PURIFICATION OF SILVER. 1 7 



tion of the chloride with invert sugar* and sodic hydroxide. While 

 it is true that this process is not the best for the final treatment of the 

 silver, the precipitation as chloride is an excellent step in the prelimi- 

 nary purification. The invert sugar was made from a filtered solution 

 of "rock sugar." No metals were found in it by electrolytic, treat- 

 ment. The sodic hydroxide used, after settling and decantation, had 

 been electrolyzed until iron was completely removed. This slow but 

 effective way of removing foreign metals was hastened by using a 

 rotating cathode, while a fixed anode stirred the solution. The 

 argentic chloride was precipitated from a somewhat dilute solution, to 

 avoid the occlusion of impurities, and must be thoroughly washed. It 

 is best to wash at first with cold water, in order to avoid the contrac- 

 tion of the precipitate. The reduction was carried out in a silver 

 dish to avoid the introduction of silicates. This precaution is not 

 needed when electrolytic purification is to follow. In some cases, 

 where the original material was not wholly free from suspicion, the 

 precipitation as chloride and reduction were repeated. After reduc- 

 tion, the washed silver was fused upon either sugar charcoal or pure 

 lime, before the blast lamp, whose nozzle was scrupulously cleaned. 

 If the large buttons of metal thus formed are not kept hot too long 

 and are cooled in a reducing flame, they are already extremely pure.f 

 For ordinary work, or even for work on atomic weights of usual 

 accuracy, its purity suffices. Nevertheless, it can not be considered as 

 perfectly pure, for it must contain, perhaps, o.ooi per cent of sulphur 

 and variable traces of carbon from the illuminating gas (or else oxy- 

 gen from the air), as well as an occasional trace of argentic chloride, 

 arising from incomplete reduction. When thus cooled it is likely to 

 contain numerous minute cells inclosing gas. 



As one stage in the purification the electrolytic method used by 

 J. L,. Hoskyns Abrahall has many advantages. | The almost pure 

 material just described is made the anode of a cell containing a con- 

 centrated solution of argentic nitrate prepared from the same silver. 

 The cathode is a pure silver wire, upon which is deposited by a 

 properly regulated current beautiful crystals of electrolytic silver. 

 The finely crystalline powder which falls from the anode may be easily 



*According to Stas, saccharose at ioo would have answered as well as invert 

 sugar at 6o. The saccharose is, of course, inverted by the strong hot alkali. 

 Oeuvres, 3, 13. 



t Richards, Proc. Am. Acad.,29, 65 (1893; Z. Anorg. Chem., 6, 98 (1893). 

 See also Proc. Am. Aoad.,38,450 (1903). This work was confirmed in the latter 

 part of the present investigation. 



J J. Oh. Soc. Trans., 61, 660 (1892). 



^Richards and Heimrod, Proc. Am. Acad., 37, 415 (1902). 



