292 Mr. J. Daniel on the Polarization upon a 



brings us to the second problem, i. e. : — The minimum 

 current-strength at which the ions are deposited visibly on the 

 gold-leaf partition for various electrolytes. 



Critical Current-Density for Gold-leaf. 



This part of the investigation consists simply in closing the 

 circuit on the voltameter with a high auxiliary resistance, 

 gradually and slowly increasing the current by decreasing 

 the resistance, and observing at what current-strength bubbles 

 of gas or deposit of metal begin to appear upon the partition. 

 Table IV. shows these values for various solutions used as 

 electrolytes, with a gold-leaf partition. Table V. gives the 

 results in 30 per cent. H 2 S0 4 for aluminium -leaf, platinum- 

 leaf, and gold-leaf respectively. 



Table v. shows the same for a gold-leaf partition in H 2 S0 4 

 of various concentrations from 100 per cent. (?) to 0*5 per cent. 



Unfortunately, the importance of the concentration of the 

 solution in affecting the value of the "critical current" was 

 not noted until after several of the solutions had been tested 

 and thrown away without measuring their concentration or 

 specific gravity, which is therefore not given for those cases. 

 The specific gravity of those solutions for which it is given 

 was determined by a very sensitive and accurate set of hydro- 

 meters reading directly to 0*001. Table V. gives the case of 

 30 per cent. H 2 S0 4 with Pt partitions of various thicknesses ; 

 also of a " thick " Pt partition perforated with 1, 2, 3, and 4 

 holes successively, each 0*5 millim. in diameter. This is 

 important as showing that the very small holes which may 

 have existed in the carefully selected gold leaves used as 

 partitions could not have had much, if even a sensible, 

 influence on the value of the critical current. 



Table VI. shows the interesting case of a 0*02 millim. 

 palladium partition in 30 per cent. H 2 S0 4 . 



The values given in the Tables are in most cases the mean 

 of several determinations which differed but slightly. 



The importance of having the edges of the partition covered 

 well up to the edge of the hole in the glass was emphasized 

 in this work on account of the fact that the anode was usually 

 placed on one side, and sometimes even at the back of the 

 inner vessel, instead of in front, in order not to obscure a free 

 inspection of the partition, and to avoid the disturbance of 

 the gas escaping from the anode, as this was considerable 

 with the strongest currents used, i. e. ten to twelve amperes. 

 Thus one edge of the exposed partition being nearer to the 

 anode than the opening, the deposit would always begin first 

 on the nearest edge. 



