminum, 2.28.

"3. Five specimens of hot-blast cast iron, No. 1, were analyzed. Two of these were from Carron, and three from the Clyde Iron-Works, where the hot-blast originally began; and where, of course, it has been longest in use. The specific gravity of these specimens was found to be as follows:

Mean..... 7.0623

"It appears from this, that the hot-blast increases the specific gravity of cast-iron by about $\backslash frac\{1\}\{2\}$nd part. It approaches nearer the specific gravity of cast iron No. 2, smelted by cold air, than to that of No. 1.

"The following table exhibits the constituents of these four specimens.

The mean of these analyses gives us,

101.285

Or, in the proportion of 6½ atoms of iron and manganese to 1 atom of carbon, silicon, and aluminum. In the cold-blast cast-iron we have,

"Thus it appears, that when iron is smelted by the hot-blast its specific gravity is increased, and it contains a greater proportion of iron, and a smaller proportion of carbon, silicon, and aluminum, than when smelted by the cold-blast.

At the request of the British Association, Mr. Eaton Hodgkinson examined the mechanical properties of hot and cold-blast iron. The following are his general results.

"Of the three columns of numbers in the table above, the first is the strength or other quality in the cold-blast iron; the second is that in the hot-blast; and the third is the ratio of these quantities.

"The results in this table contain nearly the whole information relative to the question of hot and cold-blast-iron that the preceding research affords; and before advertizing to them, it may be mentioned, that it is usual for the makers of cast-iron to divide it, when taken from the furnace, into three classes, called Nos. 1, 2, 3, differing from each other in the appearance and qualities of the material. No. 1 contains the softest and richest irons, those which have the largest crystals; No. 3, the hardest and densest irons, those with the least crystals; and No. 2, irons intermediate between the former two descriptions. Beginning with the No. 1 iron, of which we have a specimen from the Buffery Iron-Works, a few miles from Birmingham, we find the cold-blast iron somewhat surpassing the hot-blast in all the following particulars: direct tensile strength, compressive strength, transverse strength, power to resist impact, modulus of elasticity or stiffness, specific gravity; whilst the only numerical advantage possessed by the hot-blast iron is, that it bends a little more than the cold-blast before it breaks.

"In the irons of the quality No. 2, the case seems in some degree different; in these the advantages of the rival kinds seem to be more nearly balanced. They are still, however, rather in favour of the cold blast.

"Referring to the No. 2 iron, from the Carron Works in Scotland, we find the tensile, compressive, and transverse strengths, together with the modulus of elasticity and specific gravity, all higher in the cold-blast iron than the hot-blast, whilst the ultimate deflection and power of sustaining impact are greater in the hot-blast. The cold-blast iron is the better, but the difference is very small.

"In the iron No. 2, from the Coed-Talon Works in North Wales, the tensile strength is greater in the cold-blast than in the hot; but the resistance to compression is higher in the latter than the former, and that is the case with the specific gravity.

"So far as my experiments have proceeded, the irons of No. 1 have been deteriorated by the hot-blast; those of No. 2 appear also to have been slightly injured by it; while the irons of No. 3 seem to have benefited by its mollifying powers. The Carron iron No. 3, hot-blast, resists both tension and compression with considerably more energy than that made with the cold-blast; and the No. 3 hot-blast iron from the Devon Works, in Scotland, is one of the strongest cast-irons I have seen, whilst that made with the cold-blast is comparatively weak, though its specific gravity is very high, and higher than in the hot. The extreme hardness of the cold-blast Devon iron above prevented many experiments that would otherwise have been made upon it, no tools being hard enough to form the specimens. The difference of strength in the Devon irons is peculiarly striking.

"From the evidence here brought forward, it is rendered exceedingly probable that the introduction of a heated blast into the manufacture of cast-iron has injured the softer irons, whilst it has frequently mollified and improved those of a harder nature; and considering the small deterioration that the irons of the quality No. 2 have sustained, and the apparent benefit to those of No. 3, together with the great saving effected by the heated blast, there seems good reason for the process becoming as general as it has done." (C. A.)