IONIC VELOCITIES. 3 8i 



resistance, and of slightly different density, such for instance 

 as decinormal solutions of copper and ammonium chlorides 

 with enough ammonia added to each to bring out the deep 

 blue colour of the copper. The ammonium chloride solution 

 is rather lighter than the other and can be gently poured on 

 to its surface, so that a fairly sharp line of junction is obtained 

 in a vertical tube. When a current is passed across this 

 junction it carries the copper and ammonium ions forward 

 with it, while the chlorine ions in both solutions move in 

 the opposite direction. Since the blue colour depends on 

 the presence of the copper ion, the boundary between 

 the colours moves with the current and its speed can be 

 measured. It was proved by observation that the speed is 

 proportional to the potential gradient, and hence the specific 

 ionic velocity for unit gradient can be deduced. For copper 

 it came out '000309. For infinite dilution Kohlrausch gives 

 '00031, but this is not strictly comparable. It is a little 

 difficult to get a value from theory for the strength of solu- 

 tion used for this experiment, as no migration data for copper 

 chloride are known, but the number is certainly less than 

 '00030. The motion of an acid radicle was investigated by 

 using solutions of potassium bichromate and potassium car- 

 bonate. The ionic velocity of the bichromic acid group 

 (Cr 2 7 ) was found to be "00047, "00048, and "00046 by three 

 experiments. From the conductivity and migration data 

 the number •ooo^jt, can be calculated from Kohlrausch's 

 theory. The method was then extended to the case of 

 alcohol solutions, and measurements were made with solu- 

 tions of cobalt nitrate (red) and cobalt chloride (blue). The 

 agreement with theory was again fairly close. 



Further experiments are now being made ; the behaviour 

 of solutions in agar jelly i-s under investigation, and it is 

 hoped that measurements will be obtained in the case of 

 solutions of abnormally low conductivity, such as those of 

 ammonia and acetic acid. 



It seems certain then that Kohlrausch's theory of ionic 

 velocity represents the truth, at all events in the case 

 of good electrolytes in dilute solution. Let us consider 

 what happens when the concentration is greater and the 



