When we consider that the coals are thrown into the furnace in large masses, sometimes 20 lbs. in weight, it will scarcely excite surprise that the space required by the coal, before being converted entirely into coke, is above one-half of the whole depth of the furnace.

The tabulated composition of the gases further shows that the quantity of nitrogen in the gaseous mixture, taken as a depth of fourteen feet, is at a minimum, while the olefiant gas, carburetted hydrogen, and hydrogen, is at a maximum. As the latter gases are formed from coal only under the influence of an elevated temperature, we draw from this circumstance the conclusion, that the process of distillation of the coal reaches its maximum at a depth of fourteen feet. We remarked, in describing the experiments in detail, that the gases were free from the vapors of tar to a depth of fourteen feet, but that they became richly laden with them on attaining a depth of seventeen feet. The absence of these vapors from the upper part of the furnace proves that they suffer decomposition as they pass through the upper lapers of red-hot coal. The water ascending through these layers must also suffer decomposition, and this fact explains the irregularity in the proportions between the carbonic acid and carbonic oxide.

When we compare with each other the different quantities of carbonic oxide and carbonic acid at various depths of the furnace, we see a complete absence of any mutual dependence, contrary to what was observed to be the case in the smaller German furnaces fed with charcoal. In order to understand this phenomenon, it is necessary to consider attentively the conditions under which the inaterials are exposed.

We have already seen that the coal has to travel twenty-four feet, from the mouth to the boshes of the furnace, before it is deprived of its volatile carbonaceous products, hygroscopic water, and water formed by the distillation. Now, even if we admit that the temperature in this part of the furnace is never so much lowered by, the uninterrupted gasification of the coal as to prevent the reduction of the iron ore, by which carbonic oxide is converted into carbonic acid, still the ore would always be exposed, not only to the deoxidizing influence of the furnace gases, but also to the oxidizing powers of the steam evolved from the coal, which has escaped being coked. The projection of the coal into the furnace in large pieces has, therefore, the effect of subjecting the ore to a simultaneous reduction and oxidation, on account of which the relation between the carbonic acid, carbonic oxide, and hydrogen, is made to depend upon local circumstances in the upper part of the furnace. Now, when we further consider that the carbonic oxide and carbonic acid escaping from the mouth of the furnace, and from the part superior to the boshes, are almost in equal proportion, we are compelled to look for the cause of reduction of the ore in a region of the furnace still deeper. However, all doubt as to this fact disappears when we refer to the proportion between the nitrogen and oxygen of the gases collected. If the reduction of the ore and evolution of carbonic acid from the limestone had been completely effected above the point of the furnace to which we reached, the gases formed below would have contained their nitrogen and oxygen in the same proportion as in air, and would not have become richer in oxygen gas. But it will be seen that this is really

not the case, by the following table of the varying proportions of oxygen and nitrogen in the gases collected from the various depths:

79.2 79.2 79.2. 79.2 79.2 79-2 79.2 79.2 79.2 24.9 23.6 24.6 19.5 25.7 23.7 28.2 27.7 27.8

We see, from this series of numbers, that the relation of the gases, as regards their proportions from the mouth of the furnace downwards, is quite the reverse of that observed in the German furnaces. At first sight the circumstance strikes us as very inexplicable, because we do not know any chemical process in the furnace capable of diminishing the amount of oxygen contained in the gases; but we are enabled to explain this anomaly on an attentive consideration. The diminution of oxygen begins chiefly at the point where the gases generated by the combustion of coal become developed. The proportion of these gases to each other shows that, when liberated from the coal, they cannot mix quite uniformly with the column of air ascending from the lower parts of the furnace. Hence the gas collected at this region of the furnace is richer in the gaseous products of distillation of coal than would correspond to its average composition; the hydrogen, for example, actually increases to above 12 per cent. If we suppose, as we have done in the above numerical series, that the hydrogen is derived from the decomposition of water at the expense of the carbon, the quantity of oxygen could not decrease, whatever may be the proportion of the gases generated in this way at the various points of the furnace. But if, as we must suppose, the hydrogen is principally derived from the olefiant gas and empyreumatic oils decomposed by the high temperature, the calculation leads us to a smaller quantity of oxygen than really represents the truth. This fact warrants the conclusion,-That the mean composition of the gases cannot be determined at that point of the furnace where the evolution of gas by distillation is at its maximum.

The source of this uncertainty disappears in the deeper parts of the furnace, where the olefiant gas and the higher hydrocarbons are no longer present. The result obtained at the depth of twenty-three and twenty-four feet, giving the constant mean proportion 79.2: 27, proves that under the twenty-four feet, there is an evolution of carbonic acid caused either by the reduction of the ore, or by the escape of carbonic acid from the limestone, or perhaps by both these causes together. Now we conclude, from the average composition of the gases evolved from the materials used in the furnace, that this evolution of carbonic acid is really owing to the reduction of the ore, and that the process of reduction takes place only in the boshes. The average composition must be somewhere between the following numbers:

This mixture of gases contains,

1. The products of distillation of the coal.

2. The products of its combustion.

3. The carbonic acid generated during the reduction of the ore, and expelled from the limestone.

The proportion of nitrogen to oxygen, as deduced from these analyses, is 79.2: 27-33, and 79.2: 26-67, or an average of 79-2:27. The products of combustion of the coal give the proportion existing in atmospheric air 79.2: 20-8. Now as the amount of oxygen in the products of distillation of the coal is quite insignificant, and may be safely neglected in the calculation, the increase of oxygen from 20-8 to 27 must depend upon the carbonic acid of the limestone, and the oxygen of the ore given to carbon during the reduction. But the gas collected at twenty-three and twentyfour feet deep, contains 27.6 and 26.5 oxygen to 79.2 nitrogen. Hence at this depth the gas must have already accumulated all the oxygen of the iron, and the carbonic acid of the limestone. These facts warrant us in drawing the following conclusion,-That in hot-blast furnaces fed with coal, the reduction of the iron and expulsion of carbonic acid from the limestone takes place in the boshes of the furnace.

We cannot define by direct observation the exact region of the furnace in which the melting of the iron and formation of the slag are effected, but as the large masses of ironstone cannot enter the hearth in any form except

as a liquid, we may safely assume that the point of fusion is at the top of the hearth in hot-blast furnaces.

With the object of rendering these processes more intelligible, we have shaded a section of the furnace so as to represent the different parts employed in their special functions, the drawing being made to an exact scale. AB is the space in which the distillation proceeds, BC and CD show the region in which the reduction of the ore and evolution of the carbonic acid are effected, and in which the materials attain the temperature necessary for fusion.

The marked difference between the results obtained in the continental furnaces and those in this country will cease to excite surprise, when we bear in mind the different nature of the fuel employed. The principal reason of the great depression of the region of reduction in the furnaces of this country, is that almost all the body of the furnace is taken up in the process of coking; and hence the point of reduction must be still further lowered if the pieces of coal be of a large size. These pieces, often in bulk equal to a cubic foot, must remain a long time before the heat penetrates thoroughly through them, and the column of air ascending through this material must yield its heat in order to render gaseous above 30 per cent. of the fuel. Hence the depression of the temperature of the upper half of the furnace becomes so great that it does not suffice for the reduction of the ore, nor is it sufficient for the expulsion of carbonic acid from the limestone. Another important cause lowering the region of reduction, is the high pressure at which the blast is thrown into the furnace, the pressure being six or seven times the amount of that used in Germany. The materials, on this account, traverse through the furnace much more speedily, and therefore require to pass through a larger space to become heated. All these circumstances have much less influence in the German and Swedish furnaces. The charcoal with which the latter are fed is a fuel almost completely coked, and the materials, being in small fragments and thoroughly mixed, offer a heating surface at least a hundred times greater than that exposed in English furnaces. The small pressure of the blast also effects a slow combustion, so that the fuel frequently takes twice or three times the period to pass through the same region of the furnace.

To be Continued.

Description of Venitian Blinds Patented by JOHN HAMPSON, ESQ., Civ. Eng., of Carrollton, Louisiana, August 21, 1841.

The frame is made in the ordinary manner, with the exception of the inner edge of one side being rabbited, so as to allow a movable strip, a a, (see sketch,) to be placed therein, of about in. in thickness, and of such width as to allow holes to be made in it, for the pivots or journals, e e, of one end of the slats to work in. In the edge of the frame at the bottom of the rabbit, holes are bored, in which brass wire spiral springs, India rubber, or other elastic material, are placed, as shown at b b; these springs keep the strip a a close up to the end or shoulders of the slats, with just sufficient pressure to retain them in any position in which they may be placed, and prevent them from rattling or shaking down when they are

used in railroad cars or carriages. To keep the strip a a in its place, and to hide the springs, a flat strip, (of the same kind of wood as the frame is made of,) d d, is let in and fastened with screws, as shown. This flat strip need not be over in. thick; it may be stronger if required. It is evident that, if the pivots be somewhat smaller than the holes in which they work, the weather cannot affect the movement of the slats; it is also evident that the frame may be fastened together before the slats are put in, which is not the case with the common blind, and which give great facility in repairs in case of accident.

The patentee claims the method of preventing the slats from rattling, and retaining them in any position in which they may be placed, by means of the movable strip or strips pressed up by springs or other elastic sub

The patentee says:-"This blind has been in use in railroad cars, in and about New Orleans, for over seven years, and has given universal satisfaction. In simplicity, comfort, durability, and economy, it is all that can be desired. It is applicable wherever movable or rolling slat Venitian blinds can be used. Rolling slat Venitian shutters are desirable in many situations, and there is little doubt but that many more would be used were they (as commonly made) less troublesome and unmanageable. All