132 D. Forbes, Esq., on the Determination of 



which should be conducted at a boiling temperature, and car- 

 ried on without intermission until concluded. In cases where 

 even a very small amount of organic matter* is present, either 

 in the solution, or, as frequently happens, in the potash em- 

 ployed, the precipitation is not complete, or rather a portion 

 of the oxide is retained, dissolved in the solution, and is not 

 precipitated by the further addition of potash. 



Another objection also is, that when igniting the oxide a 

 portion of it may be reduced by the carbonaceous matter of 

 the filter ; this can, however, be readily obviated, either by 

 prolonged ignition, or by moistening the oxide with nitric 

 acid, and again igniting, but the operation is then necessarily 

 more tedious. 



When we likewise consider that copper and nickel are 

 generally separated in the course of analysis, by precipitation 

 as sulphuret, we shall at once see the desirability of endea- 

 vouring to determine their quantities as directly as possible 

 from that compound ; and the following experiments were 

 made with this object in view. 



The first point inquired into was, whether the precipitated 

 sulphurets of prolonged ignition, or rather calcination, could 

 not be completely deprived of all their sulphur, and converted 

 into oxides. The results obtained were, however, not at all 

 satisfactory, as the sulphurets were never completely decom- 

 posed and invariably yielded too high results, as may be seen 

 from the following experiments : — 



(a) 3*75 gr. metallic copper, equivalent to 4*70 gr. oxide, 

 yielded 5*47 gr. incinerated sulphuret ; being an ex- 

 cess of 0-77 gr., or equal to 10*38 per cent, too much 

 oxide. 

 (0) 3*63 gr. metallic copper, yielded 5-10 gr., instead of 4*55 

 gr. CuO ; equal to excess of 0*55 gr., or 12*08 per cent. 



(c) 1*76 gr. copper = 2'20 CuO, gave 224 gr., an excess of 



004 gr., or 1*81 per cent, too much. 



(d) 25*00 gr. of a mineral, containing by a previous ex- 



amination 45*31 per cent, copper, afforded 17*64 gr. 



* This is particularly apt to take place if great care is not taken to incine- 

 rate the filter perfectly on which these metals had heen precipitated pr<\ ii ill 

 to solution. 



