160 METALS AND THEIR COMBINATIONS. 



quantities, and are chiefly of scientific interest, with the exception of 

 cerium, which furnishes an official preparation. 



Occurrence in nature. Aluminum is found almost exclusively 

 in the solid mineral portion of the earth ; rarely more than traces of 

 aluminum compounds are found dissolved in water, and the occur- 

 rence of aluminum in either the vegetable or animal organism seems 

 to be purely accidental. 



By far the largest quantity of aluminum is found in combination 

 with silicic acid in the various silicated rocks forming the greater 

 mass of our earth, such as feldspar, slate, basalt, granite, mica, horn- 

 blende, etc., or in the various modifications of clay formed by their 

 decomposition. 



The minerals known as corundum, ruby, sapphire, and emery, are 

 aluminum oxide in a crystallized state, and more or less colored by 

 traces of other substances. 



Metallic aluminum may be obtained by the decomposition of 

 aluminum chloride by metallic sodium : 



A1 2 C1 6 + 6Na = 6NaCl -f 2A1. 



It is now manufactured by the electrolysis of aluminum and sodium 

 fluoride. 



Aluminum is an almost sil vej>whi tew metal of a very low specific 

 gravity (2.67) ; it is capable of assuming a high polish, and for this 

 reason is used for ornamental articles ; it is very strong, yet malleable, 

 and does not change in dry or moist air. 



Some of the alloys of aluminum are now used in the arts, as, for 

 instance, aluminum-bronze, an alloy resembling gold and composed 

 of 10 parts of aluminum with 90 of copper. 



Aluminum is trivalent, and shows, like a number of other elements 

 (iron, chromium, etc.), the peculiarity that the double atom Al 2 vi acts 

 as a single sexivalent atom. 



Alum is the general name for a group of isomorphous salts, com- 

 posed of one molecule of the sulphate of a univalent metal in combi- 

 nation with one molecule of the sulphate of a trivalent metal, 

 combined in crystallizing with 24 molecules of water. The general 

 formula of an alum is consequently M i 2 SO 4 M iii 2 (SO 4 ) 3 .24H 2 O or 

 M i 2 M iii 2 (SO 4 ) 4 .24H 2 O. M 1 represents in this case a univalent, M ai a 

 trivalent metal. 



