JUN^ ,6, 1902;] 



SCIENCE. 



887 



speetive electrodes in equivalent quantity, 

 that is, in the ease where the valency of 

 anion and cation is the same, in equal num- 

 bers, it never occurred to any one to doubt 

 that they traveled towards the electrodes 

 at the same rate, until Daniell and Miller 

 showed that this hypothesis is erroneous. 

 To follow their reasoning and that of Hit- 

 torf we may take an imaginary case, and 

 suppose an electrolyte MX with its cation 

 M and its anion A' of such character that 

 these ions when separated at the electrodes 

 can be removed from the solution, com- 

 pletely and at once, and that the electrol- 

 ysis is carried on in a vessel provided with 

 two compartments, one containing- the cath- 

 ode and the other the anode, such that what- 

 ever happens at an electrode shall affect 

 only the contents of the compartment con- 

 taining that electrode, and so arranged that 

 the liquid contained in each compartment 

 can be completely removed from it and 

 analyzed. 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 concentration of the solution in this 

 compartment 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 solu- 

 tion 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 31 X. 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 con- 

 sists not only of what enters it, but also of 

 .the excess resulting from the departure of 

 the other kind. Without taking any more 

 eases 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 elec- 

 trodes. But this really is the same thing. 

 If we imagine two processions walking with 

 the same length of step and the same num- 

 ber 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 ivalked 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 

 ■wath what has since been made out as to 

 the rate of movement of the ions. 



Hittorf 's work had to wait long for recog- 

 nition, but we now know its great impor- 

 tance, not only on account of the large num- 

 ber of accurate measurements, but also be- 

 cause of the general conclusions he drew 

 from them. He deduced from the ti-ansfer 

 numbers conclusions as to the nature of the 

 solute, showing, for instance, that solution 

 of stannic chloride electrolyzes as hydro- 

 chloric acid, the stannic chloride being com- 

 pletely hydrolyzed. He also showed that 

 such double salts as KCN,AgCN, 2KC1,- 



