RICHARDS AND HEIMROD. —THE IMPROVED VOLTAMETER. 



429 



TABLE Vm. 

 Standard Method on Platinum and on Silver. 



The only difiference is now in the opposite direction ; and this was due 

 to known experimental error. In experiment 27 a small loss of silver 

 particles in the wash-water from the silver cell produced the difference of 

 0.009 per cent. It is highly probable that but for this accident, the deposit 

 on silver would have been equal to that on platinum, as it is in No. 28. 

 These results permit us to draw two conclusions. First, it is not the greater 

 inclusion of silver salt in the crystals which increases the total weight 

 when the kathode is silver. Otherwise 27 and 28 should have grown 

 heavier in the same ratio. Secondly, it is the anode solution again 

 which is responsible. 



The increase in the deposit on a silver surface indicates the existence of 

 silver in the solution in a supersaturated state ; and this existence shows 

 that there must be present some complex gradually dissociating, with 

 metallic silver as one of its products. If this is the case, we should ex- 

 pect to find that an oxidizing environment would be capable of removing 

 this cause of inaccuracy, while substituting another easily removed by 

 nitric acid. As a matter of fact, Schuster and Crossly * have shown 

 that deposits made in vacuo are heavier than when made in air ; again 

 those made in an atmosphere of air are heavier than when made in oxy- 

 gen. Of course it is understood that in all three cases the anode was 

 only wrapped in filter paper. The solution usually contained fifteen 

 per cent of silver nitrate, but sometimes as much as thirty per cent. 

 They used the solution over and over again, thereby accumulating the 

 irregular compounds. Under reduced pressure (about " 1 ^ inch "), the 

 deposits exceeded those made in air by about 0.04 per cent, while the lat- 

 ter exceeded those in oxygen by 0.04 per cent more. Myers, f who re- 

 peated these experiments, found the difference between deposits in air 

 and in vacuo to be as much as 0.10 per cent for 20-40 percent solutions. 



* Proc. Roy. Soc, 50, 356 (1892). 



t Wied Ana., 55, 291 ff. (1895). 



