1902.] on the Ions of Electrolysis. 3^ 



once, and that the electrolysis is carried on in a vessel provided with 

 two compartments, one containing the cathode and the other the 

 anode, such that whatever happens at an electrode shall affect only 

 the contents of the compartment containing that electrode, and so 

 arranged that the liquid contained in each compartment can be 

 completely removed from it and analysed. Now, let us first suppose 

 MX to be such that its ions travel at the same rate. In the time 

 then in which one M has entered the cathode compartment one X 

 has left it. There is at this moment an excess of two M's in this 

 compartment, these are deposited at the cathode, and now the con- 

 centration of the solution in this comjDartment is diminished by one 

 MX. Similarly at the anode during the same time one X has 

 entered and one M has left, two X's have been deposited and the 

 solution has lost one MX. In this case, then, where the two sets of 

 ions travel at the same rate, the loss of solute is the same at the two 

 electrodes. Let us now suppose an extreme case in which one of 

 the sets of ions (say the cations) does not travel at all. In the 

 time in which one X leaves the cathode compartment no M enters it, 

 the excess of one M is deposited, and the solution here has lost one 

 MX. At the anode one X has entered and no M has left, the X is 

 deposited, and the solution here has lost no MX. Again, take the 

 case that the anions travel twice as fast as the cations. Here in the 

 time in which one M enters the cathode compartment two X's leave 

 it, the excess of three M's is deposited, and the solution has lost two 

 MX's. At the anode during the same time one M has left and two 

 X's have entered, the three X's have been deposited and the solution 

 has lost one MX. Of course it will be seen that the excess of one 

 kind in a compartment consists not only of what enters it, but also 

 of the excess resulting from the departure of the other kind. With- 

 out taking any more cases we at once see that the speed of the cation 

 is to that of the anion as the loss of solute at the anode is to that 

 at the cathode. This non-equivalent transfer has sometimes been 

 described in another way. It has been said that the ions go at the 

 same rate, but that at the same time the solute as a whole is being 

 moved towards one of the electrodes. But this really is the same 

 thing. If we imagine two processions walking with the same length 

 of step and the same number of steps a minute in opposite directions 

 on such a moving platform as that in the Paris Exhibition, we might 

 no doubt say that the two walked at the same rate, they could not be 

 said to travel at the same rate. Hittorf's way of putting it is not 

 only the simpler way, it is the only way that agrees with what has 

 since been made out as to the rate of movement of the ions. 



Hittorf's work had to wait long for recognition, but we now know its 

 great importance, not only on account of the large number of accurate 

 measurements, but also because of the general conclusions he drew 

 from them. He deduced from the transfer numbers conclusions as 

 to the nature of the solute, showing, for instance, that solution of 

 stannic chloride electrolyses as hydrochloric acid, the stannic chloride 



Vol. XVII. (No. 96.) d 



