1226. Clayey soils are most improved by burning. 'Many of the silicates of alumina,' says Liebig, 'which are not at all affected by acids in their natural state, acquire complete solubility when they are previously melted by heat. To this class of silicates belong pipe and potter's clay, loam, and the different varieties of clay occurring in soils. In the natural state of clay, it may be digested with concentrated sulphuric acid for hours, without dissolving in any appreciable quantity; but when the clay is slightly burnt (as is done, for example, in several alum works), it dissolves in acids with great ease, while the silica is separated in its gelatinous and soluble form. Common potter's clay forms generally very sterile soils, although it contains within it all the conditions for the luxuriant growth of plants; but the mere presence of these conditions does not suffice to render them useful to vegetation.' (Liebig's Chemistry, 4th; ed., p. 135.) When, however, soils of this nature are sufficiently pulverised to render them accessible to air and water, they obtain from them oxygen, carbonic acid, and the other elements necessary to sustain vegetable life.