CHAPTER IV 



THE EARTH'S CRUST 



NEARLY 98 per cent of the solid crust of the earth consists of 

 silicates of the six metals, aluminum, iron, calcium, potassium, 

 sodium, and magnesium (in this order of relative abundance); 

 and the remainder is largely composed of the closely related titan- 

 ates. 



Silicon. Silicon in the mineral matter constituting the earth's 

 crust corresponds to carbon in the organic matter of the vegetable 

 and animal kingdoms. In all of the great groups of organic com- 

 pounds the molecule is built up by the linking power of the four- 

 handed carbon atom, as, for example, in the hydrocarbon, hexane 

 (C 6 H 14 ): 



H H H H H H 



I I I I I I 

 H C C C C C C H 



I I I I I I 

 H H H H H H 



Silicon is the second member of the carbon group * in the periodic 

 system, as shown on page n, and its linking power is also very 

 great, although alternating with oxygen and metals and restricted 

 mainly to silicates. Thus, instead of the almost unlimited number 

 of hydrocarbons, carbohydrates, and other numerous compounds 

 of carbon, hydrogen, and oxygen (alcohols, fats, organic acids, etc.), 

 there are but four such silicon compounds known: SiH 4 , SiO 2 , 

 OSi(OH) 2 or H 2 SiO 3 , and Si(OH) 4 or H 4 SiO 4 , which differs from 

 silicon dioxid by two molecules of water. 



1 A most interesting compound is SiC, silicon carbid, so-called carborundum, 

 formed by the union of the two tetravalent elements and, next to the diamond, one 

 of the hardest known substances. 



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