ASSAYING. 137 



That the iron pyrite did not carry all the gold and silver in this porphyry 

 is shown by the fact that it contained when assayed separately only 80 

 cents (0.0001327 per cent.) gold and 65 cents (0.0017236 per cent.) silver, 

 whereas it should have assayed $6.34 (0.0010518 per cent.) gold and 

 $3.17 (0.0085062 per cent.) silver, there being 1.89 per cent, of it in the 

 porphyry, had it contained all the precious metals present in that rock. 

 There seems to be no doubt that the iron pyrite present in this porphyry 

 is a secondary product; that is to say, that it was not crystallized out of the 

 melted mass when it cooled, but that it was formed later either through the 

 action of sulphureted hydrogen or sulphur in some other form upon the 

 iron contained in the rock. It is difficult to conceive of the formation of 

 pyrite from a melted mass under conditions which would permit of the iron 

 retaining the extra atom of sulphur necessary to its composition. As it is 

 evident that this porphyry contains silver and gold independent of that in 

 the pyrite, it is highly probable that these metals were present in that rock 

 before the formation of the pyrite, and that the same causes, probably those 

 of solfataric action, which brought about the formation of the pyrite, effected 

 a partial concentration of the silver and gold in this mineral. 



This porphyry was also examined for lead. The ordinary methods of 

 analysis failed to reveal its presence, although it was thought highly prob- 

 able that it entered into combination with the rock in very minute quanti- 

 ties. I adopted the following method, founded on the well-known tendency 

 of gold to retain small quantities of lead even when in a melted state and 

 exposed to the air. Forty grammes of the finely-pulverized porphyry were 

 mixed with 150 grammes of carbonate of potash in a porcelain dish, and 

 the whole was moistened with an acid solution of terchloride of gold which 

 contained 10 grammes of gold. The mass was dried and fused for four 

 hours in a French clay crucible in a coke fire. The resulting gold button 

 was then analyzed by Dr. Melville in the following manner: The gold was 

 dissolved in aqua regia and filtered hot to remove traces of slag. The gold 

 was precipitated with oxalic acid, the solution filtered, and the filter washed 

 with hot water to remove chloride of lead. The filtrate was evaporated to 

 dryness and ignited at the lowest practicable temperature to decompose the 

 oxalate, and also to remove the excess of oxalic acid used in precipitating 



