SILICON AND ITS COMPOUNDS 105 



slowly to diffuse through animal membrane, is a physical 

 characteristic, and is occasionally made use of for wash- 

 ing and separating compounds. 



125. Silicates. Since silicon forms such a variety of 

 acids, the number of silicates found in nature is very 

 large. The hydrogen atoms of silicic acid can be re- 

 placed with different metals, forming double salts, as 

 AlKSi 3 O 8 , which is feldspar, or the double salt of trisilicic 

 acid, H 4 Si 3 O 8 . This renders the composition of the sili- 

 cates exceedingly complex. Many of the silicates con- 

 tain also water of hydration as part of the molecule ; as 

 aluminum silicate or pure clay, Al 4 (SiO 4 ) 3 .H 2 O. Since 

 rocks are composed mainly of silicates, and soils are 

 formed from the decay of rocks, it follows that soils are 

 practically a mechanical mixture of silicates with small 

 amounts of other compounds. Hence, the importance of 

 the subject of silicic acid and the silicates. Unfortunately 

 the structure and composition of the silicates have not 

 been determined as completely as of other salts and 

 acids. Pure clay is aluminum silicate, formed from the 

 disintegration of feldspar rock, a double silicate of potas- 

 sium and aluminum. Mica, hornblende, and zeolites 

 are all complex forms of silicates. 



126. Importance of Compounds of Silicon. The com- 

 pounds of silicon, as silicon dioxid, SiO 2 , and of the sili- 

 cates, are used in the manufacture of glass, porcelain and 

 brick. The element itself takes no direct part in animal 

 or plant life, but indirectly is important, for it is in 

 combination with many elements which serve as plant 

 food. Some of the simpler and more soluble silicates are 



