432 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The most striking evidence that a compound exists around the anode 

 which is capable of depositing pure silver is the existence of the " anode 

 dust." This consists of a fine powder, more or less closely adhering to 

 the anode. Examination with the microscope indicates that this powder 

 consists of minute crystals, which have every appearance of being metallic 

 silver. Rodger and Watson * analyzed the air-dried powder, and found 

 as a matter of fact that the metal is essentially pure. The contrary con- 

 clusions of Myers f and others may have been based upon results obtained 

 with small anodes, where argentic peroxide may have been formed. 



In our experience the weight of this dust is approximately propor- 

 tional to the area of the silver anode, with a given current. It seems 

 highly probable, then, that the silver at first tends to separate from the 

 anode as a polymerized ion, perhaps Ag 3 + , according to the common 

 principle that an unstable compound often forms the bridge between two 

 stable conditions. $ The greater portion of this complex ion would be 

 expected to break up at once into the normal argentic ion and metallic 

 silver (Ag 3 + = Ag + + 2Ag), the latter forming the " anode dust." The 

 last traces of the complex might, however, persist for some time, and 

 give rise to all the phenomena seeming to be due to the existence of 

 supersaturated silver in the solution. 



The argument has been so protracted that it is perhaps worth while 

 to recapitulate the way in which this interpretation would explain the 

 irregularities not to be attributed to the nitrite and oxycomplexes. 



This complex ion of polymerized silver undoubtedly unloads silver at a 

 lower potential (*. e. more easily) than the simple silver ion. Hence the 

 larger the kathode surface exposed, the greater part will the complexes 

 take in the carrying of the current, and the larger will be the deposit of 

 silver. This consequence of the theory agrees with the experience of 

 all experimenters. Moreover, since the complexes are unstable, and 

 continually tending to decompose, there must be always in solution a 

 trace of molecular unionized silver, which, being supersaturated, will 

 deposit on contact with solid silver. If the platinum bowl has been 

 previously lined with silver, this extra deposition will begin almost 

 immediately ; while if it has not been thus lined, an appreciable silver 

 surface will have to be formed before the relieving of the supersaturation 

 will begin to take place. This reasoning explains the invariable excess 

 of the deposit upon a silver kathode over and above the amount deposited 



* Phil. Trans., 186 A, 632 (1895). t Wied. Ann., 55, 295 (1895). 



\ Ostwald, Z. phys. Chera., 22, 307 (1897). 



