PARTING. 219 



Lastly, parting by sulphur is made by fusion, and 

 is, therefore, called dry parting. 



Parting gold from silver by iiitric acid or aqua 

 j^Wf^.— Although parting by nitric acid be easy, it 

 cannot succeed, or be very exact, unless we attend 

 to some essential circumstances. The gold and 

 silver must be in a proper proportion ; for if the 

 gold be irl too great a quantity-, the silver would be 

 covered and guarded by it from the action of the 

 acid ; therefore, when assayers do not know the pro- 

 portion of gold to silver in the mass, they rub the 

 mass upon a touch-stone (which is usually composed 

 of black basalt, though black pottery will do 

 very weil^) so as to leave a maiiv upon it; they 

 then make similar marks with the proof-needles, 

 (which are needles composed of gold and silver 

 alloyed together in graduated proportions,) and by 

 comparing the colour of the several marks, they 

 discover the probable sicaie of admiJiture. 



If the trial shows that in any given mass the 

 silver is not to the gold as three to one, this mass 

 is improper for the operation of parting by aqua 

 fortis. In this case, tiie quantity of silver neces- 

 sary to make any alloy of that proportimi must be 

 added. This operation is called quart<ition, be- 

 cause it reduces the gold to a fourth of the whole 

 mass. No inconvenience arises from too great 

 quantity of silver, except a waste of aqua fortis. 

 The nitric acid, or aqua fortis employed, must be 

 very pure, and especially free from mixture of sul- 

 phuric and muriatic acids. Its purity must, there- 

 fore, be ascertained; and if this be found not suffi- 

 cient, the acid must be purified by nitrate of silver. 

 If the purity of the nitric acid W' ere not attended 

 to, a quantity of silver proportionable to these two 

 foi^eign acids, would be separated during the solu- 



