122 SALTS OF THE METALS. 



combine with either potash or soda ; and besides, as 

 many stones contain silica in combination with potash 

 or soda, so these stones, in crumbling awaj, present 

 a constant source of soluble silica. 



266. Silica is found in many plants, such as, for 

 example, corn and grasses, the stalks of which mainly 

 derive the strength requisite to enable them to grow 

 erect from the silica which they contain. Plants de- 

 rive this earth from the soil, and are only able to ab- 

 sorb by their roots the silica, which, by having com- 

 bined with alkali, has become soluble in water, and is 

 consequently able to enter into the structure of the 

 plant. 



267. Silica, or silicic acid, combines with lime, 

 magnesia, and alumina, to form silicates, as well as 

 with the alkalies, potash and soda ; these silicates are 

 for the most part insoluble in water, and constitute, 

 either pure, mixed, or combined together, an immense 

 variety of different stones ; the action of the atmo- 

 sphere and other natural circumstances combine to 

 effect the gradual decomposition of such compounds. 



268. Besides the three earthy bases already spoken 

 of, and a few others of far less importance, as they 

 are very rare, and only to be found in particular 

 places, there are a numerous series of bases called 

 metallic oxides, several of which are of great import- 

 ance in the arts, and two of which are almost always 

 present in small quantity in the soil. 



269. Most of the common metals, such as iron, 

 lead, and zinc, gradually tarnish, and become rusty 



