THE sriiSTANCKS OF THE EAUTII. 23 



Potassium is a soft, light substance, whose affinity for 

 oxygen is so strong that it can only l)e kept pure in some 

 substance containing no oxygen. When placed upon a 

 ])iece of ice it burns freely. 



With oxygen it forms })otasli (KoO), and with oxygen 

 and hydrogen caustic potash (KOH). Potash unites 

 with acids jn'oducing a variety of salts of potash, as chlo- 

 rate of potash (KCIO3), sulphate of potash (K2S()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 sei)arafed. 

 In limestone and marl)le it is united with carbonic acid, 

 forming calcium carljonate (CaCOa). 



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 l)urns brilliantly, and is sometimes used for illu- 

 minating ])urposes 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 ajtjiearance. It is used to a small 

 extent in making jewelry and ornamental work. The 

 sai)phire and ruby are beautiful forms of alumina. It 

 exists largely in common clay in combination with silica. 



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

 ore, which is purified and used for mannfacturing pur- 

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

 clayey stjils their dark f)rown color. 



