326 PROCEEDINGS OF SECTION B. 
silver and silver sulphide. Metallic silver is nearly nine times 
more soluble in the latter than in the former, whilst silver sulphide 
is absolutely unaffected by ordinary solution, but is rapidly attacked 
by extra solution. As a considerable amount of silver sulphide is 
frequently left undecomposed after the chloridising roast, the extra 
solution is generally found to extract 10 per cent. to 20 per cent. of 
the total silver in the ore after as much as possible has been removed 
by ordinary solution. Cu,S is precipitated during the solution of 
Ag.S ; but no precipitation of metallic copper can take place 
during the sclution of metallic silver, owing to its ready solubility 
in sodium hypo-sulphite. 
Before passing to the subject of precipitation, it may be as well 
to discuss at this point the best materials for vats. Three kinds 
of vats suggest themselves for use, viz., plain wood, wood lead-lined, 
and concrete. Mr. Hall, in the first part of this paper, has already 
drawn your attention to the unsuitability of the first-named in the 
hot dry climate prevailing in many parts of this continent, owing 
to its rapid shrinking when allowed to get dry. This has been 
found a considerable drawback to the use of wooden vats in 
Charters Towers and elsewhere in Australia. 
In the chlorination works at Mount Morgan, the second kind 
has been used for many years with great success; but owing to 
the rapid action of extra solution on metallic lead, vats of this 
description cannot be looked upon as satisfactory for a Russell 
mill, even when the lead is protected by a coating of tar or pitch. 
In South Africa, and more recently in Queensland, wood, as a 
material for leaching vats, is being superseded by concrete, which 
seems amply to satisfy all requirements. It was used with success 
at Rivertree for the stock solution sump, and when faced with 
neat cement, should make excellent leaching vats, which would 
not be appreciably affected by any of the solutions used in the 
Russell process. 
VI. PRECIPITATION. 
The sulphide of soda used to precipitate the silver is not a simple 
compound, but a mixture of various sulphides from Na,S to Na,S,, 
the proportions in which the different sulphides occur being 
dependent on the relative quantities of soda and sulphur used in 
the preparation of the solution. Na,S precipitates silver from 
hypo solutions regenerating sodium hyposulphite thus : 
AgS,0; + NaS = AgS + Na,8,0. 
If any higher sulphide be used, a corresponding amount of free 
sulphur is precipitated with the silver thus : 
AgS,0, + NaS+x = AgS + Na,S,0, + x8. 
