1839] 



On Chemical Tests. 



309 



immediately assumes a bluish-green colour which preserves the same 

 shade even after dessication. 



b. Solutions of chromium are distinguished by their green colour, which 

 undergoes no alteration on being treated with sulphuretted hydrogen. 

 Chromium gives an emerald green before the bloio-pipe, and imparis a 

 beautiful green colour to the fluxes. This colour is discriminative ; it 

 occurs both in the inner and outer flame, and distinguishes chromium 

 from copper, ivhich produces a green bead only in the outer flame, 



49. COBAXsT. Pure oxide of cobalt has a greenish grey colour o 

 and both it and cobalt sails are easily detected before the blow-pipe by 

 producing an intensely blue glass when mixed with "20 times its weight 

 of borax. It may also be known by its solution in acid being of 

 a violet red, or rose colour ; by alkalies producing a precipitate 

 of violet blue, and by alkaline bydro-sulphurets giving a black 

 precipitate. Carbonate of potash produces in cobalt solutions, a red 

 precipitate, which, upon being boiled becomes blue. See table of 

 re-agents. 



a. The following tests act on cobalt, and produce the colours 

 specified. 



Potash. Blue; after repose green : if boiled dirty pale red. 

 Ammonia. Blue ; more ammonia turns the precipitate green, and 

 still larger quantity dissolves the precipitate and forms a green solution. 

 Carbonate of potash. Red ; if boiled blue. 



Carbonate of ammonia. In neutral solutions, red ; soluble in mu- 

 riate of ammonia, 



Phosphate of soda. In neutral solutions, blue. 

 Prussiate of potash. Green turning grey. 

 Hydrosulphuret of ammonia. In neutral solutions, black. 



b. Solutions of cobalt-salts are particularly distinguished from 

 zinc, manganese, zirconia, cerium, yttria, thorina, glucina, alumina, 

 and the earths and alkalies, by producing a black precipitate with 

 hydrosulphuret of ammonia. This precipitate is hydrosulphuret of 

 cobalt. 



c. An impure oxide of cobalt fused with a mixture of sand and 

 potassa, produces a blue glass, which, reduced to powder, is known 

 in commerce by the name of smalt. 



d. Neither cobalt nor nickel is precipitated by the carbonate of 

 lime or of magnesia. 



