ELECTROLYSIS AND ELECTRO-CHEMISTEy. 231 



hydrogen ion through a jelly solution under a known potential gradient 

 could be observed. The results of three experiments gave 0-0029, 0'0026, 

 and 0*0024 cm. per sec. as the velocity of the hydrogen ion for a poten- 

 tial gradient of one volt per centimetre. Kohlrausch's number is 0*0032 

 for the dilution corresponding to maximum conductivity. Lodge does 

 not mentioQ the concentration of his solution, but it was probably large 

 enough to appreciably reduce the velocity. 



When the current density at the kathode of a solution of copper sul- 

 phate exceeds a certain limit, the copper is deposited as a brown or black 

 hydride. C. L. "Weber ' explained this as due to the inability of the copper 

 ions to migrate fast enough to keep up the supply for carrying the 

 current, part of which will consequently be conveyed by sulphuric acid 

 formed by the action of SO4 ions on the water. By measuring the limiting 

 current density and the conductivity of the solution, he estimated the 

 speed of the copper ions when they could travel just fast enough to carry 

 all the current, and hence he deduced their specific velocity. Similar 

 methods were used for solutions of cadmium sulphate and zinc nitrate. 

 The copper sulphate measurements were repeated with an improved appa- 

 ratus by Sheldon and Downing.^ This method does not appear to be a 

 very good one, for the dilution of the liquid round the kathode makes 

 it impossible to accurately determine the conductivity of the solution 

 concerned. This source of error will make the deduced velocities too 

 great. 



Direct determinations of the velocities of a few other ions have been 

 made in another way by the present writer.^ Two solutions, having one 

 ion in common, of equivalent concentrations, different densities, different 

 colours, and nearly equal specific resistances, were placed one over the other 

 in a vertical glass tube. In one case, for example, decinormal solutions of 

 potassium carbonate and potassium bichromate were used. The colour of 

 the latter is due to the presence of the bichromate group, CrjOy. When a 

 current was passed across the junction, the anions CI and Cr207 travelled 

 in the direction opposite to that of the current, and their velocity could 

 be determined by measuring the rate at which the colour boundary moved. 

 Similar experiments were made with alcoholic solutions of cobalt salts, in 

 which the velocities of the ions were found to be much less than in water. 

 The behaviour of agar jelly was then investigated, and the velocity of an ion 

 was shown to be very little less through a solid jelly than in an ordinary 

 liquid solution. The velocities could therefore be measured by tracing 

 the change in colour of an indicator or the formation of a precipitate. 

 Thus decinormal jelly solutions of barium chloride and sodium chloride, 

 the latter containing a trace of sodium sulphate, were placed in contact. 

 Under the influence of an electromotive force, the barium ions moved up 

 the tube, and their presence was shown by the trace of insoluble barium 

 sulphate formed. 



The following table shows the velocities of all ions which have been 

 experimentally determined. A comparison is given with their values as 

 calculated, for the same concentration, on Kohlrausch's theory. 



' Zeits. physihal. Cliem. 1889, vol. iv. p. 182. 

 ^ Physical Ilevie7v. 1893, vol. i. p 51. 



= Phil. Tram. 1893, vol. clxxxiv. A, p. 337; Phil. Mag., October, 1894; Phil. 

 Trans. 1895, vol. clxxxvi. A, p. 507. 



