120 MOLYBDENA. — TUNGSTEN. 



Ores of cobalt are very valuable. Zaffre is a 

 substance produced by roasting the ores of cobalt, 

 by which the volatile matters (generally arsenic 

 and sulphur,) are driven off; the remainder is then 

 fused with sand or pounded flints. A blue glass is 

 thus formed, which, when ground and washed, con- 

 stitutes the pigment called smalt. 



MOLYBDENA, 



Molybdena is found in nature combined with 

 sulphur; forming the sulphuret of molybdena, which 

 resembles plumbago in some of its properties. This 

 mineral is of a bluish colour, more brilliant than 

 plumbago, and makes on paper a trace of a grey 

 tint. The metal has only been procured in small 

 grains, which do not differ much in their pro- 

 perties from the sulphuret. It combines with 

 oxygen, so as to form an acid called the molybdic 

 acid. The molyhdate of lead is a beautiful yellow 

 mineral. 



The protoxide of molybdena is a tasteless pow- 

 der of a brown colour. Molybdena alloys with the 

 other metals, 



TUNGSTEN. 



A mineral called tungsten^ or ponderous stone, 

 affords a peculiar metal. This metal is capable of 

 being acidified, and when in this state it is joined 

 to lime, it forms the tungstate of lime. 



The metal when pure is much like steel, and is 

 one of the hardest of the metals; a file can scarcely 

 make any impression on, it : it is also the heaviest, 



