The most ancient kind of lime-kiln is probably that which is made by excavating the earth in the form of a cone, of such a size as may be necessary ; and afterwards building up the sides, or not, according to the circumstances of the case ; the materials being then laid in, in alternate layers of fuel and stone, properly broken, until the whole is filled up. The top is then covered with sods, in order that the heat may he prevented from escaping : and the fire lighted at the bottom, and the whole of the contents burnt, in a greater or less space of time, in proportion to the nature of the stone, and the quantity that is contained in the kiln. From the circum stance, of the top parts of these kilns, in some districts, being covered over and the sides sometimes built up with sods, they are termed sod-kilns, in order to distinguish them from the other sorts. When the whole of the contents of such kilns are grown cold, they are drawn or taken out from the bottom ; and the kiln again filled, if necessary. These kilns are obviously intended for burning only cane at a time. But as the burning of lime in this way is tedious and uneconomical, other methods and forms of kilns have been had recourse to. Where lime is much wanted, either for building or other purposes, they use perpetual kilns. or what are more generally known by the name of draw-kilos. These, as all lime-kilns ought to be, are situated by the side of a rising bank, or sheltered by an artificial mound of earth. They are generally built either of stone or brick ; but the latter, as being better adapted to stand excessive degrees of heat, is considered as preferable. The outside form of such kilns is sometimes cy lindrical, but more generally square. The inside should be formed in the shape of a hogshead. or an egg, opened a little at both ends, and set on the smallest ; being small in circumference at the bottom, gradually wider towards the middle, and then contracting again towards the top. In kilns constructed in this way, it is observed, fewer coals are necessary, in consequence of the great degree of reverberation which is created, above that which takes place in kilns formed in the shape of a sugar•loaf reversed. Near the bottom, in large kilns, two or more apertures are made : these are small at the inside of the kiln, but are sloped wider, both at the sides and the top, as they extend towards the outside of the building. The uses of these apertures are for admitting the air necessary for supplying the fire, and also for permitting the labourers to approach with a drag and shovel, to draw out the calcined lime. From the bottom of the kiln within, in some cases, a small building, called a horse, is raised in the form of a wedge, and so constructed as to accelerate the operation of drawing out the burned limestone, by forcing it to fall into the apertures which have been men tioned above. In other kilns of this kind, in place of this building, there is an iron grate near the bottom, which conies close to the inside wall, except at the apertures, where the lime is drawn out. When the kiln is to be filled, a parcel of furze or faggots is laid at the bottom ; over this a layer of coals ; then a layer of limestone, which is previously bro ken into pieces, about the size of a man's fist ; and so on alternately ; ending with a layer of coals, which is sometimes, though seldom, covered with sods or turf, in order to keep the heat as intense as possible. The fire is then lighted in the apertures ; and when the limestone towards the bottom is completely calcined, the fuel being considerably exhausted, the limestone at the top subsides. The labourers then put in an addition of limestone and coal at top, and draw out at bottom as much as they find thoroughly burned ; and thus go on, till any quantity required be calcined. When lime stone is burned with coals, from two-and-a-half to three-and a-half bushels, on a medium, three bushels of calcined lime stone are produced for every bushel of coals used in the process.
A lime-kiln of this sort is described in Count Rumford's Essays, in the possession of the Dublin Society, as well as the principal objects that ought to be had in view in con structing the kiln pointed out : the first of which is, to cause the fuel to burn in such a manner as to consume the smoke, which has here been done by obliging the smoke to descend and pass through the lire, in order that as much heat as possible might be generated. Secondly, to cause the flame and hot vapour which rise from the fire to come in contact with the lime-stone by a very large surflice, in order to economize the heat, and prevent its going off into the atmos phere; which was done by making the body of the kiln in the form of a hollow truncated cone, and very high in pro portion to its diameter ; and by filling it quite up to the top with lime-stone, the fire being made to enter near the bottom of the cone.
"Thirdly, to make the process of burning lime perpetual, in order to prevent the waste of heat which unavoidably attends the cooling of the kiln, in emptying and filling it, when, to perform that operation, it is necessary to put out the fire.
"And fourthly, to contrive matters so that the lime in which the process of burning is just finished, and which, of course, is still intensely hot, may. in cooling, be made to give off its heat in such a manner as to assist in heating the fresh quantity of cold limestone with which the kiln is replenished, as often as a portion of lime is taken out of it.
"To effectuate these purposes, the fuel is not mixed with the limestone, but is burned in a close fire-place, which opens into one side of the kiln, some distance above the bottom of it. For large lime-kilns on these principles, there may be several fire-places all opening into the same cone., and situ ated on different sides of it : which fire-places may be constructed and regulated like the fire-places of the furnaces used for burning porcelain.
" At the bottom of the kiln there is a door, which is occa sionally opened to take out the lime.
" When, in consequence of a portion of lime being drawn out of the kiln, its contents settle down or subside, the empty space in the upper part of the kiln, which is occasioned by this subtraction of the burned lime, is immediately filled up with fresh limestone.
"As soon as a portion of the lime is taken away, the door by which it is removed must be immediately shut, and the joinings well closed with moist clay, to prevent a draught of cold air through the kiln. A small opening, however, must be left, for reasons which are explained below.
"As the fire enters the kiln at some distance from the bottom of it. and as the flame rises as soon as it comes into this cavity, the lower part of the kiln (that below the level of the bottom of the fire-place) is occupied by lime already burned ; and as this lime is intensely hot, when, on a portion of lime from below being removed, it descends into this part of the kiln ; and as the air in the kiln, to which it communi cates its heat, must arise upwards in consequence of its being heated, and pass off through the top of the kiln, this lime, in cooling, is by this contrivance made to assist in heating the fresh portion of cold limestone with which the kiln is charged. To facilitate this communication of heat from the red-lot lime just burned, to the limestone above in the upper part of the kiln, a gentle draft of air through the kiln, from the bottom to the top of it, must be established, by leaving an opening in the door below, by which the cold air from without may he suffered to enter the kiln. This opening (which should be furnished with some kind of a register) must be very small, otherwise it will occasion too strong a draft of cold air into the kiln, and do more harm than good ; and it will probably be found best to close it entirely, after the lime in the lower part of the kiln has parted with a certain pro portion of its heat." It is a common practice in some places to burn limestone with furze. The kilns which are made use of in these cases are commonly known by the denomination of flanze-kibis, and are built of brick ; the walls from 4 to 5 feet thick, when they are not supported by a bank or mound of earth. The inside is nearly square, being 12 feet by 13, and 11 or 15 feet high. In the front wall there are three arches, each about 1 1;)ot 10 inches wide, by 3 feet 9 inches in height. When the kiln is to be filled, three arches are formed of the largest pieces of limestone, the whole breadth of the kiln, and opposite to the filches in the front wall. When these arches are formed, the limestone is thrown promiscuously into the kiln to the height of 7 or S feet, over which are frequently laid fifteen or twenty thousand bricks, which are burned at the same time with the limestone. When the filling of the kiln is completed, the three arches in the front wall arc filled up with bricks almost to the top, room being left in each sufficient only for putting in the furze, which is done in small quantities, the object being to keep up a constant and regular flame. In the space of thirty-six or fbrty hours, the whole li mestone, about 120 or 130 quarters, together with 15,000 or 20,000 bricks, are thoroughly calcined. Kilns constructed in this way may be seen near Wellingborough, in Northamptonshire, and other places in the northern parts of the kingdom. In many of the northern counties of Scotland, which are situated at a great distance from coal, it is also a common practice to burn limestone with peat ; and, considering the rude ill-constructed kilns which are used for the purpose, it is astonishing with what success the operations are performed. In some of these districts, it is stated that limestone is sufficiently calcined with peats, laid stratum, super stratum, in kilns formed of turf ; but owing to the quantity of ashes which fall from the peat, the quality of the lime is considerably injured ; and, from the open and exposed situation of many of these kilns, the waste of fuel is immense. But the most common method of burning limestone with peat, is in kilns constructed somewhat similar to those in the districts where furze is used as the only fuel. In kilns of this description there are, in general, only two arches, or fire-places, and the peats are thrown into the bottom of these arches, the fronts of which are seldom closed up, by which means the wind has often great influence in retarding the operation, and frequently prevents the complete calcina tion of the limestone. An improvement might, it is supposed, be made on these kilns at a very trifling expense : if an iron grate were laid across the bottom of the arch, with a place below for the ashes to fall down, and the front of the arch closed up by a door made of cast-metal, one-third of the fuel might be saved, and the operation performed in a shorter time, and with a much greater certainty, than by the method now practised in such kilns.