PROPERTIES OF METALS. 253 



The intensity with which metals combine with non-metals or with acids 

 differs widely. Selecting the combinations with oxygen as a typical instance 

 we find that the affinity between the alkali metals and the alkaline earth 

 metals is so intense that these metals cannot be exposed to the atmosphere for 

 even a few hours without undergoing complete oxidation. It is for this reason 

 that these metals cannot be used in the metallic state for purposes requiring 

 constant exposure to air. Other metals, such as iron, will oxidize (rust) slowly 

 at ordinary temperature or will burn when heated sufficiently high. Yet other 

 metals retain their metallic lustre in dry or moist air at low or high tempera- 

 ture. Indeed, the oxides of these metals are decomposed into oxygen and the 

 respective metal by the mere application of heat. The metals showing this 

 behavior are often called noble mefals, while all others are designated as base 

 metals. The noble metals are gold, silver, mercury, platinum, iridium, and a 

 few other metals related to platinum. (See also page 198.) 



Manufacture of metals. Most metals may be obtained from their 

 oxides by heating the latter with charcoal, the carbon combining with 

 the oxygen of the oxide, while the metal is liberated : 



MO + C = CO + M; 

 or 



2MO + C = CO 2 + 2M. 



Also hydrogen may be used in some cases as the deoxidizing agent : 



MO + 2H = H 2 -f M. 



Some metals are found in nature chiefly as sulphides, which usually 

 are converted into oxides (before the metal can be obtained) by roast- 

 ing. The term roasting, when used in metallurgy, means heating 

 strongly in an oxidizing atmosphere, when the sulphides are con- 

 verted into sulphates or oxides, thus : 



MS + 4O = MS0 4 ; or MS + 3O = MO + SO 2 . 



A few metals are obtained by heating the chloride with metallic 

 sodium, when sodium chloride is formed, while the other metal is 

 set free. Electrolysis is also one of the means for obtaining metals 

 from their compounds. 



Recently a generally applicable method of obtaining metals has been devised, 

 which consists in the action of aluminum powder on the oxides of the metals, 

 especially of those that have a high fusing-point and form difficultly reducible 

 oxides. So much heat is developed in these reductions that the method may be 

 used for welding, for example, the joints between rails. 



Alloys are combinations or, more correctly speaking, mixtures of 

 two or more metals. Whenever mercury is a constituent of an alloy 

 it is called amalgam. All alloys exhibit metallic nature in their 

 physical properties i. e., they have metallic lustre and are more or 

 less good conductors of heat and electricity. 



