5<J8 PROTOXIDE OF COBALT. 



liquid is then decanted, and if no additional precipitate subsides 

 from it in twenty-four hours, it is free from nickel, and may be 

 evaporated to dryness. The precipitate of nickel contains a 

 little cobalt. 



Cobalt is a brittle metal, of a reddish grey colour, somewhat 

 more fusible than iron, and of the density 8.5131 (Berzelius.) 

 It is generally stated to be magnetic, even when free from iron 

 and nickel, although a minute quantity of arsenic causes it to 

 lose that property. But Mr. Faraday finds, pure cobalt not to 

 be susceptible of magnetism. Cobalt is less oxidable in the air 

 or by acids than iron, dissolving slowly in diluted hydrochloric 

 or sulphuric acid, when heated, with effervescence of hydrogen ; 

 but it is readily oxidised by nitric acid. This metal forms a 

 protoxide and peroxide, Co O and Co 2 O 3 , corresponding with 

 the oxides of iron, and also a compound oxide, Co O + Co 2 O 3 , 

 analogous to the black oxide of iron. 



Protoxide of cobalt, Co O, 469 or 37.57- Prepared by the 

 ignition of the carbonate, this oxide is a powder of an ash grey 

 colour. It is precipitated by an alkali, as a hydrate, from its 

 solutions in acids, of a fine blue. Fused with glass, the oxide 

 of cobalt colours it blue, even when in minute quantity, no 

 other colouring matter having so much intensity. Smalt blue is 

 a pounded potash glass containing cobalt. The salts of this 

 oxide have a reddish colour in solution. They are not preci- 

 pitated by sulphuretted hydrogen, when they contain a strong 

 acid, but give a black protosulphuret with an alkaline sulphuret. 

 The oxide is precipitated blue by ammonia, and redissolved by 

 an excess of that alkali. It is precipitated as a pale pink car- 

 bonate by alkaline carbonates, which is soluble in carbonate of 

 ammonia. The colour of the ammoniacal solutions of the salts 

 of cobalt is red, which is of a lively tint when the oxide is pure, 

 but becomes of a dull purple and even brown-black when oxide 

 of nickel is present in greater or less quantity. 



Oxide of cobalt appears to combine with alkalies and earths, 

 as well as with acids. It dissolves in fused potash, and imparts 

 a blue colour to the compound. Magnesia, with a drop of 

 nitrate of cob'alt, when dried and ignited, assumes a feeble but 

 characteristic rose tint, by which the presence of that earth in 

 minerals containing no metallic oxides nor alumina, is ascer- 

 tained in blowpipe experiments. A compound of oxide of 

 cobalt with alumina is obtained by mixing the solution of a salt 



