175 



Assuming then that 1^ lb. is the smallest quantity upon which any assay 

 can be satisfactorily made, it remains to be considered how such a quantity can 

 be acted upon so as to separate any gold it may contain, with such precision 

 as to attain certainty that the result shall be true to at least one-eightieth part 

 of a grain. 



Now the methods of assaying may be divided into three classes : — 1st. 

 Separating out the gold from the mass by a menstruum which will dissolve the 

 precious metal, without acting on the earthy matter. 2nd. By resolving it 

 into a form by which the earthy matter may be dissolved out, leaving the 

 metallic matters free and in moderate compass. 3rd. Fusion of the whole 

 with such a flux as would cause any gold or silver present to separate from the 

 slag, either alone or in combination with lead. 



This last practice, which is the dry assay proper, yields very certain and 

 accurate results, but since the matters would require to be mixed with five or 

 six times their weight of litharge, the quantities which I have stated as the 

 minimum which could be used with advantage, would require the use of 

 crucibles so large as to be exceedingly inconvenient and expensive ; since a 

 crucible could never be used twice. 



The second method, as ordinarily practised by the chemist, of fluxing with 

 carbonate of soda, would be still more inconvenient from the same causes, but 

 a modification of the process, which T shall presently explain, appears to me 

 the most likely one for easily obtaining reliable results. 



In the use of the first method the menstrua which will dissolve the gold 

 from the earthy mattei^s are confined to two, viz., metallic mercury, and 

 chlorine. Now the principal use for which assays of tailings are required, is 

 to ascertain whether the mercury used on the ripple-tables and in the stamp- 

 box has succeeded or not in dissolving the whole of the gold out of the 

 material. If it has failed in doing so, the most probable cause of failure is 

 that the particles of gold may have been coated with a film of some matter 

 which prevented the contact of mercury with them. But if this is so, it is 

 obvious that the same cause will be in operation to prevent combination when 

 an assay is made by the same means, and that nothing could be more futile 

 than to attempt to test the fact, whether the whole of the gold had been 

 amalgamated by the mercury on the ripple beds, by repeating the very same 

 operation on a sample, therefore I think it is demonstrable that any assay of 

 tailings by amalgamation, is absolutely delusive and worthless. 



The other method of solution, by chlorine, would be nearly perfect if the 

 gold were in a state approaching to purity, or were it alloyed only with 

 copper ; but gold mixed with from one-third to one-half its weight of silver, as 

 is the case generally with Thames gold, is precisely that modification which is 

 insoluble in chlorine, the coating of chloride of silver formed, being sufficient 

 entirely to protect the gold beneath it, from the solvent action of chlorine, 

 unless the mechanical subdivision of the particles is absolutely infinitesimal. 



We are thus left to the second method of dissolving off the eai'thy matter, 

 and this must be done without the use of crucibles. 



Now quartz is soluble in solution of caustic potash, at all temperatures, 

 and at a temperature of about 300°, and upwards, the solution takes place 

 readily and rapidly, if then the sample of earthy matters, mixed with about 

 three times its weight of caustic potash dissolved in three or four parts of 

 water, were placed in a clean iron vessel, in a steam chest, in which it could 

 be subjected for two or three hours to the action of steam at a pressure of 

 about 60 lbs. to the square inch — which corresponds to the temperature of 307° 

 — the whole of the quartz, or at least Avith the exception of a few of the larger 

 grains, would be resolved into a silicate of soda which would then readily 

 dissolve out with hot water, leaving the gold and silver with oxide of iron and 



