118 



CHAPTER IV. 



METALLIC ELEMENTS — THE EARTHS METALS — METAL- 



254. Alumina, or pure clay, is a very abundant 

 and widely- diffused substance. It occurs native both 

 pure and in combination, but it is most usually met 

 with mixed with another earth, called silex, and com- 

 bined with a quantity of water. It is sometimes 

 found pure and free from water or any acid, And con- 

 stitutes the hard gems called ruby and sapphire ; but 

 these are very rare, and seldom met with. Alumina 

 is the oxide of a metal called aluminum (202, 225). 



255. The properties of alumina, in the ordinary 

 state in which it occurs in the soil, are very diflferent 

 from those of the bases described in the last chapter ; 

 it combines with acids to form salts, but is quite in- 

 soluble in water, has no caustic powers, and does not 

 absorb carbonic acid from the air. It has a strong 

 attraction for water, and when thoroughly wet, it 

 appears in the form of a very tenacious paste, re- 

 markable for its great plasticity, and the ease with 

 which it may be moulded into any form. All the 

 varieties of clay derive their tenacious property 



