286 METALS AND THEIR COMBINATIONS. 



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 : 



A1C1 3 + 3Na SNaCl -f Al. 



It is now manufactured by the electrolysis of aluminum and sodium 

 fluoride, or of other aluminum compounds. 



Aluminum is an almost silver- white 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 ductile and malleable 

 and ranks with silver in hardness, as also in its power of conducting 

 heat and electricity. 



Aluminum is not oxidized to any great extent in dry or moist air 

 nor is it affected by hydrogen sulphide. It is not readily acted on by 

 nitric or sulphuric acid, but easily dissolves in hydrochloric acid and 

 in solutions of the alkali hydroxides. 



Aluminum forms alloys with nearly all metals, lead being an exception. 

 The hardness and elasticity of tin is increased by addition of aluminum ; 

 readily obtainable alloys with zinc are used as solders for aluminum. A small 

 quantity of aluminum added to wrought iron so increases its fusibility that it 

 may be poured as easily as cast iron. Largely used is aluminum-bronze, an alloy 

 resembling gold and composed of 10 parts of aluminum with 90 of copper. 



Aluminum would be an ideal base for artificial dentures, were it not that 

 the corrosive action of alkaline fluids upon it limits its use. 



Aluminum is trivalent, and the composition of the chloride and 

 hydroxide is therefore given as A1C1 3 and Al(OH), respectively. 



Alum is the general name for a group of isomorphous double sul- 

 phates containing an atom each of a univaletit and a trivalent metal, 

 combined in crystallizing with 12 molecules of water. The general 

 formula of an alum is consequently M i M iii (SO 4 ) 2 .12H 2 O. M i repre- 

 sents in this case a univalent, M iu a trivalent metal. 



Alums known are, for instance : 



Ammonium-aluminum sulphate, NH 4 A1(SO 4 ) 2 .12H 2 O. 

 Potassium-chromium sulphate, KCr(SO 4 ) 2 .12H 2 O. 

 Ammonium-ferric sulphate, NH 4 Fe(SO 4 ) 8 .12H 8 O. 



