REFINING METALS. 225 



the solution is diluted with water, filtered, and a 

 plate of copper hung in it, till no more precipitate 

 falls down. Then the weight of the precipitate, 

 when edulcorated, is compared v/ith that of the 

 whole alloyed metal put to trial. 



This silver dust well washed, and mixed with 

 gum water, serves as a pigment in water painting. 



Separating Silver from Copper hy an Alcaline 

 Sulphuret. 



The affinity of copper with sulphur is stronger 

 than that of silver. Upon this ground, liver of 

 sulphur (sulphuret of potash) has been proposed as 

 an expedient to free silver from copper ; for if 

 silver holding copper be fused with alcaline sul- 

 phuret, the base metal combines with the latter, 

 and is converted into scoriae floating on the silver, 



Mr. Keir*s Mode of separating Silver from Copper* 



Chemists have long been acquainted with the 

 compound acid, called aqiia regia (nitro-muriatic 

 acid), which has the exclusive property of dissolv- 

 ing gold. The discovery of a compound acid, act- 

 ing exclusively upon silver, is owing to Mr. Keir. 



This compound acid is made by dissolving one 

 pound of nitrate of potash (common nitre or salt- 

 petre), in eight or ten pounds of sulphuric acid (oil 

 of vitriol), or by mixing together sulphuric and 

 nitric acids. This acid dissolves silver easily, 

 while it will not attack copper, iron, lead, gold, or 

 platina. 



These properties have rendered it capable of a 

 very useful application in the arts. Among the 

 manufactures at Birmingham, that of making ves- 



VOL. II. Q 



