Silica
A very abundant mineral in the soil, both in the pure state, and in combination with various elements forming Silicates. Pure Silica is made up of the two elements Silicon, or Silicium, and Oxygen, in the proportion of twenty-eight parts, by weight, of the former to thirty-two of the latter. It occurs in several conditions, of which the most frequent are quartz and flint, and the less common rock-crystal and calcedony. Amethyst and cornelian are forms of Silica, tinged red with iron oxides. Silica, in the form of quartz, is the chief ingredient of all sands and sandstones, as well as of granites and other minerals of the same general structure; and veins or masses of pure quartz of considerable size also exist. But almost more plentiful than Silica are the Silicates, chiefly of Potassium, Sodium, Calcium, and Magnesium. These are familiar to everyone as clay, mica, felspar, and other substances that make up the mass of most soils.
Silica is not soluble in pure water; but a little Carbonic Acid gas, dissolved in the water, permits of a little Silica being dissolved; and almost all water that gains access to the roots of plants contains an appreciable amount of Silica, either pure or, more frequently, combined with alkalies. Silica exhibits the properties of a weak acid, in its power of combining with various metals. It combines with these, in more than one proportion, certain compounds, called normal or basic Silicates, containing a larger proportion of the alkali than do the others. These normal Silicates are the ones that dissolve in water.