CLASSIFICATION OF METAI-3. 231 



CHAPTER II. 



METALS. 



569. CLASSIFICATION. The metals may 



How may the 



metals be das- be arranged in groups, or classes, according 

 to their affinity for oxygen. Those which 

 tarnish, or rust most readily, come first in order, while 

 the last group is made up of the noble metals, which 

 retain their brilliancy, unimpaired. 



570. CLASS i. POTASSIUM AND SODIUM. 



Describe the 



metal* of These two metals combine with oxygen so 

 eagerly, as to tarnish instantaneously, on 

 exposure to the air. They even seize on that which 

 is contained in water and expel its hydrogen. The 

 hypothetical metal, ammonium, is described in connec- 

 tion with this group, because of the similar properties 

 of its compounds. 



Describe Class 571. CLASS II. BARIUM, STRONTIUM, CALCI- 



IL UM, MAGNESIUM. The metals of this class 



show their affinity for oxygen, in the same manner as 

 those of Class I. But they are inferior, in this respect, to 

 both potassium and sodium. Either of these metals 

 can deprive them of the oxygen with which they may 

 have combined. 



Describe Class 572. CLASS III. MANGANESE, ALUMINIUM, 

 IIL IRON, CHROMIUM, COBALT, NICKEL. The 



metals of this class tarnish less rapidly than the fore- 

 going, by exposure to the air. In order that they may 

 decompose water, and appropriate its oxygen, they re- 



