ELEMENTS 201 



The metal aluminium (Al) is a very important ele- 

 ment; it occurs as a part of some of the most abundant 

 kinds of rocks. As a metal, Al is silver-white, strong, 

 but very light in weight. The oxide of Al, A1 2 O 3 , 

 occurs in nature as sapphires and rubies ; it is also 

 powdered and used for polishing, being called emery. 

 Common alum is a sulphate of Al and K, A1K(SO 4 ) 2 . 



234. Silicon. The element silicon (Si) never occurs 

 free in nature, though in compounds it is very abundant 

 in the earth. The most common compound, SiO 2 , is 

 called silica or quartz ; it is a white or colorless rock, 

 and in a finely broken form it is white sand. Silicates 

 are salts of silicic acid, H 2 SiO 3 . 



QUESTIONS 



1. What sort of a substance is O? In what does it occur? 

 How common is O ? What two important uses does it serve ? 



2. Describe the element H. Why is H used in balloons ? What 

 chemical property makes it a very important element ? In what 

 substances does it occur? 



3. In what does N occur? Is it an active element? What 

 purpose does it serve in the air? 



4. Name substances that are nearly pure carbon. How are 

 these substances made ? In what sorts of matter is C very impor- 

 tant? Name two different crystalline forms of C. 



5. Describe the element S. Where does it occur in nature, 

 and in what form ? What important compounds contain S ? For 

 what is free S used? 



6. For what chemical property is P a valuable element? Ex- 

 plain its use in matches. Why cut P under water? Why not 

 breathe its fumes? 



7. Describe chlorine. What important compounds does it form ? 

 What uses are made of the element ? 



