This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1852. Excerpt: ... T. Silica.--In flint and rock ' crystals, we have examples of this earth. It is harder than glass; and, in the. common form in which it is'"'-found, is insoluble. It appears, in this form, to be one of the most indifferent of all bodies. Yet it is found in such combinations that it is freely taken up by the roots of grasses, and plants of this family. 4t constitutes a very large proportion of the earth's crust. In soils, itvaries in quantity. Its proper proportion is about 85 per cent;; but good soils often contain less, and two 01 three per cent. more. The office which silex performs is to preserve a due amount of the coarser matter: for, when a soil is composed of impalpable matter, it is comparatively barren; and silex, from its excessive hardness, resists the wear and action of the elements. By being commingled with clay, it imparts porosity and looseness; permits the roots to penetrate deeply; while, at the same time, air and moisture permeate through the mass as far as roots can find their way. The use of silex is, therefore, partly mechanical and partly physiological; being necessary m soils to preserve porosity, and particularly necessary to the cereals to protect the straw and kernels, and give elasticity and strength to the whole plant. So abundant is this substance, however, that it is never necessary to add it to the soil, except for mechanical use _--potash and lime are often added for the purpose of freeing it from its insoluble combinations, when the grains are special objects of culture. 8. Oxide Of Iron.--Analyses of organic bodies prove the existence of iron in them; and in those animals which have red blood, it is satisfactorily demonstrated that it serves to maintain the heat of the body. In addition to this, its salutaiy effects upon...