426 Introduction to the Study of Science 



twenty-eight per cent of the total composition of the earth's 

 crust or lithosphere. It is second only to oxygen, which com- 

 poses forty-seven per cent of the earth's rocky crust. Next to 

 silicon is aluminium, 7.84 per cent, and then follows iron, 

 4.44 per cent of the total crust (see page 433). 



The table, given for reference, indicates the percentage of 

 silica and other common substances in the different kinds of 

 rocks and shales, and shows the relative abundance of certain 

 useful minerals in nature. It is adapted from the complete 

 table in Bulletin 491, United States Geological Survey, pages 

 27-32. 



204. Carborundum. Silica is an essential material in the 

 manufacture of what is commercially known as carborundum. 

 This is a compound made from silica and carbon. A mixture 

 of sand, coke, common salt, and sawdust is arranged in an 

 electric furnace. The electric current must pass through this 

 mass. After about eight hours of heating at the high tempera- 

 ture produced by the resistance of the coke to the electric 

 current, the product is seen to be a mass of black, brilliant, and 

 often iridescent crystals which have a degree of hardness next 

 in the scale to that of the diamond. Hard steel is readily cut 

 by a wheel covered with the crystals of carborundum. What 

 are some of its uses in your neighborhood ? 



SUMMARY 



$15,000,000 worth of lime is produced annually in the United States. 



Quicklime, or calcium oxid, is slaked with water, forming calcium 

 hydroxid. 



Slaked lime is used to make whitewash and mortar, which harden 

 by the absorption of carbon dioxid from the air and the liberation of 

 water. 



A knowledge of chemistry is necessary to understand many of the 

 important household and manufacturing processes upon which our 

 material welfare depends. 



Cement to the value of $70,000,000 is produced annually in the 

 United States. 



Cement sets by combining with enough water to crystallize. 



