OF ARTS AND SCIENCES. 55 



chloride of silver is effected, or from the sulphuric acid which is used 

 to excite the reaction. 



The method of analysis employed was as follows. The coin having 

 been dissolved in moderately strong, pure, nitric acid, the cooled solu- 

 tion was treated with an excess of a solution of pure caustic ammonia, 

 added by small portions, — no notice being taken of the small, dark- 

 colored residue (AgS, etc.) insoluble in nitric acid. The precipitated 

 oxide of silver was re-dissolved in nitric acid, enough of the latter being 

 added to render the solution distinctly acid. The whole was then 

 heated in a water-bath, and a strong solution of pure chloride of ammo- 

 nium added, until chloride of silver was no longer precipitated, care 

 being taken to avoid adding an unnecessary excess of chloride of am- 

 monium. It will be seen, that the chloride of silver was thus pre- 

 cipitated in presence of a large excess of nitrate of ammonia; and 

 since chloride of lead is readily soluble in a solution of nitrate of 

 ammonia,* any lead which the coin may have contained would remain 

 in solution. It may be mentioned in passing, that the small amount 

 of chloride of silver which, as has already been stated by Mulder,t is 

 retained in solution by nitrate of ammonia, — especially by hot solu- 

 tions, — did not interfere in the least with the subsequent steps of the 

 analysis, or in any way to influence the amount of lead obtained. 

 The precipitated chloride of silver was collected upon a filter, and 

 thoroughly washed with hot water, the lumps of chloride of silver 

 being broken down as much as possible with a glass rod. The filtrate 

 and wash waters were evaporated in a porcelain dish to the consist- 

 ence of a thin syrup, and then transferred to a flask of hard German 

 glass, in which the evaporation was continued until the nitrate of am- 

 monia had been entirely destroyed. This operation requires care in 

 the application of the heat, lest violent decomposition of the nitrate of 

 ammonia ensue, and portions of the substance be projected from the 

 flask. When properly conducted, the process goes on tranquilly, and 

 no loss occurs. The residue, consisting of nitrate of copper, together 

 with nitrate of lead, and more or less gold, should be washed with hot 

 water into a porcelain dish, where it is mixed with a slight excess of 

 pure sulphuric acid. It may be necessary to use a little nitric acid to 

 dissolve the last portions of the substance in the flask. The mixture 



* Bolley, Ann. Ch. u. Pharm., 1854, XCI. 115. 

 t Die Silber-Probirmethode, (Leipzig, 1859,) p. 28. 



