THE SUBSTANCES OF THE EARTH. 23 



Potassium is a soft, light substance, whose affinity for 

 oxygen is so strong that it can only be kept pure in some 

 substance containing no oxygen. When placed upon a 

 piece of ice it burns freely. 



With oxygen it forms potash (K 2 0), and with oxygen 

 and hydrogen caustic potash ^KOH). Potash unites 

 with acids producing a variety of salts of potash, as chlo- 

 rate of potash (KC10 3 ), sulphate of potash (K 2 S0 4 ), etc. 



Sodium is somewhat similar to potassium. It forms a 

 great variety of salts, of which common salt (NaCl) is a 

 familiar example. Caustic soda is prepared in large 

 quantities for manufacturing soap. 



Calcium is a common substance, found in combination 

 with other elements, from which it is not easily separated. 

 In limestone and marble it is united with carbonic acid, 

 forming calcium carbonate (CaC0 3 ). 



Compounds of calcium constitute a large part of the 

 shells of clams, oysters, and other shell-fish, and also of 

 the bones of all animals. 



Magnesium is a metal found in some rocks. The pure 

 metal burns brilliantly, and is sometimes used for illu- 

 minating purposes when a very strong light is required. 

 It is found to some extent in plants and animals. With 

 oxygen it forms magnesia (MgO). 



Aluminium is somewhat similar to magnesium. It 

 resembles silver in appearance. It is used to a small 

 extent in making jewelry and ornamental work. The 

 sapphire and ruby are beautiful forms of alumina. It 

 exists largely in common clay in combination with silica. 



Iron is found in many parts of the earth in the form of 

 ore, which is purified and used for manufacturing pur- 

 poses. It exists to some extent in all soils. It gives 

 clayey soils their dark brown color. 



