This is derived from limestones containing about 10 to 20 per cent of clay or silica, which is intimately mixed with the carbonate of lime in the structure of the stone. During the process of burning, some of the lime combines with the clay (or the silica) so as to form the aluminate or silicate of lime. The excess of lime becomes quicklime as before. During the process of slaking, which should be done by mere sprinkling, the lime having been intimately mixed with the clay or silica, the expansion of the lime completely disintegrates the whole mass. This slaking is done by the manufacturer. The lime having a much greater avidity for the water than the aluminate or the silicate, the small amount of water used in the slaking is absorbed entirely by the lime, and the aluminate or the silicate is not affected. The setting of hydraulic lime appears to be due to the crystallizing of the aluminate and silicate; and since this will be accomplished even when the masonry is under water, it receives from this property its name of hydraulic lime. It is used but little in this country, and is all imported.