COBALT. 119 



contains one-third of its weight of oxygen. This 

 oxide is sometimes very beautifully crystallized. 

 Manganese is also employed by glass-makers to 

 destroy the greenish tint of glass, and for making 

 violet-coloured glass. 



Almost all the salts of manganese are soluble in 

 water. 



COBALT. 



Cobalt is never found but in a state of combin- 

 ation. It is met with united to sulphur, arsenic, and 

 other metallic substances. 



The ores of cobalt had been long used for giving 

 a blue colour to glass, before its metallic nature 

 was known. 



The metal itself is not applied to any use. When 

 pure, it is a reddish grey colour ; rather soft and 

 brittle. Like iron, it is attracted by the magnet. 

 It is not oxidized by the air nor by water. It is 

 converted into a deep blue by exposure to heat 

 and flame. 



There are two oxides of cobalt. The 'protoxide 

 may be formed by precipitating by potass, a solu- 

 tion of cobalt in nitric acid. It is of a blue colour 

 when first precipitated, but becomes black by 

 absorbing oxygen. To recover the blue colour, it 

 must be heated red hot, by which the oxygen is 

 expelled. This oxide dissolves in acids. The mu- 

 riate of cobalt is green, and forms a sympathetic 

 ink. Letters written with it are invisible, until they 

 are warmed, and then they appear of a fine green j 

 when cold they disappear again. 



The peroxide of cobalt is procured by drying in 

 the air, with heat, the protoxide just precipitated; 

 by this the protoxide absorbs oxygen, and becomes 

 the peroxide. It is black. 



I 4 



