PARTING. 219 



Lastly, parting by sulphur is made by fusion, and 

 is, therefore, called dry parting. 



Parting gold from silver by nitric acid or aqua 

 fortis. 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 it proper proportion ; for if the 

 gold be in too great a quantity, the silver would be 

 covered and guarded by it from the action of tHfe 

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

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

 mass upon a touch-stone (which Is u'sually composed 

 of black basalt, though black pottery will do 

 very well,) so as to leave a mark upon it; they 

 then make similar marks with the pro of -nee dies, 

 (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 scale of admixture. 



If the trial shovvs that in any given mass the 

 silver is not to the gold ag three to one, this inass 

 is improper for the operation of parting by aqua 

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

 sary to make any alloy of that proportion must be 

 added. This operation is called quaftation, 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 were not attended 

 to, k quantity of silver proportionable to these two 

 foreign acids, would be separated during the solu- 



