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redness, in a glass tube with a very small orifice, the residue which is obtained by evaporating to dryness the green muriate of iron. It is a fixed substance, requiring a red heat for its fusion. It has a grayish variegated color, a metallic splendor, and a lamellar texture. It absorbs chlorine when heated in this gas, and becomes entirely converted into the volatile deutochio

Iron
Chlorine

According to Mr. Porrett.

2 primes iron

1 prime chlorine

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46.57 . 53'43

3.5 43.75 100.0 4.5 56.25 128.7

It is

duced to the oxygen scale, is 6.86, one-half of which, 343, is very nearly the determination of Berzelius. But Mr. Porrett, in an ingenious paper published in the Annals of Philosophy for October 1819, conceives, that, to make the theoretical proportions relative to iron harmoaize with the experimental results, we must consider 1.75, or the half of 35, as its true prime equivalent, or lowest term of combination. The ride. It consists, according to Dr. Davy, of protoxide will then consist of two primes of iron bo one of oxygen. M. Thenard, in his Traité, ol. ii. p. 73, says, 'The above oxide, obtained oy decomposing protosulphate of iron by potash or soda, and washing the precipitate in close vessels with water deprived of its air, consists, according to M. Gay Lussac, of 100 parts of iron, and 25 of oxygen. This determination would make the atom of iron 4.0; and is probably incorrect. This proportion is proved,' he adds, by dissolving a certain quantity of iron in dilute sulphuric acid, and collecting the evolved hydrogen. Now by this method extreme precision should be ensured.' 2d. Deutoxide of M. Gay Lussac. He forms it by exposing a coil of fine iron wire, placed in an ignited porcelain tube, to a current of steam, as long as any hydrogen comes over. There is no danger, he says, of generating peroxide in this experiment, because iron once in the state of deutoxide has no such affinity for oxygen as to enable it to decompose water. It may also, he states, be procured by calcining strongly a mixture of one part of iron and three parts of the red oxide in a stone-ware crucible, to the neck of which a tube is adapted to cut off the contact of air. But this process is less certain than the first, because a portion of peroxide may escape the reaction of the iron. But we may dispense with the trouble of making it,' adds M. Thenard, 'because it is found abundantly in nature.' He refers to this oxide, the crystallised specular iron ore of Elba, Corsica, Dalecarlia, and Sweden. He also classes under this oxide all the magnetic iron ores; and says, that the above-described protoxide does not exist in nature. From the synthesis of this oxide by steam, M. Gay Lussac has determined its composition to be,

The deutochloride may be formed by the combustion of iron wire in chlorine gas, or by gently heating the green muriate in a glass tube. the volatile compound described by Sir H. Davy in his celebrated Bakerian lecture on oxymuriatic acid. It condenses after sublimation, in the form of small brilliant iridescent plates. It consists, according to Dr. Davy, of Iron Chlorine

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35.1 64.9

100.00

By Mr. Porrett
4 primes iron
7.0 34.14
3 primes chlorine. 13.5 65.86 192.85
3. For the iodide of iron, see IODINE.

4. Sulphurets of iron; of which, according to Mr. Porrett, there are four, though only two are usually described, his protosulphuret and persulphuret.

The persulphuret of iron exists in nature. It has the metallic appearance of bronze, but its powder is blackish-gray. It is in fact the magnetic pyrites of mineralogy, which see among the ORES of IRON. By the analysis of Mr. Hatchett, and Professor Proust, it seems to consist of iron

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sulphur
Mr. Porrett represents it as composed of
2 primes iron
1 prime sulphur

.3.5 63.75
2.0 36.25

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63 . 37

100 57

His deutosulphate and tritosulphate are as

:

follows:-
Deutos. 3 primes iron

2 primes sulphur

5.25 57 100 4.00 43 76

Tritos.

4 primes iron.

7.0

3 primes sulphur

6.0

54 100 46 86

He conceives, that in Proust's experiments, as related in the first volume of Nicholson's 8vo.

Journal, descriptions of compounds corresponding to those two sulphurets are given.

the mineralogist. It consists, according to Mr. The persulphuret is the cubic iron pyrites of Porrett, of

1 prime iron. . 1.75 46.5 100.0 1 prime sulphur 12:00 53.5 114.2; and the mean of Mr. Hatchett's celebrated experiments on pyrites, published in the Philosophical Transactions for 1804, gives of iron 100 sulphur. 113

5. Carburets of iron. These compounds form steel, and probably cast-iron; though the latter contains also some other ingredients.

The

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Graphite or plumbago is also a carburet of iron, containing about ten per cent. of metal, which, calling the prime of iron 1.75, makes it a compound of twenty-one primes of carbon to one of metal.

Salts of iron. These salts have the following general characters :

1. Most of them are soluble in water; those with the protoxide for a base are generally crystallisable; those with the peroxide are generally not; the former are insoluble, the latter soluble in alcohol. 2. Ferroprussiate of potash throws down a blue precipitate, or one becoming blue in the air. 3. Infusion of galls gives a dark purple precipitate, or one becoming so in the air. 4. Hydrosulphuret of potash or ammonia gives a black precipitate; but sulphureted hydrogen merely deprives the solutions of iron of their yellow-brown color. 5. Phosphate of soda gives a whitish precipitate. 6. Benzoate of ammonia, yellow. 7. Succinate of ammonia, flesh-colored with the peroxide. Protacetate of iron forms small prismatic crystals, of a green color, a sweetish styptic taste, and a specific gravity 1.368.

Peracetate of iron forms a reddish-brown uncrystallisable solution much used by the calico printers, and prepared by keeping iron turnings, or pieces of old iron, for six months immersed in redistilled pyrolignous acid.

Protarseniate of iron exists native in crystals, and may be formed in a pulverulent state, by pouring arseniate of ammonia into sulphate of It is insoluble, and consists, according to Chenevix, of 38 acid, 43 oxide, and 19 water, in 100 parts.

iron.

Perarseniate of iron may be formed by pouring arseniate of ammonia into peracetate of iron; or by boiling nitric acid on the protarseniate. It is insoluble.

Antimoniate of iron is white, becoming yellow, insoluble.

Borate, pale yellow, insoluble.
Benzoate, yellow, insoluble.
Protocarbonate, greenish, soluble.
Percarbonate, brown, insoluble.
Chromate, blackish, insoluble.
Protocitrate, brown crystals, soluble.
Protoferroprussiate, white, insoluble.
Perferroprussiate, white, insoluble.

This constitutes the beautiful pigment called prussian blue. When exposed to a heat of about 400° it takes fire in the open air; but in close vessels it is decomposed, apparently, into carbureted hydrogen, water, and hydrocyanate of

ammonia, which come over; while a mixture of charcoal and oxide of iron remains in the state of pulverulent pyrophorus, ready to become inflamed with contact of air. See PRUSSIC ACID. Protogallate, colorless, soluble. Pergallate, purple, insoluble.

Protomuriate, green crystals, very soluble. Permuriate, brown, uncrystallisable, very soluble. See the chlorides of iron previously described. Protonitrate, pale green, soluble. Pernitrate, brown, soluble. Protoxalate, green prisms, soluble. Peroxalate, yellow, scarcely soluble. Protophosphate, blue, insoluble. Perphosphate, white, insoluble. Protosuccinate, brown crystals, soluble. Persuccinate, brownish-red, insoluble.

Protosulphate, green vitriol, or copperas. It is generally formed by exposing native pyrites to air and moisture, when the sulphur and iron both absorb oxygen, and form the salt. There is, however, an excess of sulphuric acid, which must be saturated by digesting the lixivium of the decomposed pyrites with a quantity of iron plates or turnings.

It forms beautiful green crystals, which are transparent rhomboidal prisms, whose faces are rhombs with angles of 79° 50′ and 100° 10′, inclined to each other at angles of 98° 37′ and 81° 23. Specific gravity 1-84. Its taste is harsh and styptic. It reddens vegetable blues. Two parts of cold and three-fourths of boiling water dissolve it. It does not dissolve in alcohol. Exposure to air converts the surface of the crystals into a red by separating the water of crystallisation, and a stronger heat drives off the sulphuric acid. Its constituents are 28.9 acid, 28.3 protoxide, and 45 water, according to Berzelius: consisting, according to Mr. Porrett's views, of 1 prime acid + 2 oxide + 7 water.

Persulphate. Of this salt there seems to be four or more varieties, having a ferreous base, which consists, by Mr. Porrett, of 4 primes iron +3 oxygen 10 in weight, from which their constitution may be learned.

The tartrate and pertartrate of iron may also be formed; or, by digesting cream of tartar with water on iron filings, a triple salt may be obtained, formerly called tartarised tincture of Mars.

Iron is one of the most valuable articles of the materia medica. The protoxide acts as a genial stimulant and tonic, in all cases of chronic debility not connected with organic congestion or inflammation. It is peculiarly efficacious in chlorosis. It appears to me, says Dr. Ure, that the peroxide and its combinations are almost uniformly irritating, causing heartburn, febrile heat, and quickness of pulse. Many chalybeate mineral waters contain an exceedingly minute quantity of protocarbonate of iron, and yet exercise an astonishing power in recruiting the exhausted frame. I believe their virtue to be derived simply from the metal being oxidized to a minimum, and diffused by the agency of a mild acid through a great body of water, in which state it is rapidly taken up by the lacteals, and speedily imparts a ruddy hue to the wan countenance. I find that these qualities may be imitated exactly, by dissolving three grains of

sulphate of iron, and sixty of bicarbonate of potash, in a quart of cool water, with agitation in a close vessel.

IRON BRIDGES, in modern engineering, are an invention exclusively British; and one of which the metropolis of this country contains two of the most complete specimens.

The first iron bridge erected was that of cast iron over the Severn, about two miles below Colebrook Dale, Shropshire, between the villages of Brosely and Madeley. It consists of five ribs forming the segment of a circle; and having its chord line 100 feet in length, and its versed sine forty-five feet; making its curve almost a semicircle. The arch springs at about ten feet from low water mark, which makes the entire height from the water to the vertex of the soffit fifty-five feet On the arch-shaped ribs the roadway is formed by pieces of cast iron and plates which carry the road. This bridge was cast at the Colbrooke Dale foundries by Mr. Abiah Darby, and erected in 1777. The curvature of the exterior concentric arches, which assist in supporting the adway, though somewhat too great for the most favorable exertion of their resistance, leaves them still sufficiently strong for the purpose intended; and the partial failure, which accidentally occurred, bears testimony rather to the merits than to the faults of the bridge, as they would be estimated in any other situation: for the lateral thrust, which it is desirable to reduce as much as possible, was here actually too small, and the abutments were forced inwards, by the pressure of the loose external materials, forming the banks, against which the abutments pressed.

On the whole, if not so elegant a structure as some that have succeeded it, this is a most respectable and scientific edifice: we subjoin a sketch of it.

Colebrook Dale Bridge.

The next cast iron bridge seen in this country was designed by the celebrated Thomas Paine: it was intended to be erected in America, and was an imitation of a catenarian curve: the Messrs. Walkers of Rotherham were the founders. Paine, however, became involved in his circumstances, and the bridge, after being exhibited at Pancras, was taken to pieces and the materials used in the bridge at Wearmouth, erected under the direction of R. Burdon, esq. and Mr. Thomas Wilson, and which was completed in 1796. It is near Sunderland, and is often called by the name of that place. This beautiful edifice springs seventy feet above low water mark; and the arch rises thirty feet, leaving a height of 100 feet in the whole for the passage ships in the middle

of the stream: the span is 240. The abutments rest on a solid rock, but their own internal solidity appears to be doubtful. The weight of iron in it is 250 tons; 210 of them being of cast, and forty of wrought iron.

In the same year a bridge was erected at Buildwas, near Colebrook Dale by Mr. Telford; 130 feet in span, weighing 174 tons; and rising only seventeen feet in the roadway, but furnished on each side with a stronger arch, of about twice the depth. This indeed extends to the top of the railing, and assists both in suspending the part of the road which is below it by means of king-posts, and supporting the part nearer the abutments by braces and shores. The breadth is eighteen feet; and the construction would not be so easily applicable to a wider bridge.

Another iron bridge was erected in 1796 on the Parrot at Bridgewater, by the Colebrook Dale Company, consisting of an elliptic arch of seventy-five feet span, and twenty-three feet height, and resembling the bridge at Wearmouth in the mode of filling the haunches with circular rings.

Even failures may be useful to record as warnings. Two occurred at this time, one at Yarn in Staffordshire, and another in Herefordshire. The former was in a bridge of 180 feet span over the Tees; and the latter on a similar erection on the Tarne: both fell to pieces on the removal of their centres.

In 1803 an iron bridge of 181 feet span, and sixteen and a half rise, was completed at Staines, on the general model of that at Wearmouth, but its parts were connected somewhat differently. It began to sink, and some of the transverse pieces broke in a short time after it was finished, when upon examination it was found that one of the abutments had failed: and when this was repaired the other gave way in a similar manner. It was pushed outwards horizontally; and the architect seems to have trusted to the firmness of the iron, and the excellence of the workmanship, neglecting the calculation of the lateral thrust. The derangement, however, was not material.

Mr. Rennie's beautiful bridge over the Witham at Boston in Lincolnshire, we believe, was next in order of time. There is not a more elegant structure of this kind in the kingdom. The span is eighty-six feet, and the rise five and a half only; but the abutments are well constructed, and it has stood securely, notwithstanding the fracture of some of the cross pieces of the frames, which had been weakened by the unequal contraction of the metal in cooling.

Messrs. Jessop have erected two iron bridges at Bristol, of 100 feet span, rising fifteen; each of them contains 150 tons of gray iron: the expense of each was about £4000. The chords of their arches are segments of circles, of 100 feet in dimension, their versed sine fifteen; the diameter of the complete circle, of which the arch is a segment, 182 feet; and the height of the frame work of the arch at the vertex, two feet four. See plate II. fig. 5.!

In 1803 light iron bridge, for foot passengers only, was thrown across the Seine, opposite to the gate of the Louyre. Narrow stone piers, leaving the lateral thrust uncompensated, are its

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supporters, and there is great apparent deficiency in strength; but it is improbable than any failure should occur in such a situation, supposing the construction of the bridge itself to be sound. See plate I. fig. 2.

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In 1811 Mr. Telford threw an iron bridge over an arm of the sea, at Bonar in Sutherlandshire, Scotland. It consists of an arch whose chord is 150 feet, versed sine twenty feet, diameter of the circle, of which the arch is an abscissa, 301 feet, and the height of the frame-work of the arch at the vertex three feet.

But the two noble erections of this kind on the Thames have attracted the principal attention of engineers. Vauxhall Bridge, the first of them, was opened in August 1816: it consists of nine arches of cast iron, each of seventy-eight feet span, and between eleven and twelve feet rise. The architect was Mr. Walker. The breadth of

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the roadway is thirty-six feet clear. The arches resemble those of Messrs. Jessop's bridges at Bristol; but the bridge has, on the whole, a lighter appearance, while the abutments are more compact and solid. See plate I. figs. 3, 4.

The Southwark, or Trafalgar Bridge, at the bottom of Queen Street, Cheapside, has been considered the finest iron bridge in the world. It consists of three magnificent arches, resting on granite piers and abutments. Mr. Rennie was the architect: and the arches were cast by Messrs. Walker and Yates, late of Rotherham in Yorkshire. The chord of the middle arch is 240 feet. Its curve is the segment of a circle of 624 feet diameter; its versed sine twenty-four feet; and the height of the frame-work of the arch at the vertex, six feet. See plate II. figs. 1, 2, 3.

The following is the weight of half of the middle arch of this bridge :

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Span 240 feet. Rise twenty-four. Depth of the blocks or plates at the crown six feet; at the pier eight feet.

We may notice that a still more splendid iron bridge of one arch was projected by Mr. Telford, and for some time under the consideration of a committee of the house of commons, as a substitute for London Bridge. The chord of the arch was to be 600 feet; and it was to have its centre not less than sixty-five feet high in the clear: but the opinions of respectable architects were so divided as to its merits that the plan was, relinquished.

Minor erections of this kind are now common, and Mr. Telford has erected several aqueduct bridges on a large scale. One of these, near Wellington in Shropshire, cast by Messrs. Reynolds, was completed in 1796. It is 180 feet long, and twenty feet above the water of the river, being supported on iron pillars.

A large one was cast by Mr. Hazledine, for carrying the Ellesmere Canal over the river Dee, at Pontcysylte near Llangollen.. It is supported 126 feet above the surface of the river, by twenty stone pillars, and is 1020 feet in length and twelve feet wide. See plate II. fig. 4.

IRON FOUNDRY. See IRON MANUFACTURE. IRON MANUFACTURE. This important branch of our manufactures has tended most materially towards establishing the commercial superiority of Great Britain over every other competitor. It is true that there are, many parts of Europe that excel us both in the richness and quantity

of their iron ores; and we remember one instance in which a large mountain of pure metallic iron was found in an extensive mineralogical district in Sweden. But, generally speaking, the excellence and abundance of our fuel, with the vast capitals employed by British iron masters, have enabled us to materially improve the foundry process.

We may commence our examination of this subject by a reference to the process employed by the Romans.

The ancient mode of reducing the ores of iron is thus described by Agricola. A mass of brick-work was raised five feet in length and breadth and three feet and a half high, resembling a smith's hearth, except that in the middle of this was sunk a cup-shaped cavity or crucible, one foot in depth and half a foot wide, in the upper part of which was made a hole opening into a channel through the brick-work. This hole being closed with clay, the crucible was filled with lighted charcoal, heaped up so as to be above the level of the hearth; a blast of air was then admitted through a pipe let into the wall in the same manner as a smith's forge, and so contrived that the focus of the blast should be just above the centre of the crucible. Charcoal alone was added from time to time, till the heap became thoroughly hot, and then, at the discretion of the workmen, the ore, in very small

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