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hot air blast, they require more power with the latter, a larger quantity of air being necessary in running the furnace. It is not said that the dimensions of the blast pipe nozzle have been changed. At Ancy-le-Franc in August, 1834, the pressure at the governor remaining constant, that at the tugere was observed to fall to one-half, when the air was heated. The size of the nozzle was increased, but there was not an adequate power to supply the air required, and the yield of the furnace diminished.

Tuyeres, cooled by water, have been substituted for the ordinary ones in furnaces using the heated air blast; the cooling effect of the blast being taken away, the ordinary tuyere is rapidly burned out. Cast-iron water tuyeres have been found to last longer than those of wrought iron; they wear out in from three to six months.

many establishments the blast pipe nozzle is permanently attached to the tuyere, an arrangement which answers well when it is not necessary to clean out the tuyeres. When this is necessary, the common arrangement is to be preferred, and this is generally the case in the French works even where charcoal is used.

When the nozzle is not closely fitted to the tuyere, the blast is slightly cooled before it gets into the furnace, and part of it does not pass in.

II. On the EFFECTS AND ADVANTAGES OF THE Hot Air Blast. The effect appears to be to increase the heat within the furnace, so that a refractory ore is fused; any stoppage in the furnace is prevented, and the working is more readily resumed after the furnace has been cooled. Less fusible ores may be used, less flux is required for their reduction, the slags are more fusible, and become spongy if water is thrown upon them when incandescent. This property has been observed only in the Styrian furnaces and others where charcoal is used as a fuel, and the ore is a manganesian carbonate of iron. Further, grey pig iron is obtained with every kind of ore, this variety of iron requiring a high temperature for its production. Generally the heated air and combustible gases which issue' from the trunnel head, are diminished in quantity and the heat is more concentrated in the lower parts of the furnace; a source of great advantage, but which causes a more rapid destruction of the hearth and boshes.

The working of the furnace when heated air is used is comparatively easy, there are fewer cases of clogging and they are readily remedied; the tugeres are almost always free, no slag collecting and hardening about them. Frequently, a clogging in the furnace may be removed by raising the temperature of the blast. The advantages may be succinctly enumerated as follows.

1. A change in the iron which becomes more grey, and even black, and the slag is more fluid than when cold air is used.

2. An increase in the quantity of ore which a given weight of fuel will bear, whence results a diminution in the quantity of fuel required to produce a ton of metal, after the fuel required to heat the air has been taken into account.

3. A diminution in the quantity of flux, to which there are, however, exceptions.

4. An increase in the daily yield of a furnace.

We do not enumerate among the advantages that of using crude coal, because it has been ascertained both in Wales and at Decazeville, that this may be done with the cold air blast.

In regard to the quality of the iron produced by the hot air blast, the following facts have been collected. Vol. XVII. No. 1.- JANUARY, 1836.

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It has been asserted that iron, thus obtained, requires to be remelted when it becomes duly tenacious, and yet the Lyonese founders complain that the iron of Vienne is weak. On the other hand, iron from the Torteron furnace was cast into shells which required more powder to burst them than similar ones made from iron procured by the cold blast, the strength having been nearly double, in the former case, of that in the latter.

In England there appears to have been no sensible difference between the castings made from iron obtained by the two different methods.

The same uncertainty prevails in regard to the forged iron obtained from pigs reduced by the aid of the hot air blast

. M. Dufrenoy and M. Debilly, consider the notions prevalent on this subject in England, to be founded in prejudice. My observations in the South of France have shown that there is, if any, a very slight difference in the quality of the iron in favour of that made by the cold blast. At one of the furnaces it was suggested that silicious ores gave a worse iron by this process than by the cold air blast, the great heat facilitating the union of the silicium with the carbon and iron. A careful analysis is required to demonstrate this theory, in favour of the probability of which it may be stated that at Firmy, where a very silicious ore is used, the iron made by the hot air blast is worse than that by the other process, and when refined, produces a worse malleable iron. It is remarkable, moreover, that the best iron is obtained when the ore is in excess in charging the fnroace, in which case the iron is reduced at the lowest temperature.

[TO BE CONTINUED IN OUR NEXT.]

Composition and specific gravity of different kinds of Glass. Ordinary flint-glass, according to Mr. Faraday's analysis, consists, in 100 parts, of silica 51.93, oxide of lead 33.28, potash 13.77, with minute portions of other substances. A specimen of the same kind of glass, manufactured for telescopes by the late M. Guinand, yielded the same chemist, silica 44.3, oxide of lead 43.05, and potash 11.75. Mr. Faraday found the specific gravity of M. Guinand's glass to be about 3.616, that of ordinary flint-glass 3.290, that of plate-glass 2.5257, and that of crown glass 2.5448.

Glass has usually been considered, without much actual inquiry into the subject, to be strictly a chemical combination of its ingredients, and in all respects a very perfect artificial compound. This, however, is far from being the truth, as will appear from the following facts. That the alkali in common glass of all kinds is in a very imperfect state of combination, many circumstances concur to evince. For example, Mr. Griffiths has shown, that if a small quantity either of flint-glass, or of plate-glass, be very finely pulverized in an agate inortar, then placed upon a piece of turmeric paper and moistened with a drop of pure water, strong indications of free alkali will be obtained; and that if the pulverization be very perfect, the alkali can be detected in other kinds of glass, containing far smaller quantities of it. This proves, that in whatever state of combination the alkali may be, it is still subject to the action of moisture. That flint-glass is by no means a compound resulting from very strong chemical affinities, and that the oxide of lead which it contains is as imperfectly combined as the alkali, has been shown experimentally by Mr. Faraday, and also appears from the tarnish which is produced on its surface by exposure to sulphuretted vapours, owing to the combination of sulphur with the lead, Glass which has long been exposed to the weather, frequently exhibits a beautiful iridescent appearance, and is so far decayed, that it may be scratched with the nail. The glass of some bottles of wine which had lain in a wet cellar near the Bank of London upwards of 150 years, examined by Mr. Brande, was soft, and greatly corroded upon the surface, in consequence of the partial abstraction of its alkali. After reciting some of these facts, and others of a similar description, Mr. Faraday observes, "Glass may be considered rather as a solution of different substances one in another, than as a strong chemical compound; and it owes its power of resisting (chemical] agents generally, to its perfectly compact state, and the existence of an insoluble and unchangeable film of silica, or highly silicated matter, upon its surface." See Mr. Faraday's Bakerian Lecture on the manufacture of glass for optical purposes; Phil. Trans. 1850, pp. 46-50.Parke's Chem. Cal, by Brayley. (Arcana, &c. 1835.)

Carriage Wheels. On the 27th of May, at the Institution of Civil Engineers, Mr. Walker's paper, on the subject of the most advantageous form for wheels of different kinds of carriages, having been read, a member remarked that there were some practical objections to the use of horizontal axles, which were not alluded to in Mr. Walker's communication-one, the difficulty of making the wheel perfectly secure from coming off the axle, as a greater strain is unavoidably thrown on the linchpin. The wheels of ordinary country wagons are usually much dished, and the axles slightly inclined downwards, by which arrangement, the principal strain is thrown on the shoulder of the axletree, and a very ordinary description of linchpin will answer the purpose. As far as regards friction, and, consequently, an easy draught for the horse, the straight axle and cylindrical wheel have the preference; but, for safety, strength, and durability, he thought the inclined axle and dished wheel superior; besides which, there exists much practical difficulty in constructing carriages with horizontal axles and cylindrical wheels. It was remarked, that one reason for the conical wheel being so much adhered to in practice, was the greater liability of the tire on the cylindrical wheel to get loose; by the constant rolling of a heavy weight frequently on a small

extent of surface, the tire becomes slightly elongated, and, on a cylindrical wheel, gets loose, and may occasion accidents; the conical wheel provides against this, by its greater elasticity, and the tendency it has to become more flat in the dishing, and, in a slight degree, to stretch out the periphery. It was stated that, at first, the cylindrical shape was adopted in Jones' patent iron wheels; but it was found that, with upright wheels, the width of track was required to be seven feet, and some of the streets do not admit of such a vehicle passing; also, in crowded thoroughfares, the nave is exposed, and liable to come in contact with other carriages.-It was stated, that a wheel of a new construction had lately been attempted, and was likely to become an improvement; the rim and nave are of cast-iron, and the spokes of wroughtiron; a wooden band is put round the cast-iron rim, which again is surrounded and fastened on by a wrought-iron tire, secured in the ordinary manner.-It was mentioned that, in Austria, cylindrical wheels are invariably used for wagons and heavy carriages, but for light vehicles the dished wheel is generally

preferred. – A member stated, that in the neighbourhood of Edinburgh, the common stone carts belonging to the Cragleith, and other quarries, are generally made with broad cylindrical wheels.

On June 3, the conversation on the subject of the best form for wheels of carriages was resumed. An ingenious method was adopted by Messrs.

Jones, to exhibit the friction occasioned by conical wheels: a carriage was run upon the edges of loose boards, placed side by side; it was shown that, while the board under the middle part of the wheel remained stationary, that at the outside was pushed forward, and the board on the inside, backward; such, however, can only occur when the whole breadth of the wheel touches the ground, which is seldom the case, a wheel of nine inches having frequently a bearing of only three inches, in consequence of the middle tire being made of larger diameter.-Arcana, &c., 1835.

Rutler's Heat Process. Dr. Daubeny brought before the meeting the economical employment of coal-tar in connexion with water as fuel, according to the method lately suggested by Mr. Rutter. * A discussion then arose as to whether the water in this case acts chemically or mechanically, or both, in facilitating the combustion of the tar. Mr. Macintosh stated, that by repeated experiments he had found, that coal-tar gave no more heat when burned than an equal weight of splint coal, the kind preferred, where a long continued heat is required. Mr. Low also stated, that from long experience he could affirm, that the use of water along with coal-tar was productive of no benefit whatever, and that 3 gallons, or 33 lbs. of coal-tar, give an equal amount of heating effect, fully, to 40 lbs. of coke, made from the Newcastle coal of the Hutton seam. From the discussion on this subject, which was protracted for some time, it appears to be established, 1. That tar may be used as fuel, but that it does not give much more heat than the same weight of the best coal. 2. That when mixed with water, it flows more easily through tubes, but does not appear to evolve more heat than when used alone.--Jameson's Journal.

Progress of Physical Science.

On some elementary laws of Electricity, by W. Snow Harris, Esq. F. R.s.&c.

Mr. Harris has opened some new views of Electricity by decidedly original experiments; he concludes that many of the phenomena treated in the course of his paper do not seem to have been contemplated in the more perfect theories of Electricity,

The new instruments invented by Mr. Harris to render research more easy and results certain and measurable are 1. A new electrometer by divergence. 2. An electrometer measuring directly the attractive force of an electrified body in terms of a known standard of weight, estimated in degrees upon a graduated arc. 3. A unit of measure for electrical charges, furnished by a small jar with an attached discharging electrometer. 4. A balance electrometer. By the aid of these instruments, or of a combination of them with other general means of experiment, and with the electro-thermometer previously invented, the following laws have been established.

1. When the amount of surface of an electrical conductor is varied, or the quantity of electricity distributed upon a given surface, the figure of the surface being the same, the force of attraction for other bodies varies inversely as the square of the relative superficies. Thus a given quantity of electricity being divided between two conductors

• Originally suggested by Capt. Morey, of New Hampshire.

of equal surface, each one exerts an attractive force of only fourth that which is exerted when the same quantity is distributed upon a single conductor,

This law Mr. Harris considers to be connected with the following facts. That the force of attraction exerted between the two bodies, one of which is electrified, is more or less diminished by the presence of a neutral, or other body, sharing the attraction. This leads him to distinguish three elements peculiar to the conditions of electrical accumulation. 1st. The comparative quantity actually accumulated. And. The quantity not sensible to the electrometer. 3d. The quantity appreciable by the electrometer. The first he calls quantity; the second controlled quantity or controlled action; and the third free quantity or free action. Mr. Harris limits the term electrical tension to the electric force of a given quantity accumulated in a given space." The term intensity is different from this being "applied to the indications of an electrometer and immediately referable to what [he] has called the free action," that is to the operation of either a part, or the whole of the total force in a given direction, up to the point of discharge.” The "intensity” by the law above stated varies as the square of the density of the electrical stratum or as the square of the tension.” In connexion with this subject Mr. Harris examines Singer's theory that the diminished intensity observed in disposing a given quantity of electricity on an extended surface is referable to the attractive force of the atmosphere, to the influence of which the electric particles become more extensively exposed. This view he finds to be inconsistent with direct experiment as it is also with the law above stated,

2. The heating effects of the same quantity of electricity is the same, the number of jars, and length of circuit through which the discharge is made, being the same. A change in the tension of the electricity produces no change in heating effect. This fact had previously been shown by Faraday, in regard to the magnetic and chemical effects of electricity.

3. The distances of discharge between two electrified conductors varies directly as the quantity accumulated upon a given surface. The distance through which an electrical accumulation can discharge itself in air of a given density is an accurate measure of the comparative quantity in a unit of surface. The same law of relation holds when the surfaces are varied, thus the distance of discharge is invariably as the discharging surface, for a given quantity of electricity. This third law is consistent with the first.

4. It follows from the third law that the resistance of the atmosphere to the passage of electricity, is the same for all distances, being produced simply by its pressure.

When the density of the air is diminished, the distance of discharge (striking distance) varies inversely as the density, the quantity of electricity accumulated on a given surface being the same. The quantities of electricity required to overcome a given striking distance vary directly as the density of the air. These conclusions accord with the first law.

The temperature of air has no effect on its conducting power for electricity otherwise than as it changes the density. If therefore heat be material it is a non-conductor of electricity, not impairing the insulating power of air which contains it.

5. The resistance of conductors to the passage of electricity is of a different kind from that of air and other non-conductors; it is, other things being the same, directly as the length of the circuit.

Heat impairs the conducting powers of conductors for electricity, as has been asserted by Davy and Professor Christie, but denied by Professor Ritchie.

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