SILICON AND BORON. 191 



in quartz and flint. Various precious stones, carnelian, am- 

 ethyst, opal, jasper, etc., are silica, their different colors be- 

 ing caused by the presence of metallic oxides. Common 

 sand is silica, generally rendered yellow by the hydrated 

 oxides of iron or iron rust. Then silica is present in many 

 salts called silicates, constituting part of an abundant class 

 of rocks. In the granite rocks we have mingled with the 

 quartz, which is silica, two silicates feldspar and mica. 

 There are silicates in many other rocks also. In clays there 

 are variable quantities of various silicates ; but the silicate 

 of alumina is largely predominant, and is the essential basis 

 of all clays. The best porcelain clay, which is perfectly 

 white, is nearly pure silicate of alumina. As earthenware 

 is made of clay, it is composed of silicates. The same thing 

 is true of the various kinds of glass. 



260. Silica in "Water and in Plants. Through the agency 

 of potash silica is rendered soluble to some extent, and 

 therefore is found in water and in plants. If spring-water 

 be evaporated, what remains in solid form is in part silica ; 

 and so if we burn plants, it is found in their ashes. There 

 is considerable silica in grasses and the various kinds of 

 grain, and they have therefore been called silicious plants. 

 Absorbed by the root, it goes up in the plant dissolved in 

 the sap, and is deposited chiefly in the stalks, giving to them 

 their requisite firmness. It is to them what the mineral 

 matter, the phosphate of lime, in our bones is to us. Silica 

 is also present to a considerable extent, especially in the 

 frame- work of those minute animals, which can be seen only 

 by means of the microscope, called infusoria. 



261. Silicified Wood. A singular result occurs when 

 wood decays in water that has considerable silica dissolved 

 in it. The water, of course, soaks into every part of the 

 wood, taking the silica along with it. Now, as the parti- 

 cles of wood are loosened one after another and carried 



