According as these various felspars are present in a soil, so will the quality of the soil be. It stands to reason that as the presence of potash in a soil is one of the distinguishing features of its fertility, much will depend on the extent to which the orthoclase felspar is present; and also, not only on the extent, but on the state and degree of its disintegration. It is important to note the method of this disintegration. It is effected by the absorption of water. This water is not merely absorbed mechanically, but actually enters into the composition of the mineral. It is not present as moisture merely, capable of being expelled at ordinary boiling temperature, but it forms what is known as water of composition. In this process of hydration, the mineral loses its lustre and crystalline appearance, crumbles away into a more or less--according to its state of disintegration--powdery mass. A very great change is also effected in its chemical composition; it loses nearly all its base. This is effected in the following way. As water enters into the mineral's composition, it sets free a certain portion of the base; there is thus formed a basic silicate, which, being soluble in water, is washed away in solution. This change may be illustrated by quoting the analysis of a kaolin clay formed by the disintegration of orthoclase felspar. _Kaolin Clay formed by disintegration of Orthoclase._ Silica 46.80 Alumina 36.83 Peroxide of iron 3.11 Carbonate of lime 0.55 Potash 0.27 Water 12.44 ------ 100.00 ------ The chief difference here is the almost total loss of potash and a portion of the silica, and the gain of water. The other constituents practically remain insoluble. Another important mineral is _Mica_. Its composition is not unlike felspar. It contains silica, alumina, and iron, in considerable quantities, also magnesia and potash. There are two kinds of mica--that containing potash, and that containing magnesia, in excess. The analyses of these two kinds are as follows (by the late Dr Anderson):-- MICAS. (_a_) Potash. (_b_) Magnesia. Silica 46.36 42.65 Alumina 36.80 12.96 Peroxide of iron 4.53 none Protoxide of iron none 7.11 Oxide of manganese 0.02 1.06 Magnesia none 25.75 Potash 9.22 6.03 Hydrofluoric acid 0.70 0.62 Water 1.84 3.17 ----- ----- 99.47 99.35 ----- -----