71 Dr. H. J. S. Sand on the Concentration 



densities were employed, the metal requiring the higher 

 E.M.F. for its liberation was always deposited in relative 

 quantities which far exceeded its relative concentration in 

 the solutions. 



Firstly, as regards the values of the lowest current-den- 

 sities used by the author, it seems probable that these would 

 have been sufficient to make the concentrations at his cathode 

 go down to zero in times varying from ten to one-tenth of a 

 second if no convection-currents had interfered. This can 

 he taken from the values given for concentrations and current- 

 density in the thesis referred to, if, in order to obtain nume- 

 rical values, we assume that diffusion-coefficients &c. had been 

 the same in the author's solutions as in mine. None of his 

 current-densities can therefore be described as small in the 

 sense used in this paper; and I do not think there can be any 

 doubt that if sufficiently small values had been used, the 

 metal requiring the lower E.M.F. for its deposition would 

 have preponderated in the alloy formed. As regards the 

 actual preponderance of the baser of the two metals, a great 

 deal of light could no doubt be thrown on this by determi- 

 nations of transport values in the solutions examined, as will 

 be seen from what has been stated above. If, as Toepffer 

 assumes, the nobler metal has the tendency to form complex 

 anions to a greater extent than the baser, this would certainly 

 greatly influence the transport values in the direction required 

 to explain the results. 



The main purpose of the experiments I am now about to 

 describe is to show to how great an extent the relative 

 quantity of hydrogen, given off in the electrolysis of an acid 

 solution of copper sulphate, is influenced by convection- 

 currents ; and also to show that when these are artificially 

 increased by stirring, the hydrogen can be made to disappear 

 altogether, even in cases in which otherwise more of its 

 equivalents than of those of copper would have been 

 liberated. 



For this purpose the apparatus was devised, shown in the 

 accompanying figure (fig. 5), which allowed the course of 

 the electrolysis to be continually followed by measuring the 

 quantity of hydrogen given off, and in which, at the same 

 time, the solution above the cathode could, when desired, be 

 rapidly renewed by vigorous stirring. It will be seen that it 

 resembles a desiccator in general appearance, but has two 

 openings in the side and two in the lid. Through one of the 

 side-openings the tube a passes leading to the cup c, which is 

 rilled with mercury, forming the cathode, uniform deposition 

 of the copper being thus ensured. The glass tube b passing 



