b. In Fig. 145, the other arrangement of the heating pipe is represented. It differs in no respect from the one just exhibited,

except in the fact that the heating pipes are straight and set almost vertically, connected at the top by a short curved pipe. The straight pipes are frequently from six to eight feet in length at large furnaces; diameter inside five inches; metal one inch thick.

Other forms of heating apparatus are but little in use, and are less useful than those described. For small charcoal furnaces, an apparatus five feet long, consisting of eight pipes, is sufficient. At large anthracite coke furnaces, we meet with an apparatus of thirty and more pipes for each furnace. No general rule with respect to the number of pipes, and the extent of surface required, can be given; but conclusions derived from experience prove that a surface from three to four feet square is, when exposed to the heating flame, large enough to heat an amount of air sufficient

for one ton of iron per week. The heat, in the interior of such a stove, is not very great; but the furnace, and frequently the whole apparatus, are lined with fire brick. This is more durable, and saves more fuel than red or common brick. Heavy brickwork is very advantageous at hot air furnaces, for changes in the temperature of the blast should be avoided.

c. At a blast furnace, where hot blast is employed, the tuyere can not be left open around the nozzle. We are thus deprived of the means of getting at it directly. As it is necessary that the tuyere should be cleaned, an arrangement is made to get at it from the cap of the pipe near a, Fig. 146. A small hole in the cap Fig. 146.

admits a three-quarter inch round rod, which is pushed in until it reaches the tuyere. This hole is closed by a short iron stopper. The cap a, on which the nozzle is fastened, must be light and easily movable, above the valve; for, as we have stated, some work is necessary to be done at the tuyere. The cap and nozzle should be light, and movable with facility, and therefore the best material from which they can be made is sheet iron. In case the heatingstove is at the top of the blast furnace, the blast pipe may be led from above to the tuyere. This is advantageous where but one tuyere is to be supplied; but where two or more tuyeres are in

the furnace, it will be found preferable to lay the pipes below the bottom stone. In case there is no surplus of heat, and we wish to preserve, as much as possible, the heat in the blast, the hot airpipe, leading from the stove to the tuyeres, may be packed in sand, and laid in a wooden box; or, it may be surrounded by a coating of loam. The pipes closest to the fire must be secured against the violent action of the flame, by a protection of brickwork or fireclay. It is advisable to have a direct communication between the tuyere and the blast machine; cold blast can thus be led directly, or around the heating stove, into the furnace when it is required. The hot air-pipes are sometimes injured without apparent cause, and the stopping up of the furnace would result from this accident, were we to stop the blast machine.

d. In the cap of each nozzle there is a hole, to which the manometer is attached; this enables us, by introducing a thermometer for a short time, to measure the degree of heat of the blast. The thermometer must be sufficiently long to indicate the degrees of heat below the boiling point of mercury; a greater length would be superfluous. Glass instruments are very liable to be broken, particularly at furnaces; and as it is a matter of considerable importance to know the temperature of the blast, a more practical means of measuring heat, namely, by alloys of metal, is resorted to. A small quantity of such a composition, or of pure lead, is put into a small vessel of copper or iron, which is fastened to a thin wire, and let down into the centre of the blast pipe. After obtaining a composition which melts at a certain temperature, no further experiments are necessary; and such a test may be applied when a doubt arises in our minds as to the heat of the blast. For practical purposes it is unnecessary to know exactly the degree of this heat. An approximation to correctness, will, in this case, answer every purpose. Pure lead melts at 470°; mercury boils at 660°; tin melts at 380°, tinner's solder at 410°, type metal at 350°, and a mixture of type metal and tinner's solder at 300°. We shall annex a few alloys, which may be used to measure lower temperatures; but with the caution that the melting point of the alloys is somewhat raised after each experiment. A mixture of five parts of lead, three parts of tin, and eight parts of bismuth, melts at the heat of boiling water, or 212°. The addition of mercury makes it still more fusible. Three parts of bismuth, one of lead, and one of tin, melt at 200°. Bismuth melts at 480°; but its

alloys are very fusible. With the addition of bismuth to tinner's solder, all the degrees between 200° and 500° may be produced. Nevertheless, a mercury thermometer, inclosed in a metal capsule, and suspended on wire, to prevent the contact of the glass and metal, is the best and easiest method of measuring the heat of the blast.

II. Theory of Hot Blast.

Hot air, for the alimentation of combustion, has been employed for a number of years; still, a clear and comprehensive demonstration of the cause and effects of its action, in the manufacture of iron, remains yet a desideratum. We shall endeavor to elucidate the subject as clearly as possible. The effect of the application of hot blast is threefold: First, it saves fuel in the direct proportion of the temperature of the aliment to the temperature of combustion. Secondly, the supply of hot air to smelting operations increases the reducing power of the gases, by promoting the combination of oxygen and carbon. Thirdly, it promotes

the chemical union of the particles of fluxes. This union is occasioned by heat, and may therefore be considered a mechanical operation.

a. The first of these advantages is limited in its extent; for, if we heat the air, designed for the nourishment of fuel, beyond a certain degree, it ceases to add to the increase of temperature, as well as to economize fuel. A theoretical explanation of this fact may be found in the greater facility with which matter combines when warm with excited polarity. This polarity occasions a saving of fuel, because, where other conditions exist, particles of cold air may escape uncombined, and, of course, absorb heat and reduce temperature. Hot blast in some measure prevents this result. But if all the oxygen is combined with the carbon, no further rise of temperature is possible. In fact, were all the oxygen of cold air to combine with carbon and hydrogen, and were no air to escape but that which formed water or carbonic acid, there would be no rational basis upon which a rise of temperature, or economy of fuel by hot blast, could be hypothecated. Several authors have attempted, by very elaborate theoretical investigations, to demonstrate that hot air produces a higher temperature, independently of the above cause; and they have endeavored to show that the temperature of combustion may be raised, by a judicious application of hot blast, 500° beyond the point at which combustion takes

place with cold air. But we must be permitted to doubt the correctness of a conclusion, arrived at by means of mathematical calculus, in relation to a subject concerning which the laws of chemistry alone are competent guides. These are simple and comprehensive, and based upon actual experiments; and they do not favor such abstract views concerning the increase of temperature by means of hot air, but they clearly explain an increase caused by a given combustion. If the increase of temperature depends upon the facilities which hot air imparts to combustion, it is evident that there must be a point in the temperature of the hot air at which its economical advantages cease-that there must be a point, in fact, at which the largest quantity of oxygen is converted into carbonic acid. If the alimentary air is cold, a diminution of temperature in consequence of imperfect combustion, will take place; if it is too hot, a lower temperature will result, because the combustion is so thorough, that carbonic oxide gas, instead of carbonic acid gas, will be formed. By referring to the second chapter, this matter may be more clearly understood. The highest heat obtained from the combustion of fuel and the application of hot air, which results in advantage, has been found to be about 500°. or, the melting point of lead. This number, of course, varies according to the kind of fuel employed. A higher heat will be advantageous if we burn anthracite, and a far lower temperature, if we burn charcoal. If we do not need a high temperature for a given operation, we may economize fuel by spreading the heat over a large, surface.

Hot air effects a saving of fuel in proportion as its temperature is the complement of that in the furnace. In nearly every instance of combustion, unburnt air passes through the fuel. The lower the temperature of combustion, the greater the quantity of air which will pass uncombined; that is to say, the heat of the blast or the nourishing air, which adds directly to the heat of combustion, will be the more apparent, the lower the heat in the furnace.

From these considerations, we may conclude that, where the highest possible temperature is desirable, the application of hot air in combustion is indispensable. But there is a limit to this application; and the air suitable for one kind of fuel may be too hot or too cold for another kind. There is no direct saving of fuel in cases in which the air is to be heated by separate fuel, but only in cases where it can be heated by the waste heat of the furnace itself, or by some other means. The percentage of fuel saved, in the latter case, is equal to the temperature applied to it. It thus follows that,