73 On Assaying Silver. [April 



furnace, and through which there flows a current of air ; the 

 lead, and the alloy in the silver become oxidized, and are ab- 

 sorbed by the crucible, leaving, when the operation is com- 

 plete, a button of pure silver, and the difference between the 

 weight of this pure silver, and the original weight of 15 or 20 

 grains, points out the quantity of alloy with which the silver 

 was contaminated, 



4. This method of assay is defective. In the first place, 

 if any gold be contained in the silver, its quantity remains 

 unknown, and it continues to be alloyed with the silver : I 

 once obtained from a rupee as much gold as was equal to 

 T Vth its value. In the next place, the crucible absorbs a por- 

 tion of the silver, and an allowance is accordingly made for 

 it ; but in nice experiments some assay ers choose to report the 

 actual weight of the button of pure silver, noticing only that 

 no absorption allowance has been made, which seems to im- 

 ply, that the exact quantum of allowance cannot be precisely 

 fixed : and in the third place, the heat of the fire materially 

 affects results ; so that although specimens of silver bullion 

 of the same known quality may be assayed together, the 

 whole of them do not result, as they should do, of equal and 

 uniform fineness. 



5. I stated that if gold be contained in silver, the assay 

 by fire does not detect it ; the seller of bullion thereby suffers 

 a loss in proportion to the quantity of gold that his silver 

 contains. Silver from the eastward is sometimes rich in 

 gold, and indeed almost all silver contains gold in greater or 

 less quantity. This is a fact well known and acted on in Eu- 

 rope. Silver is not there, now, refined by fire as it used to be 

 formerly, but is dissolved in sulphuric acid, which acts upon 

 the silver, and its base alloy, but leaves the gold untouched 

 in a metallic state, resembling a brown residue, which, when 

 washed and submitted to a red heat, regains the yellow colour 

 of pure gold. The silver which is held in solution by the sul- 

 phuric acid is precipitated by iron or by zinc, and when col- 

 lected, washed, and melted, is found to be silver of fine qua- 

 lity. The gold by this process is therefore saved, and I un- 

 derstand it to be the practice in England for the refiner, if 

 silver is rich in gold, to retain as his fee 8 grains of gold for 



