ON THE SILVER VOLTAMETER. 549 



L, M, and N are platinum vessels with lateral projecting ears to support them 

 during electrolysis. L is nearly cylindrical in shape, M is a cylindrical ring only, and 

 N is hemispherical. The vessels N and were very kindly lent to the National 

 Physical Laboratory by Professor G. VAN DIJK ; N is the small bowl referred to as B 

 in Professor VAN DIJK'H papers,* and O is one of the crucibles indicated by L and II. 

 For the vessel E we are indebted to Professor AYKTON. 



The Anodes. 



An anode usually consisted of a pure silver plate, 5 'Ox 5 '0x0 '4 centims., held by 

 a silver rod riveted through its centre. The silver was obtained from Messrs. 

 Johnson, Matthey and Co. Before employing it for a determination of the electro- 

 chemical equivalent of silver, about 10 grammes of silver were deposited electrically 

 on the plate, and on these occasions a platinum bowl with a deposit of silver on its 

 inner surface was generally used as the anode. In this way the silver was always 

 removed from the platinum bowls, but not from those of silver. When new, the 

 surface of a platinum vessel is free from scratches, and silver deposited on it adheres 

 much better than to a scratched surface ; it is important, therefore, to avoid the use 

 of a spatula. The electrical method removes the deposit, cleans the platinum, and 

 at the same time prepares the anode. Shortly before using, the latter was washed 

 with distilled water and dried in an electric oven. The platinum bowls were rinsed 

 with distilled water and strong nitric acid, and if much of the dark silver salt 

 Ag 7 NO u adhered to the platinum the washing with nitric acid was repeated. 

 Distilled water was finally used and the bowls dried in an electric oven at 1 60 C. 



For the Richards form of voltameter silver rods of two sizes were used ; some of 

 the rods were 1 centim. in diameter and the others 2'5 centims. These rods were also 

 coated with electrically deposited silver. 



The Electrolyte. 



For the comparison of various types of voltameters the electrolyte was usually a 

 15 per cent, solution of pure silver nitrate in water. The silver nitrate was sometimes 

 purchased and sometimes recovered from used solutions. A description of the 

 preparation of the pure salt is given in Part II., p. 585, of this communication. 



Form of Voltameters. 

 The following types were used : 



(1) The Rayleigh Form (fig. l). In this the kathode was usually a platinum bowl 

 and the anode a silver plate or rod coated with electrically deposited silver. The 

 anode was inserted in a cup made of filter paper, the folds in the paper being secured 



* VAN DIJK and KUXST, 'Ann. der Phys.,' U, p. 569, 1904. VAN DIJK, 'Ann. der Phys.,' 19, 

 p. 249, 1906. 



