LIME KILNS. The description of ovens in which limestones are subjected to the process of calcination are known by the name of lime kilns. These structures are usually built of brickwork lined with fire bricks, and in shape are either rectangular straight prisms, portions of cylinders, cylinders surmounted by truncated cones, reversed straight sided cones, or cones produced by the revolution of an ellipsoid. The first three forms are used for what are called intermittent kilns, whether of wood or coal ; the next two forma are used for what are called running and arc those in which coal is burnt ; whilst the last of all is most generally used when peat is the fuel employed. lo the intermittent kilns the main body of the fuel is applied below the charge, and after it has been consumed the fire is let out and the kiln 'pooled ; in the running kilns tho fuel is intermixed with the charge, and the calcined lime, falling to the bottom, is gradually withdrawn as it is burnt, so that the operation is, technically speaking, continuous. Local con siderations of the cost of constriction, of fuel, and of the nature of the demand, must regulate the choice of the particular description of kiln.
It is calculated that the degree of heat required in a lime kiln is about from 15 to 30 of Wedgewood's pyrometer, and that in inter mittent kilns this heat must be maintained for between 3 or 4 days. It is supposed that about 60 cubic feet of oak, 117 feet of fir, about the same quantity of peat, and 9 cubic feet of coal are required to calcine 35 cubic feet of limestone, in intermittent kilns; and that 7 cubic feet of coal are able to calcine that quantity of lime-stone in running kilns. The lime-stones lose, on the average, 45 per cent. of their weight by the evaporation of the water, and the expulsion of the carbonie acid gas ; but they hardly diminish in volume more than from 1 to 2 per cent. A great deal of the lots of weight by calcination is permanent ; for even after the lime has been hydrated thoroughly, it remains about from 15 to 17 per cent. less in weight than it had previously been, when in the form of the stone.