316 ScientiflG Proceedings, Royal Dublin Society. 



an equivalent portion of the metallic iron being consumed on each 

 precipitation :-— 



Ou SO +Fe = Fe SO4+CU, 



From this it appears that the chief reason for the excessive 

 consumption of iron is the presence of these ferric salts, and fur- 

 thermore, M. Zoppi found that in the presence of cast-iron this 

 ferric sulphate is converted into ferrous sulphate, and a basic sul- 

 phate (Fe^ SOfi) that coats the metals and prevents the copper 

 solution acting upon it. The means adopted at Schmollnitz for 

 overcoming this latter difficulty have been described (page Sll), 

 also that proposed for the Ovoca district (pages 308 and 309), but 

 it is often more advantageous to get rid of these salts as much 

 as possible. In order to precipitate them, it has been proposed to 

 add to the solution either calcic hydrate (milk of lime) or car- 

 bonate of lime in sufficient quantity to nearly neutralize the 

 liquid without precipitating the copper, by this means arsenic and 

 antimony are also removed. 



At Agordo the method adopted was to saturate the solution 

 with sulphurous acid, and thus reduce the ferric salts : — 



Fe, (S0,)3+ S0,+2 H, = 2 Fe S0,+ 2 H SO, 



Here, however, there is an introduction of free acid which would 

 also cause an excessive consumption of iron, but on the other hand 

 the resultant precipitate is very pure. (See Table D.) 



The removal of the silver from solution varies according to the 

 precipitant subsequently used for the copper, if that be iron, as is 

 most genera], a soluble iodide (as potassic iodide, Glaudet) is added 

 in sufficient quantity to precipitate the silver, and after the preci- 

 pitate has subsided, the clear liquors are run off to be subsequently 

 treated for the contained copper. It has also been pro- 

 posed to blow finely divided iron dust into the liquid, till nineteen 

 per cent, of the copper is precipitated, which then contains about 

 eighty per cent, of the silver originally present. (Snelus.) 



When sulphuretted hydrogen is used for precipitating the 

 copper, the greater part of the silver is contained in the first six 

 per cent, of the precipitate, which is accordingly first thrown 

 down and treated separately for the contained silver (Oihhs). 



Alkaline sulphites have also been proposed. 



