338 Facts upon which to found a History 



they are then red coIaureJ and opaque. In this state they 

 can support a red heat without being decomposed, except 

 in those points at which they touch the retort. 



2d. When we mix sulphate of potash with the fore- 

 going-, we obtain more vohmiinous crystals, which are 

 rhomboidal cubes. This triple salt is less soluble than the 

 simple 5 it only loses 0:26 of water upon distillation. 



Carbonate, — The carbonate of potash gives from 0*40 to 

 0*42 of carbonate of cobalt with the simple sulphate. An 

 excess of alkali dissolves a great part of the precipitate. 

 Ebullition and cold water decompose this solution. 



Oxide at the minimjim. — 100 parts of carbonate leave,. 

 after the separation of the water and carbonic acid, from 

 0*60 to 0*62 of greenish gr;iy oxide. In order to have it 

 very pure, we must charge the retorts as full as possible, and 

 heat them gradually. Without these precautions, we ob- 

 tain an oxide mixed with oxide at the maximum, which, in 

 this case, gives oxygenated gas with the muriatic acid, while 

 that which is pure does not give an atom of it. 



The gray oxide is dissolved with effervescence in the nitric 

 acid, without giving nitrous gas ; when heated in the open 

 air. It 'becomes black immediately. We easily discover aa 

 oxide, some parts of which are raised to the maximum by 

 the application of a weak acid, which dissolves only the 

 minor oxide. Ammonia operates the same separation as 

 Thenaid remarked. 



Oxide by Precipitation, — 1st. Some drops of nitrate of 

 cobalt, poured into boiling water with a little potash in it, 

 give a blue precipitate, which at last becomes red if the 

 ebullition be continued :. in this case a hydrate is formed ! 



2d. If we employ alkali^Jed water cold, the blue precipi- 

 tate is formed ; but in place of making a hydrate it passes 

 to the green, without the contact of the air being able to 

 obscure its shade : it preserves this colour after being dried» 



3d. If v/e boil this green precipitate, while it is fresh, ia 

 water with a little potash, it becomes a reddish gray, and 

 does not change any more. 



The weak aci^s, vinegar for instance, dissolve the first 

 ' i • precipitate 



