Giirpsland Auriferous Ores. 7 



Limonite evolves water and becomes spongy ; but at too high 

 a temperature it will contract. 



Siderite evolves carl^on dioxide and carbon monoxide, and 

 becomes spongy ferric oxide. 



The last three minerals should be treated by themselves on the 

 last hearth of the furnace, since they require a temperature just 

 sufficient to dehydrate them and render them fit for leaching. 



Metallic iron is wholly oxidised if not in pieces over one-eighth 

 of an inch diameter. 



Barite passes through the furnace unchanged, and offers no 

 difficulties. 



When oxygen is supplied to ores in the manner suggested, 

 there need not be any fear of antimoniates or arseniates inter- 

 fering with subsequent work ; nor would I recommend the 

 addition of carbon to eliminate these. Some fine charcoal 

 accidentally became mixed with a small parcel of roasted ore at 

 our works, and it was found, in burning this out, that a great 

 deal of ferric oxide became reduced to triferric tetroxide, which 

 we could not convert back again into the higher form. 



In order to test whether the material has been thoroughly 

 roasted, several tests have been recommended. KusteP recom- 

 mends throwing the heated ore into water and then plunging 

 a bright iron rod in ; if the rod is darkened, then sulphates are 

 still present. 



Rose^ has pointed out that this method is worthless, since it 

 will not show the presence of sulphate of iron ; he, however, 

 suggests the addition of barium chloride as a test for sulphates. 

 But this test is also valueless, since sulphate of zinc, or sodium, 

 may exist in an ore without interfering with the action of 

 chlorine ; and when it is considered that sulphuric acid in 

 certain cases is actually mixed with the ore, it is certainly not 

 necessary to test specially for this. 



Eissler^ recommends the use of ferrocyanide of potassium to a 

 solution of any salts in the ore, to which there is no objection ; 

 but it should be pointed out that cobalt and nickel salts will give 

 a greenish precipitate with this rejagent. The main objection to 



1 Roasting of Gold and Silver Ores, San Francisco, 



2 The Metallurgy of Gold, Rose, p. 229. 



3 The Metalhirgy of Gold, Eissler, p. 279. 



