ON THE SILVER VOLTAMETER. 593 



< )ur experiments have shown that the abnormally heavy deposits cannot be explained 

 HH clue to sulphide acting in presence of copper or of iron. 



Two solutions were prepared by dissolving 60 grammes of silver nitrate in 

 60 grammes of water, and to each of these was added 0'12 gramme of commercial 

 pure copper foil ; this was left in contact with the solution until the copper had 

 dissolved as nitrate by displacing an equivalent quantity of metallic silver. Hydrogen 

 sulphide was then added and the solution filtered and diluted as before ; very little 

 sulphide was precipitated by diluting, and the electrochemical equivalent was found 

 to be in the case of the first solution 



1-11850 (67c); 



in the case of the second solution, to which hydrogen sulphide had been lavishly 

 added, a nearly normal value was obtained, 



1-11824* (74a). 



Similar experiments were made with iron. This could not be introduced in the 

 same way as the copper, for the metal appeared to become passive in contact with the 

 strong nitrate solution and refused to dissolve. Two solutions were prepared by 

 adding 1 gramme of crystallised ferric nitrate to GO grammes of silver and adding 

 sulphuretted hydrogen as before. The first solution gave the perfectly normal electro- 

 chemical equivalent 



1-11825 (67d), 



and the second, to which much more hydrogen sulphide was added, gave the value 



1-11834 (74c), 



also substantially normal. The ferric nitrate was strongly acid, and this fact must 

 be taken into account in discussing the above result, but it is clear that the 

 extraordinarily high electrochemical equivalent of the mother-liquors cannot be 

 attributed to the presence either of iron or of copper. Addition of ferric nitrate to 

 the mother-liquor produced no marked change in the electrochemical equivalent, the 

 value obtained being 1-12141 (276) and 1 '12055 (30c) before and 1 '121 71 (74</) after 

 the addition of 1 gramme of ferric nitrate to about 400 cub. centime, of 15 per cent, 

 mother-liquor. 



J. Influence of Nitrite and Hyponitrite. 



The abnormally high deposits obtained with the silver voltameter have usually 

 been attributed to anodic impurities. Such impurities would normally be oxidised 

 substances comparable with the persulphuric acids, with lead peroxide, or with silver 

 peroxynitrate. RICHARDS has, however, made the suggestion that reduction may 



* The fact that this figure is somewhat lower than the normal may be due to the trace of acid which 

 is liberated by the sulphide 2AgNO s + SHj = AgjS + 2HN0 8 . The solution did not, however, appear acid 

 to litmus. 



VOL. CCVII. A. 4 O 



